2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
8 @node Simple Data Types
9 @section Simple Generic Data Types
11 This chapter describes those of Guile's simple data types which are
12 primarily used for their role as items of generic data. By
13 @dfn{simple} we mean data types that are not primarily used as
14 containers to hold other data --- i.e.@: pairs, lists, vectors and so on.
15 For the documentation of such @dfn{compound} data types, see
16 @ref{Compound Data Types}.
18 @c One of the great strengths of Scheme is that there is no straightforward
19 @c distinction between ``data'' and ``functionality''. For example,
20 @c Guile's support for dynamic linking could be described:
24 @c either in a ``data-centric'' way, as the behaviour and properties of the
25 @c ``dynamically linked object'' data type, and the operations that may be
26 @c applied to instances of this type
29 @c or in a ``functionality-centric'' way, as the set of procedures that
30 @c constitute Guile's support for dynamic linking, in the context of the
34 @c The contents of this chapter are, therefore, a matter of judgment. By
35 @c @dfn{generic}, we mean to select those data types whose typical use as
36 @c @emph{data} in a wide variety of programming contexts is more important
37 @c than their use in the implementation of a particular piece of
38 @c @emph{functionality}. The last section of this chapter provides
39 @c references for all the data types that are documented not here but in a
40 @c ``functionality-centric'' way elsewhere in the manual.
43 * Booleans:: True/false values.
44 * Numbers:: Numerical data types.
45 * Characters:: Single characters.
46 * Character Sets:: Sets of characters.
47 * Strings:: Sequences of characters.
48 * Regular Expressions:: Pattern matching and substitution.
50 * Keywords:: Self-quoting, customizable display keywords.
51 * Other Types:: "Functionality-centric" data types.
59 The two boolean values are @code{#t} for true and @code{#f} for false.
61 Boolean values are returned by predicate procedures, such as the general
62 equality predicates @code{eq?}, @code{eqv?} and @code{equal?}
63 (@pxref{Equality}) and numerical and string comparison operators like
64 @code{string=?} (@pxref{String Comparison}) and @code{<=}
74 (equal? "house" "houses")
82 In test condition contexts like @code{if} and @code{cond} (@pxref{if
83 cond case}), where a group of subexpressions will be evaluated only if a
84 @var{condition} expression evaluates to ``true'', ``true'' means any
85 value at all except @code{#f}.
98 A result of this asymmetry is that typical Scheme source code more often
99 uses @code{#f} explicitly than @code{#t}: @code{#f} is necessary to
100 represent an @code{if} or @code{cond} false value, whereas @code{#t} is
101 not necessary to represent an @code{if} or @code{cond} true value.
103 It is important to note that @code{#f} is @strong{not} equivalent to any
104 other Scheme value. In particular, @code{#f} is not the same as the
105 number 0 (like in C and C++), and not the same as the ``empty list''
106 (like in some Lisp dialects).
108 In C, the two Scheme boolean values are available as the two constants
109 @code{SCM_BOOL_T} for @code{#t} and @code{SCM_BOOL_F} for @code{#f}.
110 Care must be taken with the false value @code{SCM_BOOL_F}: it is not
111 false when used in C conditionals. In order to test for it, use
112 @code{scm_is_false} or @code{scm_is_true}.
115 @deffn {Scheme Procedure} not x
116 @deffnx {C Function} scm_not (x)
117 Return @code{#t} if @var{x} is @code{#f}, else return @code{#f}.
121 @deffn {Scheme Procedure} boolean? obj
122 @deffnx {C Function} scm_boolean_p (obj)
123 Return @code{#t} if @var{obj} is either @code{#t} or @code{#f}, else
127 @deftypevr {C Macro} SCM SCM_BOOL_T
128 The @code{SCM} representation of the Scheme object @code{#t}.
131 @deftypevr {C Macro} SCM SCM_BOOL_F
132 The @code{SCM} representation of the Scheme object @code{#f}.
135 @deftypefn {C Function} int scm_is_true (SCM obj)
136 Return @code{0} if @var{obj} is @code{#f}, else return @code{1}.
139 @deftypefn {C Function} int scm_is_false (SCM obj)
140 Return @code{1} if @var{obj} is @code{#f}, else return @code{0}.
143 @deftypefn {C Function} int scm_is_bool (SCM obj)
144 Return @code{1} if @var{obj} is either @code{#t} or @code{#f}, else
148 @deftypefn {C Function} SCM scm_from_bool (int val)
149 Return @code{#f} if @var{val} is @code{0}, else return @code{#t}.
152 @deftypefn {C Function} int scm_to_bool (SCM val)
153 Return @code{1} if @var{val} is @code{SCM_BOOL_T}, return @code{0}
154 when @var{val} is @code{SCM_BOOL_F}, else signal a `wrong type' error.
156 You should probably use @code{scm_is_true} instead of this function
157 when you just want to test a @code{SCM} value for trueness.
161 @subsection Numerical data types
164 Guile supports a rich ``tower'' of numerical types --- integer,
165 rational, real and complex --- and provides an extensive set of
166 mathematical and scientific functions for operating on numerical
167 data. This section of the manual documents those types and functions.
169 You may also find it illuminating to read R5RS's presentation of numbers
170 in Scheme, which is particularly clear and accessible: see
171 @ref{Numbers,,,r5rs,R5RS}.
174 * Numerical Tower:: Scheme's numerical "tower".
175 * Integers:: Whole numbers.
176 * Reals and Rationals:: Real and rational numbers.
177 * Complex Numbers:: Complex numbers.
178 * Exactness:: Exactness and inexactness.
179 * Number Syntax:: Read syntax for numerical data.
180 * Integer Operations:: Operations on integer values.
181 * Comparison:: Comparison predicates.
182 * Conversion:: Converting numbers to and from strings.
183 * Complex:: Complex number operations.
184 * Arithmetic:: Arithmetic functions.
185 * Scientific:: Scientific functions.
186 * Primitive Numerics:: Primitive numeric functions.
187 * Bitwise Operations:: Logical AND, OR, NOT, and so on.
188 * Random:: Random number generation.
192 @node Numerical Tower
193 @subsubsection Scheme's Numerical ``Tower''
196 Scheme's numerical ``tower'' consists of the following categories of
201 Whole numbers, positive or negative; e.g.@: --5, 0, 18.
204 The set of numbers that can be expressed as @math{@var{p}/@var{q}}
205 where @var{p} and @var{q} are integers; e.g.@: @math{9/16} works, but
206 pi (an irrational number) doesn't. These include integers
210 The set of numbers that describes all possible positions along a
211 one-dimensional line. This includes rationals as well as irrational
214 @item complex numbers
215 The set of numbers that describes all possible positions in a two
216 dimensional space. This includes real as well as imaginary numbers
217 (@math{@var{a}+@var{b}i}, where @var{a} is the @dfn{real part},
218 @var{b} is the @dfn{imaginary part}, and @math{i} is the square root of
222 It is called a tower because each category ``sits on'' the one that
223 follows it, in the sense that every integer is also a rational, every
224 rational is also real, and every real number is also a complex number
225 (but with zero imaginary part).
227 In addition to the classification into integers, rationals, reals and
228 complex numbers, Scheme also distinguishes between whether a number is
229 represented exactly or not. For example, the result of
230 @m{2\sin(\pi/4),2*sin(pi/4)} is exactly @m{\sqrt{2},2^(1/2)}, but Guile
231 can represent neither @m{\pi/4,pi/4} nor @m{\sqrt{2},2^(1/2)} exactly.
232 Instead, it stores an inexact approximation, using the C type
235 Guile can represent exact rationals of any magnitude, inexact
236 rationals that fit into a C @code{double}, and inexact complex numbers
237 with @code{double} real and imaginary parts.
239 The @code{number?} predicate may be applied to any Scheme value to
240 discover whether the value is any of the supported numerical types.
242 @deffn {Scheme Procedure} number? obj
243 @deffnx {C Function} scm_number_p (obj)
244 Return @code{#t} if @var{obj} is any kind of number, else @code{#f}.
253 (number? "hello there!")
256 (define pi 3.141592654)
261 @deftypefn {C Function} int scm_is_number (SCM obj)
262 This is equivalent to @code{scm_is_true (scm_number_p (obj))}.
265 The next few subsections document each of Guile's numerical data types
269 @subsubsection Integers
271 @tpindex Integer numbers
275 Integers are whole numbers, that is numbers with no fractional part,
276 such as 2, 83, and @minus{}3789.
278 Integers in Guile can be arbitrarily big, as shown by the following
282 (define (factorial n)
283 (let loop ((n n) (product 1))
286 (loop (- n 1) (* product n)))))
292 @result{} 2432902008176640000
295 @result{} -119622220865480194561963161495657715064383733760000000000
298 Readers whose background is in programming languages where integers are
299 limited by the need to fit into just 4 or 8 bytes of memory may find
300 this surprising, or suspect that Guile's representation of integers is
301 inefficient. In fact, Guile achieves a near optimal balance of
302 convenience and efficiency by using the host computer's native
303 representation of integers where possible, and a more general
304 representation where the required number does not fit in the native
305 form. Conversion between these two representations is automatic and
306 completely invisible to the Scheme level programmer.
308 The infinities @samp{+inf.0} and @samp{-inf.0} are considered to be
309 inexact integers. They are explained in detail in the next section,
310 together with reals and rationals.
312 C has a host of different integer types, and Guile offers a host of
313 functions to convert between them and the @code{SCM} representation.
314 For example, a C @code{int} can be handled with @code{scm_to_int} and
315 @code{scm_from_int}. Guile also defines a few C integer types of its
316 own, to help with differences between systems.
318 C integer types that are not covered can be handled with the generic
319 @code{scm_to_signed_integer} and @code{scm_from_signed_integer} for
320 signed types, or with @code{scm_to_unsigned_integer} and
321 @code{scm_from_unsigned_integer} for unsigned types.
323 Scheme integers can be exact and inexact. For example, a number
324 written as @code{3.0} with an explicit decimal-point is inexact, but
325 it is also an integer. The functions @code{integer?} and
326 @code{scm_is_integer} report true for such a number, but the functions
327 @code{scm_is_signed_integer} and @code{scm_is_unsigned_integer} only
328 allow exact integers and thus report false. Likewise, the conversion
329 functions like @code{scm_to_signed_integer} only accept exact
332 The motivation for this behavior is that the inexactness of a number
333 should not be lost silently. If you want to allow inexact integers,
334 you can explicitely insert a call to @code{inexact->exact} or to its C
335 equivalent @code{scm_inexact_to_exact}. (Only inexact integers will
336 be converted by this call into exact integers; inexact non-integers
337 will become exact fractions.)
339 @deffn {Scheme Procedure} integer? x
340 @deffnx {C Function} scm_integer_p (x)
341 Return @code{#t} if @var{x} is an exact or inexact integer number, else
359 @deftypefn {C Function} int scm_is_integer (SCM x)
360 This is equivalent to @code{scm_is_true (scm_integer_p (x))}.
363 @defvr {C Type} scm_t_int8
364 @defvrx {C Type} scm_t_uint8
365 @defvrx {C Type} scm_t_int16
366 @defvrx {C Type} scm_t_uint16
367 @defvrx {C Type} scm_t_int32
368 @defvrx {C Type} scm_t_uint32
369 @defvrx {C Type} scm_t_int64
370 @defvrx {C Type} scm_t_uint64
371 @defvrx {C Type} scm_t_intmax
372 @defvrx {C Type} scm_t_uintmax
373 The C types are equivalent to the corresponding ISO C types but are
374 defined on all platforms, with the exception of @code{scm_t_int64} and
375 @code{scm_t_uint64}, which are only defined when a 64-bit type is
376 available. For example, @code{scm_t_int8} is equivalent to
379 You can regard these definitions as a stop-gap measure until all
380 platforms provide these types. If you know that all the platforms
381 that you are interested in already provide these types, it is better
382 to use them directly instead of the types provided by Guile.
385 @deftypefn {C Function} int scm_is_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
386 @deftypefnx {C Function} int scm_is_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
387 Return @code{1} when @var{x} represents an exact integer that is
388 between @var{min} and @var{max}, inclusive.
390 These functions can be used to check whether a @code{SCM} value will
391 fit into a given range, such as the range of a given C integer type.
392 If you just want to convert a @code{SCM} value to a given C integer
393 type, use one of the conversion functions directly.
396 @deftypefn {C Function} scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
397 @deftypefnx {C Function} scm_t_uintmax scm_to_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
398 When @var{x} represents an exact integer that is between @var{min} and
399 @var{max} inclusive, return that integer. Else signal an error,
400 either a `wrong-type' error when @var{x} is not an exact integer, or
401 an `out-of-range' error when it doesn't fit the given range.
404 @deftypefn {C Function} SCM scm_from_signed_integer (scm_t_intmax x)
405 @deftypefnx {C Function} SCM scm_from_unsigned_integer (scm_t_uintmax x)
406 Return the @code{SCM} value that represents the integer @var{x}. This
407 function will always succeed and will always return an exact number.
410 @deftypefn {C Function} char scm_to_char (SCM x)
411 @deftypefnx {C Function} {signed char} scm_to_schar (SCM x)
412 @deftypefnx {C Function} {unsigned char} scm_to_uchar (SCM x)
413 @deftypefnx {C Function} short scm_to_short (SCM x)
414 @deftypefnx {C Function} {unsigned short} scm_to_ushort (SCM x)
415 @deftypefnx {C Function} int scm_to_int (SCM x)
416 @deftypefnx {C Function} {unsigned int} scm_to_uint (SCM x)
417 @deftypefnx {C Function} long scm_to_long (SCM x)
418 @deftypefnx {C Function} {unsigned long} scm_to_ulong (SCM x)
419 @deftypefnx {C Function} {long long} scm_to_long_long (SCM x)
420 @deftypefnx {C Function} {unsigned long long} scm_to_ulong_long (SCM x)
421 @deftypefnx {C Function} size_t scm_to_size_t (SCM x)
422 @deftypefnx {C Function} ssize_t scm_to_ssize_t (SCM x)
423 @deftypefnx {C Function} scm_t_int8 scm_to_int8 (SCM x)
424 @deftypefnx {C Function} scm_t_uint8 scm_to_uint8 (SCM x)
425 @deftypefnx {C Function} scm_t_int16 scm_to_int16 (SCM x)
426 @deftypefnx {C Function} scm_t_uint16 scm_to_uint16 (SCM x)
427 @deftypefnx {C Function} scm_t_int32 scm_to_int32 (SCM x)
428 @deftypefnx {C Function} scm_t_uint32 scm_to_uint32 (SCM x)
429 @deftypefnx {C Function} scm_t_int64 scm_to_int64 (SCM x)
430 @deftypefnx {C Function} scm_t_uint64 scm_to_uint64 (SCM x)
431 @deftypefnx {C Function} scm_t_intmax scm_to_intmax (SCM x)
432 @deftypefnx {C Function} scm_t_uintmax scm_to_uintmax (SCM x)
433 When @var{x} represents an exact integer that fits into the indicated
434 C type, return that integer. Else signal an error, either a
435 `wrong-type' error when @var{x} is not an exact integer, or an
436 `out-of-range' error when it doesn't fit the given range.
438 The functions @code{scm_to_long_long}, @code{scm_to_ulong_long},
439 @code{scm_to_int64}, and @code{scm_to_uint64} are only available when
440 the corresponding types are.
443 @deftypefn {C Function} SCM scm_from_char (char x)
444 @deftypefnx {C Function} SCM scm_from_schar (signed char x)
445 @deftypefnx {C Function} SCM scm_from_uchar (unsigned char x)
446 @deftypefnx {C Function} SCM scm_from_short (short x)
447 @deftypefnx {C Function} SCM scm_from_ushort (unsigned short x)
448 @deftypefnx {C Function} SCM scm_from_int (int x)
449 @deftypefnx {C Function} SCM scm_from_uint (unsigned int x)
450 @deftypefnx {C Function} SCM scm_from_long (long x)
451 @deftypefnx {C Function} SCM scm_from_ulong (unsigned long x)
452 @deftypefnx {C Function} SCM scm_from_long_long (long long x)
453 @deftypefnx {C Function} SCM scm_from_ulong_long (unsigned long long x)
454 @deftypefnx {C Function} SCM scm_from_size_t (size_t x)
455 @deftypefnx {C Function} SCM scm_from_ssize_t (ssize_t x)
456 @deftypefnx {C Function} SCM scm_from_int8 (scm_t_int8 x)
457 @deftypefnx {C Function} SCM scm_from_uint8 (scm_t_uint8 x)
458 @deftypefnx {C Function} SCM scm_from_int16 (scm_t_int16 x)
459 @deftypefnx {C Function} SCM scm_from_uint16 (scm_t_uint16 x)
460 @deftypefnx {C Function} SCM scm_from_int32 (scm_t_int32 x)
461 @deftypefnx {C Function} SCM scm_from_uint32 (scm_t_uint32 x)
462 @deftypefnx {C Function} SCM scm_from_int64 (scm_t_int64 x)
463 @deftypefnx {C Function} SCM scm_from_uint64 (scm_t_uint64 x)
464 @deftypefnx {C Function} SCM scm_from_intmax (scm_t_intmax x)
465 @deftypefnx {C Function} SCM scm_from_uintmax (scm_t_uintmax x)
466 Return the @code{SCM} value that represents the integer @var{x}.
467 These functions will always succeed and will always return an exact
471 @deftypefn {C Function} void scm_to_mpz (SCM val, mpz_t rop)
472 Assign @var{val} to the multiple precision integer @var{rop}.
473 @var{val} must be an exact integer, otherwise an error will be
474 signalled. @var{rop} must have been initialized with @code{mpz_init}
475 before this function is called. When @var{rop} is no longer needed
476 the occupied space must be freed with @code{mpz_clear}.
477 @xref{Initializing Integers,,, gmp, GNU MP Manual}, for details.
480 @deftypefn {C Function} SCM scm_from_mpz (mpz_t val)
481 Return the @code{SCM} value that represents @var{val}.
484 @node Reals and Rationals
485 @subsubsection Real and Rational Numbers
486 @tpindex Real numbers
487 @tpindex Rational numbers
492 Mathematically, the real numbers are the set of numbers that describe
493 all possible points along a continuous, infinite, one-dimensional line.
494 The rational numbers are the set of all numbers that can be written as
495 fractions @var{p}/@var{q}, where @var{p} and @var{q} are integers.
496 All rational numbers are also real, but there are real numbers that
497 are not rational, for example @m{\sqrt2, the square root of 2}, and
500 Guile can represent both exact and inexact rational numbers, but it
501 can not represent irrational numbers. Exact rationals are represented
502 by storing the numerator and denominator as two exact integers.
503 Inexact rationals are stored as floating point numbers using the C
506 Exact rationals are written as a fraction of integers. There must be
507 no whitespace around the slash:
514 Even though the actual encoding of inexact rationals is in binary, it
515 may be helpful to think of it as a decimal number with a limited
516 number of significant figures and a decimal point somewhere, since
517 this corresponds to the standard notation for non-whole numbers. For
523 -5648394822220000000000.0
527 The limited precision of Guile's encoding means that any ``real'' number
528 in Guile can be written in a rational form, by multiplying and then dividing
529 by sufficient powers of 10 (or in fact, 2). For example,
530 @samp{-0.00000142857931198} is the same as @minus{}142857931198 divided by
531 100000000000000000. In Guile's current incarnation, therefore, the
532 @code{rational?} and @code{real?} predicates are equivalent.
535 Dividing by an exact zero leads to a error message, as one might
536 expect. However, dividing by an inexact zero does not produce an
537 error. Instead, the result of the division is either plus or minus
538 infinity, depending on the sign of the divided number.
540 The infinities are written @samp{+inf.0} and @samp{-inf.0},
541 respectivly. This syntax is also recognized by @code{read} as an
542 extension to the usual Scheme syntax.
544 Dividing zero by zero yields something that is not a number at all:
545 @samp{+nan.0}. This is the special `not a number' value.
547 On platforms that follow @acronym{IEEE} 754 for their floating point
548 arithmetic, the @samp{+inf.0}, @samp{-inf.0}, and @samp{+nan.0} values
549 are implemented using the corresponding @acronym{IEEE} 754 values.
550 They behave in arithmetic operations like @acronym{IEEE} 754 describes
551 it, i.e., @code{(= +nan.0 +nan.0)} @result{} @code{#f}.
553 The infinities are inexact integers and are considered to be both even
554 and odd. While @samp{+nan.0} is not @code{=} to itself, it is
555 @code{eqv?} to itself.
557 To test for the special values, use the functions @code{inf?} and
560 @deffn {Scheme Procedure} real? obj
561 @deffnx {C Function} scm_real_p (obj)
562 Return @code{#t} if @var{obj} is a real number, else @code{#f}. Note
563 that the sets of integer and rational values form subsets of the set
564 of real numbers, so the predicate will also be fulfilled if @var{obj}
565 is an integer number or a rational number.
568 @deffn {Scheme Procedure} rational? x
569 @deffnx {C Function} scm_rational_p (x)
570 Return @code{#t} if @var{x} is a rational number, @code{#f} otherwise.
571 Note that the set of integer values forms a subset of the set of
572 rational numbers, i. e. the predicate will also be fulfilled if
573 @var{x} is an integer number.
575 Since Guile can not represent irrational numbers, every number
576 satisfying @code{real?} also satisfies @code{rational?} in Guile.
579 @deffn {Scheme Procedure} rationalize x eps
580 @deffnx {C Function} scm_rationalize (x, eps)
581 Returns the @emph{simplest} rational number differing
582 from @var{x} by no more than @var{eps}.
584 As required by @acronym{R5RS}, @code{rationalize} only returns an
585 exact result when both its arguments are exact. Thus, you might need
586 to use @code{inexact->exact} on the arguments.
589 (rationalize (inexact->exact 1.2) 1/100)
595 @deffn {Scheme Procedure} inf? x
596 @deffnx {C Function} scm_inf_p (x)
597 Return @code{#t} if @var{x} is either @samp{+inf.0} or @samp{-inf.0},
601 @deffn {Scheme Procedure} nan? x
602 @deffnx {C Function} scm_nan_p (x)
603 Return @code{#t} if @var{x} is @samp{+nan.0}, @code{#f} otherwise.
606 @deffn {Scheme Procedure} nan
607 @deffnx {C Function} scm_nan ()
611 @deffn {Scheme Procedure} inf
612 @deffnx {C Function} scm_inf ()
616 @deffn {Scheme Procedure} numerator x
617 @deffnx {C Function} scm_numerator (x)
618 Return the numerator of the rational number @var{x}.
621 @deffn {Scheme Procedure} denominator x
622 @deffnx {C Function} scm_denominator (x)
623 Return the denominator of the rational number @var{x}.
626 @deftypefn {C Function} int scm_is_real (SCM val)
627 @deftypefnx {C Function} int scm_is_rational (SCM val)
628 Equivalent to @code{scm_is_true (scm_real_p (val))} and
629 @code{scm_is_true (scm_rational_p (val))}, respectively.
632 @deftypefn {C Function} double scm_to_double (SCM val)
633 Returns the number closest to @var{val} that is representable as a
634 @code{double}. Returns infinity for a @var{val} that is too large in
635 magnitude. The argument @var{val} must be a real number.
638 @deftypefn {C Function} SCM scm_from_double (double val)
639 Return the @code{SCM} value that representats @var{val}. The returned
640 value is inexact according to the predicate @code{inexact?}, but it
641 will be exactly equal to @var{val}.
644 @node Complex Numbers
645 @subsubsection Complex Numbers
646 @tpindex Complex numbers
650 Complex numbers are the set of numbers that describe all possible points
651 in a two-dimensional space. The two coordinates of a particular point
652 in this space are known as the @dfn{real} and @dfn{imaginary} parts of
653 the complex number that describes that point.
655 In Guile, complex numbers are written in rectangular form as the sum of
656 their real and imaginary parts, using the symbol @code{i} to indicate
671 Polar form can also be used, with an @samp{@@} between magnitude and
675 1@@3.141592 @result{} -1.0 (approx)
676 -1@@1.57079 @result{} 0.0-1.0i (approx)
679 Guile represents a complex number with a non-zero imaginary part as a
680 pair of inexact rationals, so the real and imaginary parts of a
681 complex number have the same properties of inexactness and limited
682 precision as single inexact rational numbers. Guile can not represent
683 exact complex numbers with non-zero imaginary parts.
685 @deffn {Scheme Procedure} complex? z
686 @deffnx {C Function} scm_complex_p (z)
687 Return @code{#t} if @var{x} is a complex number, @code{#f}
688 otherwise. Note that the sets of real, rational and integer
689 values form subsets of the set of complex numbers, i. e. the
690 predicate will also be fulfilled if @var{x} is a real,
691 rational or integer number.
694 @deftypefn {C Function} int scm_is_complex (SCM val)
695 Equivalent to @code{scm_is_true (scm_complex_p (val))}.
699 @subsubsection Exact and Inexact Numbers
700 @tpindex Exact numbers
701 @tpindex Inexact numbers
705 @rnindex exact->inexact
706 @rnindex inexact->exact
708 R5RS requires that a calculation involving inexact numbers always
709 produces an inexact result. To meet this requirement, Guile
710 distinguishes between an exact integer value such as @samp{5} and the
711 corresponding inexact real value which, to the limited precision
712 available, has no fractional part, and is printed as @samp{5.0}. Guile
713 will only convert the latter value to the former when forced to do so by
714 an invocation of the @code{inexact->exact} procedure.
716 @deffn {Scheme Procedure} exact? z
717 @deffnx {C Function} scm_exact_p (z)
718 Return @code{#t} if the number @var{z} is exact, @code{#f}
734 @deffn {Scheme Procedure} inexact? z
735 @deffnx {C Function} scm_inexact_p (z)
736 Return @code{#t} if the number @var{z} is inexact, @code{#f}
740 @deffn {Scheme Procedure} inexact->exact z
741 @deffnx {C Function} scm_inexact_to_exact (z)
742 Return an exact number that is numerically closest to @var{z}, when
743 there is one. For inexact rationals, Guile returns the exact rational
744 that is numerically equal to the inexact rational. Inexact complex
745 numbers with a non-zero imaginary part can not be made exact.
752 The following happens because 12/10 is not exactly representable as a
753 @code{double} (on most platforms). However, when reading a decimal
754 number that has been marked exact with the ``#e'' prefix, Guile is
755 able to represent it correctly.
759 @result{} 5404319552844595/4503599627370496
767 @c begin (texi-doc-string "guile" "exact->inexact")
768 @deffn {Scheme Procedure} exact->inexact z
769 @deffnx {C Function} scm_exact_to_inexact (z)
770 Convert the number @var{z} to its inexact representation.
775 @subsubsection Read Syntax for Numerical Data
777 The read syntax for integers is a string of digits, optionally
778 preceded by a minus or plus character, a code indicating the
779 base in which the integer is encoded, and a code indicating whether
780 the number is exact or inexact. The supported base codes are:
785 the integer is written in binary (base 2)
789 the integer is written in octal (base 8)
793 the integer is written in decimal (base 10)
797 the integer is written in hexadecimal (base 16)
800 If the base code is omitted, the integer is assumed to be decimal. The
801 following examples show how these base codes are used.
820 The codes for indicating exactness (which can, incidentally, be applied
821 to all numerical values) are:
830 the number is inexact.
833 If the exactness indicator is omitted, the number is exact unless it
834 contains a radix point. Since Guile can not represent exact complex
835 numbers, an error is signalled when asking for them.
845 ERROR: Wrong type argument
848 Guile also understands the syntax @samp{+inf.0} and @samp{-inf.0} for
849 plus and minus infinity, respectively. The value must be written
850 exactly as shown, that is, they always must have a sign and exactly
851 one zero digit after the decimal point. It also understands
852 @samp{+nan.0} and @samp{-nan.0} for the special `not-a-number' value.
853 The sign is ignored for `not-a-number' and the value is always printed
856 @node Integer Operations
857 @subsubsection Operations on Integer Values
866 @deffn {Scheme Procedure} odd? n
867 @deffnx {C Function} scm_odd_p (n)
868 Return @code{#t} if @var{n} is an odd number, @code{#f}
872 @deffn {Scheme Procedure} even? n
873 @deffnx {C Function} scm_even_p (n)
874 Return @code{#t} if @var{n} is an even number, @code{#f}
878 @c begin (texi-doc-string "guile" "quotient")
879 @c begin (texi-doc-string "guile" "remainder")
880 @deffn {Scheme Procedure} quotient n d
881 @deffnx {Scheme Procedure} remainder n d
882 @deffnx {C Function} scm_quotient (n, d)
883 @deffnx {C Function} scm_remainder (n, d)
884 Return the quotient or remainder from @var{n} divided by @var{d}. The
885 quotient is rounded towards zero, and the remainder will have the same
886 sign as @var{n}. In all cases quotient and remainder satisfy
887 @math{@var{n} = @var{q}*@var{d} + @var{r}}.
890 (remainder 13 4) @result{} 1
891 (remainder -13 4) @result{} -1
895 @c begin (texi-doc-string "guile" "modulo")
896 @deffn {Scheme Procedure} modulo n d
897 @deffnx {C Function} scm_modulo (n, d)
898 Return the remainder from @var{n} divided by @var{d}, with the same
902 (modulo 13 4) @result{} 1
903 (modulo -13 4) @result{} 3
904 (modulo 13 -4) @result{} -3
905 (modulo -13 -4) @result{} -1
909 @c begin (texi-doc-string "guile" "gcd")
910 @deffn {Scheme Procedure} gcd x@dots{}
911 @deffnx {C Function} scm_gcd (x, y)
912 Return the greatest common divisor of all arguments.
913 If called without arguments, 0 is returned.
915 The C function @code{scm_gcd} always takes two arguments, while the
916 Scheme function can take an arbitrary number.
919 @c begin (texi-doc-string "guile" "lcm")
920 @deffn {Scheme Procedure} lcm x@dots{}
921 @deffnx {C Function} scm_lcm (x, y)
922 Return the least common multiple of the arguments.
923 If called without arguments, 1 is returned.
925 The C function @code{scm_lcm} always takes two arguments, while the
926 Scheme function can take an arbitrary number.
929 @deffn {Scheme Procedure} modulo-expt n k m
930 @deffnx {C Function} scm_modulo_expt (n, k, m)
931 Return @var{n} raised to the integer exponent
932 @var{k}, modulo @var{m}.
941 @subsubsection Comparison Predicates
946 The C comparison functions below always takes two arguments, while the
947 Scheme functions can take an arbitrary number. Also keep in mind that
948 the C functions return one of the Scheme boolean values
949 @code{SCM_BOOL_T} or @code{SCM_BOOL_F} which are both true as far as C
950 is concerned. Thus, always write @code{scm_is_true (scm_num_eq_p (x,
951 y))} when testing the two Scheme numbers @code{x} and @code{y} for
952 equality, for example.
954 @c begin (texi-doc-string "guile" "=")
955 @deffn {Scheme Procedure} =
956 @deffnx {C Function} scm_num_eq_p (x, y)
957 Return @code{#t} if all parameters are numerically equal.
960 @c begin (texi-doc-string "guile" "<")
961 @deffn {Scheme Procedure} <
962 @deffnx {C Function} scm_less_p (x, y)
963 Return @code{#t} if the list of parameters is monotonically
967 @c begin (texi-doc-string "guile" ">")
968 @deffn {Scheme Procedure} >
969 @deffnx {C Function} scm_gr_p (x, y)
970 Return @code{#t} if the list of parameters is monotonically
974 @c begin (texi-doc-string "guile" "<=")
975 @deffn {Scheme Procedure} <=
976 @deffnx {C Function} scm_leq_p (x, y)
977 Return @code{#t} if the list of parameters is monotonically
981 @c begin (texi-doc-string "guile" ">=")
982 @deffn {Scheme Procedure} >=
983 @deffnx {C Function} scm_geq_p (x, y)
984 Return @code{#t} if the list of parameters is monotonically
988 @c begin (texi-doc-string "guile" "zero?")
989 @deffn {Scheme Procedure} zero? z
990 @deffnx {C Function} scm_zero_p (z)
991 Return @code{#t} if @var{z} is an exact or inexact number equal to
995 @c begin (texi-doc-string "guile" "positive?")
996 @deffn {Scheme Procedure} positive? x
997 @deffnx {C Function} scm_positive_p (x)
998 Return @code{#t} if @var{x} is an exact or inexact number greater than
1002 @c begin (texi-doc-string "guile" "negative?")
1003 @deffn {Scheme Procedure} negative? x
1004 @deffnx {C Function} scm_negative_p (x)
1005 Return @code{#t} if @var{x} is an exact or inexact number less than
1011 @subsubsection Converting Numbers To and From Strings
1012 @rnindex number->string
1013 @rnindex string->number
1015 The following procedures read and write numbers according to their
1016 external representation as defined by R5RS (@pxref{Lexical structure,
1017 R5RS Lexical Structure,, r5rs, The Revised^5 Report on the Algorithmic
1018 Language Scheme}). @xref{The ice-9 i18n Module, the @code{(ice-9
1019 i18n)} module}, for locale-dependent number parsing.
1021 @deffn {Scheme Procedure} number->string n [radix]
1022 @deffnx {C Function} scm_number_to_string (n, radix)
1023 Return a string holding the external representation of the
1024 number @var{n} in the given @var{radix}. If @var{n} is
1025 inexact, a radix of 10 will be used.
1028 @deffn {Scheme Procedure} string->number string [radix]
1029 @deffnx {C Function} scm_string_to_number (string, radix)
1030 Return a number of the maximally precise representation
1031 expressed by the given @var{string}. @var{radix} must be an
1032 exact integer, either 2, 8, 10, or 16. If supplied, @var{radix}
1033 is a default radix that may be overridden by an explicit radix
1034 prefix in @var{string} (e.g. "#o177"). If @var{radix} is not
1035 supplied, then the default radix is 10. If string is not a
1036 syntactically valid notation for a number, then
1037 @code{string->number} returns @code{#f}.
1040 @deftypefn {C Function} SCM scm_c_locale_stringn_to_number (const char *string, size_t len, unsigned radix)
1041 As per @code{string->number} above, but taking a C string, as pointer
1042 and length. The string characters should be in the current locale
1043 encoding (@code{locale} in the name refers only to that, there's no
1044 locale-dependent parsing).
1049 @subsubsection Complex Number Operations
1050 @rnindex make-rectangular
1057 @deffn {Scheme Procedure} make-rectangular real imaginary
1058 @deffnx {C Function} scm_make_rectangular (real, imaginary)
1059 Return a complex number constructed of the given @var{real} and
1060 @var{imaginary} parts.
1063 @deffn {Scheme Procedure} make-polar x y
1064 @deffnx {C Function} scm_make_polar (x, y)
1066 Return the complex number @var{x} * e^(i * @var{y}).
1069 @c begin (texi-doc-string "guile" "real-part")
1070 @deffn {Scheme Procedure} real-part z
1071 @deffnx {C Function} scm_real_part (z)
1072 Return the real part of the number @var{z}.
1075 @c begin (texi-doc-string "guile" "imag-part")
1076 @deffn {Scheme Procedure} imag-part z
1077 @deffnx {C Function} scm_imag_part (z)
1078 Return the imaginary part of the number @var{z}.
1081 @c begin (texi-doc-string "guile" "magnitude")
1082 @deffn {Scheme Procedure} magnitude z
1083 @deffnx {C Function} scm_magnitude (z)
1084 Return the magnitude of the number @var{z}. This is the same as
1085 @code{abs} for real arguments, but also allows complex numbers.
1088 @c begin (texi-doc-string "guile" "angle")
1089 @deffn {Scheme Procedure} angle z
1090 @deffnx {C Function} scm_angle (z)
1091 Return the angle of the complex number @var{z}.
1094 @deftypefn {C Function} SCM scm_c_make_rectangular (double re, double im)
1095 @deftypefnx {C Function} SCM scm_c_make_polar (double x, double y)
1096 Like @code{scm_make_rectangular} or @code{scm_make_polar},
1097 respectively, but these functions take @code{double}s as their
1101 @deftypefn {C Function} double scm_c_real_part (z)
1102 @deftypefnx {C Function} double scm_c_imag_part (z)
1103 Returns the real or imaginary part of @var{z} as a @code{double}.
1106 @deftypefn {C Function} double scm_c_magnitude (z)
1107 @deftypefnx {C Function} double scm_c_angle (z)
1108 Returns the magnitude or angle of @var{z} as a @code{double}.
1113 @subsubsection Arithmetic Functions
1126 The C arithmetic functions below always takes two arguments, while the
1127 Scheme functions can take an arbitrary number. When you need to
1128 invoke them with just one argument, for example to compute the
1129 equivalent od @code{(- x)}, pass @code{SCM_UNDEFINED} as the second
1130 one: @code{scm_difference (x, SCM_UNDEFINED)}.
1132 @c begin (texi-doc-string "guile" "+")
1133 @deffn {Scheme Procedure} + z1 @dots{}
1134 @deffnx {C Function} scm_sum (z1, z2)
1135 Return the sum of all parameter values. Return 0 if called without any
1139 @c begin (texi-doc-string "guile" "-")
1140 @deffn {Scheme Procedure} - z1 z2 @dots{}
1141 @deffnx {C Function} scm_difference (z1, z2)
1142 If called with one argument @var{z1}, -@var{z1} is returned. Otherwise
1143 the sum of all but the first argument are subtracted from the first
1147 @c begin (texi-doc-string "guile" "*")
1148 @deffn {Scheme Procedure} * z1 @dots{}
1149 @deffnx {C Function} scm_product (z1, z2)
1150 Return the product of all arguments. If called without arguments, 1 is
1154 @c begin (texi-doc-string "guile" "/")
1155 @deffn {Scheme Procedure} / z1 z2 @dots{}
1156 @deffnx {C Function} scm_divide (z1, z2)
1157 Divide the first argument by the product of the remaining arguments. If
1158 called with one argument @var{z1}, 1/@var{z1} is returned.
1161 @c begin (texi-doc-string "guile" "abs")
1162 @deffn {Scheme Procedure} abs x
1163 @deffnx {C Function} scm_abs (x)
1164 Return the absolute value of @var{x}.
1166 @var{x} must be a number with zero imaginary part. To calculate the
1167 magnitude of a complex number, use @code{magnitude} instead.
1170 @c begin (texi-doc-string "guile" "max")
1171 @deffn {Scheme Procedure} max x1 x2 @dots{}
1172 @deffnx {C Function} scm_max (x1, x2)
1173 Return the maximum of all parameter values.
1176 @c begin (texi-doc-string "guile" "min")
1177 @deffn {Scheme Procedure} min x1 x2 @dots{}
1178 @deffnx {C Function} scm_min (x1, x2)
1179 Return the minimum of all parameter values.
1182 @c begin (texi-doc-string "guile" "truncate")
1183 @deffn {Scheme Procedure} truncate x
1184 @deffnx {C Function} scm_truncate_number (x)
1185 Round the inexact number @var{x} towards zero.
1188 @c begin (texi-doc-string "guile" "round")
1189 @deffn {Scheme Procedure} round x
1190 @deffnx {C Function} scm_round_number (x)
1191 Round the inexact number @var{x} to the nearest integer. When exactly
1192 halfway between two integers, round to the even one.
1195 @c begin (texi-doc-string "guile" "floor")
1196 @deffn {Scheme Procedure} floor x
1197 @deffnx {C Function} scm_floor (x)
1198 Round the number @var{x} towards minus infinity.
1201 @c begin (texi-doc-string "guile" "ceiling")
1202 @deffn {Scheme Procedure} ceiling x
1203 @deffnx {C Function} scm_ceiling (x)
1204 Round the number @var{x} towards infinity.
1207 @deftypefn {C Function} double scm_c_truncate (double x)
1208 @deftypefnx {C Function} double scm_c_round (double x)
1209 Like @code{scm_truncate_number} or @code{scm_round_number},
1210 respectively, but these functions take and return @code{double}
1215 @subsubsection Scientific Functions
1217 The following procedures accept any kind of number as arguments,
1218 including complex numbers.
1221 @c begin (texi-doc-string "guile" "sqrt")
1222 @deffn {Scheme Procedure} sqrt z
1223 Return the square root of @var{z}. Of the two possible roots
1224 (positive and negative), the one with the a positive real part is
1225 returned, or if that's zero then a positive imaginary part. Thus,
1228 (sqrt 9.0) @result{} 3.0
1229 (sqrt -9.0) @result{} 0.0+3.0i
1230 (sqrt 1.0+1.0i) @result{} 1.09868411346781+0.455089860562227i
1231 (sqrt -1.0-1.0i) @result{} 0.455089860562227-1.09868411346781i
1236 @c begin (texi-doc-string "guile" "expt")
1237 @deffn {Scheme Procedure} expt z1 z2
1238 Return @var{z1} raised to the power of @var{z2}.
1242 @c begin (texi-doc-string "guile" "sin")
1243 @deffn {Scheme Procedure} sin z
1244 Return the sine of @var{z}.
1248 @c begin (texi-doc-string "guile" "cos")
1249 @deffn {Scheme Procedure} cos z
1250 Return the cosine of @var{z}.
1254 @c begin (texi-doc-string "guile" "tan")
1255 @deffn {Scheme Procedure} tan z
1256 Return the tangent of @var{z}.
1260 @c begin (texi-doc-string "guile" "asin")
1261 @deffn {Scheme Procedure} asin z
1262 Return the arcsine of @var{z}.
1266 @c begin (texi-doc-string "guile" "acos")
1267 @deffn {Scheme Procedure} acos z
1268 Return the arccosine of @var{z}.
1272 @c begin (texi-doc-string "guile" "atan")
1273 @deffn {Scheme Procedure} atan z
1274 @deffnx {Scheme Procedure} atan y x
1275 Return the arctangent of @var{z}, or of @math{@var{y}/@var{x}}.
1279 @c begin (texi-doc-string "guile" "exp")
1280 @deffn {Scheme Procedure} exp z
1281 Return e to the power of @var{z}, where e is the base of natural
1282 logarithms (2.71828@dots{}).
1286 @c begin (texi-doc-string "guile" "log")
1287 @deffn {Scheme Procedure} log z
1288 Return the natural logarithm of @var{z}.
1291 @c begin (texi-doc-string "guile" "log10")
1292 @deffn {Scheme Procedure} log10 z
1293 Return the base 10 logarithm of @var{z}.
1296 @c begin (texi-doc-string "guile" "sinh")
1297 @deffn {Scheme Procedure} sinh z
1298 Return the hyperbolic sine of @var{z}.
1301 @c begin (texi-doc-string "guile" "cosh")
1302 @deffn {Scheme Procedure} cosh z
1303 Return the hyperbolic cosine of @var{z}.
1306 @c begin (texi-doc-string "guile" "tanh")
1307 @deffn {Scheme Procedure} tanh z
1308 Return the hyperbolic tangent of @var{z}.
1311 @c begin (texi-doc-string "guile" "asinh")
1312 @deffn {Scheme Procedure} asinh z
1313 Return the hyperbolic arcsine of @var{z}.
1316 @c begin (texi-doc-string "guile" "acosh")
1317 @deffn {Scheme Procedure} acosh z
1318 Return the hyperbolic arccosine of @var{z}.
1321 @c begin (texi-doc-string "guile" "atanh")
1322 @deffn {Scheme Procedure} atanh z
1323 Return the hyperbolic arctangent of @var{z}.
1327 @node Primitive Numerics
1328 @subsubsection Primitive Numeric Functions
1330 Many of Guile's numeric procedures which accept any kind of numbers as
1331 arguments, including complex numbers, are implemented as Scheme
1332 procedures that use the following real number-based primitives. These
1333 primitives signal an error if they are called with complex arguments.
1335 @c begin (texi-doc-string "guile" "$abs")
1336 @deffn {Scheme Procedure} $abs x
1337 Return the absolute value of @var{x}.
1340 @c begin (texi-doc-string "guile" "$sqrt")
1341 @deffn {Scheme Procedure} $sqrt x
1342 Return the square root of @var{x}.
1345 @deffn {Scheme Procedure} $expt x y
1346 @deffnx {C Function} scm_sys_expt (x, y)
1347 Return @var{x} raised to the power of @var{y}. This
1348 procedure does not accept complex arguments.
1351 @c begin (texi-doc-string "guile" "$sin")
1352 @deffn {Scheme Procedure} $sin x
1353 Return the sine of @var{x}.
1356 @c begin (texi-doc-string "guile" "$cos")
1357 @deffn {Scheme Procedure} $cos x
1358 Return the cosine of @var{x}.
1361 @c begin (texi-doc-string "guile" "$tan")
1362 @deffn {Scheme Procedure} $tan x
1363 Return the tangent of @var{x}.
1366 @c begin (texi-doc-string "guile" "$asin")
1367 @deffn {Scheme Procedure} $asin x
1368 Return the arcsine of @var{x}.
1371 @c begin (texi-doc-string "guile" "$acos")
1372 @deffn {Scheme Procedure} $acos x
1373 Return the arccosine of @var{x}.
1376 @c begin (texi-doc-string "guile" "$atan")
1377 @deffn {Scheme Procedure} $atan x
1378 Return the arctangent of @var{x} in the range @minus{}@math{PI/2} to
1382 @deffn {Scheme Procedure} $atan2 x y
1383 @deffnx {C Function} scm_sys_atan2 (x, y)
1384 Return the arc tangent of the two arguments @var{x} and
1385 @var{y}. This is similar to calculating the arc tangent of
1386 @var{x} / @var{y}, except that the signs of both arguments
1387 are used to determine the quadrant of the result. This
1388 procedure does not accept complex arguments.
1391 @c begin (texi-doc-string "guile" "$exp")
1392 @deffn {Scheme Procedure} $exp x
1393 Return e to the power of @var{x}, where e is the base of natural
1394 logarithms (2.71828@dots{}).
1397 @c begin (texi-doc-string "guile" "$log")
1398 @deffn {Scheme Procedure} $log x
1399 Return the natural logarithm of @var{x}.
1402 @c begin (texi-doc-string "guile" "$sinh")
1403 @deffn {Scheme Procedure} $sinh x
1404 Return the hyperbolic sine of @var{x}.
1407 @c begin (texi-doc-string "guile" "$cosh")
1408 @deffn {Scheme Procedure} $cosh x
1409 Return the hyperbolic cosine of @var{x}.
1412 @c begin (texi-doc-string "guile" "$tanh")
1413 @deffn {Scheme Procedure} $tanh x
1414 Return the hyperbolic tangent of @var{x}.
1417 @c begin (texi-doc-string "guile" "$asinh")
1418 @deffn {Scheme Procedure} $asinh x
1419 Return the hyperbolic arcsine of @var{x}.
1422 @c begin (texi-doc-string "guile" "$acosh")
1423 @deffn {Scheme Procedure} $acosh x
1424 Return the hyperbolic arccosine of @var{x}.
1427 @c begin (texi-doc-string "guile" "$atanh")
1428 @deffn {Scheme Procedure} $atanh x
1429 Return the hyperbolic arctangent of @var{x}.
1432 C functions for the above are provided by the standard mathematics
1433 library. Naturally these expect and return @code{double} arguments
1434 (@pxref{Mathematics,,, libc, GNU C Library Reference Manual}).
1436 @multitable {xx} {Scheme Procedure} {C Function}
1437 @item @tab Scheme Procedure @tab C Function
1439 @item @tab @code{$abs} @tab @code{fabs}
1440 @item @tab @code{$sqrt} @tab @code{sqrt}
1441 @item @tab @code{$sin} @tab @code{sin}
1442 @item @tab @code{$cos} @tab @code{cos}
1443 @item @tab @code{$tan} @tab @code{tan}
1444 @item @tab @code{$asin} @tab @code{asin}
1445 @item @tab @code{$acos} @tab @code{acos}
1446 @item @tab @code{$atan} @tab @code{atan}
1447 @item @tab @code{$atan2} @tab @code{atan2}
1448 @item @tab @code{$exp} @tab @code{exp}
1449 @item @tab @code{$expt} @tab @code{pow}
1450 @item @tab @code{$log} @tab @code{log}
1451 @item @tab @code{$sinh} @tab @code{sinh}
1452 @item @tab @code{$cosh} @tab @code{cosh}
1453 @item @tab @code{$tanh} @tab @code{tanh}
1454 @item @tab @code{$asinh} @tab @code{asinh}
1455 @item @tab @code{$acosh} @tab @code{acosh}
1456 @item @tab @code{$atanh} @tab @code{atanh}
1459 @code{asinh}, @code{acosh} and @code{atanh} are C99 standard but might
1460 not be available on older systems. Guile provides the following
1461 equivalents (on all systems).
1463 @deftypefn {C Function} double scm_asinh (double x)
1464 @deftypefnx {C Function} double scm_acosh (double x)
1465 @deftypefnx {C Function} double scm_atanh (double x)
1466 Return the hyperbolic arcsine, arccosine or arctangent of @var{x}
1471 @node Bitwise Operations
1472 @subsubsection Bitwise Operations
1474 For the following bitwise functions, negative numbers are treated as
1475 infinite precision twos-complements. For instance @math{-6} is bits
1476 @math{@dots{}111010}, with infinitely many ones on the left. It can
1477 be seen that adding 6 (binary 110) to such a bit pattern gives all
1480 @deffn {Scheme Procedure} logand n1 n2 @dots{}
1481 @deffnx {C Function} scm_logand (n1, n2)
1482 Return the bitwise @sc{and} of the integer arguments.
1485 (logand) @result{} -1
1486 (logand 7) @result{} 7
1487 (logand #b111 #b011 #b001) @result{} 1
1491 @deffn {Scheme Procedure} logior n1 n2 @dots{}
1492 @deffnx {C Function} scm_logior (n1, n2)
1493 Return the bitwise @sc{or} of the integer arguments.
1496 (logior) @result{} 0
1497 (logior 7) @result{} 7
1498 (logior #b000 #b001 #b011) @result{} 3
1502 @deffn {Scheme Procedure} logxor n1 n2 @dots{}
1503 @deffnx {C Function} scm_loxor (n1, n2)
1504 Return the bitwise @sc{xor} of the integer arguments. A bit is
1505 set in the result if it is set in an odd number of arguments.
1508 (logxor) @result{} 0
1509 (logxor 7) @result{} 7
1510 (logxor #b000 #b001 #b011) @result{} 2
1511 (logxor #b000 #b001 #b011 #b011) @result{} 1
1515 @deffn {Scheme Procedure} lognot n
1516 @deffnx {C Function} scm_lognot (n)
1517 Return the integer which is the ones-complement of the integer
1518 argument, ie.@: each 0 bit is changed to 1 and each 1 bit to 0.
1521 (number->string (lognot #b10000000) 2)
1522 @result{} "-10000001"
1523 (number->string (lognot #b0) 2)
1528 @deffn {Scheme Procedure} logtest j k
1529 @deffnx {C Function} scm_logtest (j, k)
1530 Test whether @var{j} and @var{k} have any 1 bits in common. This is
1531 equivalent to @code{(not (zero? (logand j k)))}, but without actually
1532 calculating the @code{logand}, just testing for non-zero.
1535 (logtest #b0100 #b1011) @result{} #f
1536 (logtest #b0100 #b0111) @result{} #t
1540 @deffn {Scheme Procedure} logbit? index j
1541 @deffnx {C Function} scm_logbit_p (index, j)
1542 Test whether bit number @var{index} in @var{j} is set. @var{index}
1543 starts from 0 for the least significant bit.
1546 (logbit? 0 #b1101) @result{} #t
1547 (logbit? 1 #b1101) @result{} #f
1548 (logbit? 2 #b1101) @result{} #t
1549 (logbit? 3 #b1101) @result{} #t
1550 (logbit? 4 #b1101) @result{} #f
1554 @deffn {Scheme Procedure} ash n cnt
1555 @deffnx {C Function} scm_ash (n, cnt)
1556 Return @var{n} shifted left by @var{cnt} bits, or shifted right if
1557 @var{cnt} is negative. This is an ``arithmetic'' shift.
1559 This is effectively a multiplication by @m{2^{cnt}, 2^@var{cnt}}, and
1560 when @var{cnt} is negative it's a division, rounded towards negative
1561 infinity. (Note that this is not the same rounding as @code{quotient}
1564 With @var{n} viewed as an infinite precision twos complement,
1565 @code{ash} means a left shift introducing zero bits, or a right shift
1569 (number->string (ash #b1 3) 2) @result{} "1000"
1570 (number->string (ash #b1010 -1) 2) @result{} "101"
1572 ;; -23 is bits ...11101001, -6 is bits ...111010
1573 (ash -23 -2) @result{} -6
1577 @deffn {Scheme Procedure} logcount n
1578 @deffnx {C Function} scm_logcount (n)
1579 Return the number of bits in integer @var{n}. If @var{n} is
1580 positive, the 1-bits in its binary representation are counted.
1581 If negative, the 0-bits in its two's-complement binary
1582 representation are counted. If zero, 0 is returned.
1585 (logcount #b10101010)
1594 @deffn {Scheme Procedure} integer-length n
1595 @deffnx {C Function} scm_integer_length (n)
1596 Return the number of bits necessary to represent @var{n}.
1598 For positive @var{n} this is how many bits to the most significant one
1599 bit. For negative @var{n} it's how many bits to the most significant
1600 zero bit in twos complement form.
1603 (integer-length #b10101010) @result{} 8
1604 (integer-length #b1111) @result{} 4
1605 (integer-length 0) @result{} 0
1606 (integer-length -1) @result{} 0
1607 (integer-length -256) @result{} 8
1608 (integer-length -257) @result{} 9
1612 @deffn {Scheme Procedure} integer-expt n k
1613 @deffnx {C Function} scm_integer_expt (n, k)
1614 Return @var{n} raised to the power @var{k}. @var{k} must be an exact
1615 integer, @var{n} can be any number.
1617 Negative @var{k} is supported, and results in @m{1/n^|k|, 1/n^abs(k)}
1618 in the usual way. @math{@var{n}^0} is 1, as usual, and that includes
1622 (integer-expt 2 5) @result{} 32
1623 (integer-expt -3 3) @result{} -27
1624 (integer-expt 5 -3) @result{} 1/125
1625 (integer-expt 0 0) @result{} 1
1629 @deffn {Scheme Procedure} bit-extract n start end
1630 @deffnx {C Function} scm_bit_extract (n, start, end)
1631 Return the integer composed of the @var{start} (inclusive)
1632 through @var{end} (exclusive) bits of @var{n}. The
1633 @var{start}th bit becomes the 0-th bit in the result.
1636 (number->string (bit-extract #b1101101010 0 4) 2)
1638 (number->string (bit-extract #b1101101010 4 9) 2)
1645 @subsubsection Random Number Generation
1647 Pseudo-random numbers are generated from a random state object, which
1648 can be created with @code{seed->random-state}. The @var{state}
1649 parameter to the various functions below is optional, it defaults to
1650 the state object in the @code{*random-state*} variable.
1652 @deffn {Scheme Procedure} copy-random-state [state]
1653 @deffnx {C Function} scm_copy_random_state (state)
1654 Return a copy of the random state @var{state}.
1657 @deffn {Scheme Procedure} random n [state]
1658 @deffnx {C Function} scm_random (n, state)
1659 Return a number in [0, @var{n}).
1661 Accepts a positive integer or real n and returns a
1662 number of the same type between zero (inclusive) and
1663 @var{n} (exclusive). The values returned have a uniform
1667 @deffn {Scheme Procedure} random:exp [state]
1668 @deffnx {C Function} scm_random_exp (state)
1669 Return an inexact real in an exponential distribution with mean
1670 1. For an exponential distribution with mean @var{u} use @code{(*
1671 @var{u} (random:exp))}.
1674 @deffn {Scheme Procedure} random:hollow-sphere! vect [state]
1675 @deffnx {C Function} scm_random_hollow_sphere_x (vect, state)
1676 Fills @var{vect} with inexact real random numbers the sum of whose
1677 squares is equal to 1.0. Thinking of @var{vect} as coordinates in
1678 space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
1679 the coordinates are uniformly distributed over the surface of the unit
1683 @deffn {Scheme Procedure} random:normal [state]
1684 @deffnx {C Function} scm_random_normal (state)
1685 Return an inexact real in a normal distribution. The distribution
1686 used has mean 0 and standard deviation 1. For a normal distribution
1687 with mean @var{m} and standard deviation @var{d} use @code{(+ @var{m}
1688 (* @var{d} (random:normal)))}.
1691 @deffn {Scheme Procedure} random:normal-vector! vect [state]
1692 @deffnx {C Function} scm_random_normal_vector_x (vect, state)
1693 Fills @var{vect} with inexact real random numbers that are
1694 independent and standard normally distributed
1695 (i.e., with mean 0 and variance 1).
1698 @deffn {Scheme Procedure} random:solid-sphere! vect [state]
1699 @deffnx {C Function} scm_random_solid_sphere_x (vect, state)
1700 Fills @var{vect} with inexact real random numbers the sum of whose
1701 squares is less than 1.0. Thinking of @var{vect} as coordinates in
1702 space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
1703 the coordinates are uniformly distributed within the unit
1705 @c FIXME: What does this mean, particularly the n-sphere part?
1708 @deffn {Scheme Procedure} random:uniform [state]
1709 @deffnx {C Function} scm_random_uniform (state)
1710 Return a uniformly distributed inexact real random number in
1714 @deffn {Scheme Procedure} seed->random-state seed
1715 @deffnx {C Function} scm_seed_to_random_state (seed)
1716 Return a new random state using @var{seed}.
1719 @defvar *random-state*
1720 The global random state used by the above functions when the
1721 @var{state} parameter is not given.
1726 @subsection Characters
1729 In Scheme, a character literal is written as @code{#\@var{name}} where
1730 @var{name} is the name of the character that you want. Printable
1731 characters have their usual single character name; for example,
1732 @code{#\a} is a lower case @code{a}.
1734 Most of the ``control characters'' (those below codepoint 32) in the
1735 @acronym{ASCII} character set, as well as the space, may be referred
1736 to by longer names: for example, @code{#\tab}, @code{#\esc},
1737 @code{#\stx}, and so on. The following table describes the
1738 @acronym{ASCII} names for each character.
1740 @multitable @columnfractions .25 .25 .25 .25
1741 @item 0 = @code{#\nul}
1742 @tab 1 = @code{#\soh}
1743 @tab 2 = @code{#\stx}
1744 @tab 3 = @code{#\etx}
1745 @item 4 = @code{#\eot}
1746 @tab 5 = @code{#\enq}
1747 @tab 6 = @code{#\ack}
1748 @tab 7 = @code{#\bel}
1749 @item 8 = @code{#\bs}
1750 @tab 9 = @code{#\ht}
1751 @tab 10 = @code{#\nl}
1752 @tab 11 = @code{#\vt}
1753 @item 12 = @code{#\np}
1754 @tab 13 = @code{#\cr}
1755 @tab 14 = @code{#\so}
1756 @tab 15 = @code{#\si}
1757 @item 16 = @code{#\dle}
1758 @tab 17 = @code{#\dc1}
1759 @tab 18 = @code{#\dc2}
1760 @tab 19 = @code{#\dc3}
1761 @item 20 = @code{#\dc4}
1762 @tab 21 = @code{#\nak}
1763 @tab 22 = @code{#\syn}
1764 @tab 23 = @code{#\etb}
1765 @item 24 = @code{#\can}
1766 @tab 25 = @code{#\em}
1767 @tab 26 = @code{#\sub}
1768 @tab 27 = @code{#\esc}
1769 @item 28 = @code{#\fs}
1770 @tab 29 = @code{#\gs}
1771 @tab 30 = @code{#\rs}
1772 @tab 31 = @code{#\us}
1773 @item 32 = @code{#\sp}
1776 The ``delete'' character (octal 177) may be referred to with the name
1779 Several characters have more than one name:
1781 @multitable {@code{#\backspace}} {Original}
1782 @item Alias @tab Original
1783 @item @code{#\space} @tab @code{#\sp}
1784 @item @code{#\newline} @tab @code{#\nl}
1785 @item @code{#\tab} @tab @code{#\ht}
1786 @item @code{#\backspace} @tab @code{#\bs}
1787 @item @code{#\return} @tab @code{#\cr}
1788 @item @code{#\page} @tab @code{#\np}
1789 @item @code{#\null} @tab @code{#\nul}
1793 @deffn {Scheme Procedure} char? x
1794 @deffnx {C Function} scm_char_p (x)
1795 Return @code{#t} iff @var{x} is a character, else @code{#f}.
1799 @deffn {Scheme Procedure} char=? x y
1800 Return @code{#t} iff @var{x} is the same character as @var{y}, else @code{#f}.
1804 @deffn {Scheme Procedure} char<? x y
1805 Return @code{#t} iff @var{x} is less than @var{y} in the @acronym{ASCII} sequence,
1810 @deffn {Scheme Procedure} char<=? x y
1811 Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
1812 @acronym{ASCII} sequence, else @code{#f}.
1816 @deffn {Scheme Procedure} char>? x y
1817 Return @code{#t} iff @var{x} is greater than @var{y} in the @acronym{ASCII}
1818 sequence, else @code{#f}.
1822 @deffn {Scheme Procedure} char>=? x y
1823 Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
1824 @acronym{ASCII} sequence, else @code{#f}.
1828 @deffn {Scheme Procedure} char-ci=? x y
1829 Return @code{#t} iff @var{x} is the same character as @var{y} ignoring
1830 case, else @code{#f}.
1834 @deffn {Scheme Procedure} char-ci<? x y
1835 Return @code{#t} iff @var{x} is less than @var{y} in the @acronym{ASCII} sequence
1836 ignoring case, else @code{#f}.
1840 @deffn {Scheme Procedure} char-ci<=? x y
1841 Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
1842 @acronym{ASCII} sequence ignoring case, else @code{#f}.
1846 @deffn {Scheme Procedure} char-ci>? x y
1847 Return @code{#t} iff @var{x} is greater than @var{y} in the @acronym{ASCII}
1848 sequence ignoring case, else @code{#f}.
1852 @deffn {Scheme Procedure} char-ci>=? x y
1853 Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
1854 @acronym{ASCII} sequence ignoring case, else @code{#f}.
1857 @rnindex char-alphabetic?
1858 @deffn {Scheme Procedure} char-alphabetic? chr
1859 @deffnx {C Function} scm_char_alphabetic_p (chr)
1860 Return @code{#t} iff @var{chr} is alphabetic, else @code{#f}.
1863 @rnindex char-numeric?
1864 @deffn {Scheme Procedure} char-numeric? chr
1865 @deffnx {C Function} scm_char_numeric_p (chr)
1866 Return @code{#t} iff @var{chr} is numeric, else @code{#f}.
1869 @rnindex char-whitespace?
1870 @deffn {Scheme Procedure} char-whitespace? chr
1871 @deffnx {C Function} scm_char_whitespace_p (chr)
1872 Return @code{#t} iff @var{chr} is whitespace, else @code{#f}.
1875 @rnindex char-upper-case?
1876 @deffn {Scheme Procedure} char-upper-case? chr
1877 @deffnx {C Function} scm_char_upper_case_p (chr)
1878 Return @code{#t} iff @var{chr} is uppercase, else @code{#f}.
1881 @rnindex char-lower-case?
1882 @deffn {Scheme Procedure} char-lower-case? chr
1883 @deffnx {C Function} scm_char_lower_case_p (chr)
1884 Return @code{#t} iff @var{chr} is lowercase, else @code{#f}.
1887 @deffn {Scheme Procedure} char-is-both? chr
1888 @deffnx {C Function} scm_char_is_both_p (chr)
1889 Return @code{#t} iff @var{chr} is either uppercase or lowercase, else
1893 @rnindex char->integer
1894 @deffn {Scheme Procedure} char->integer chr
1895 @deffnx {C Function} scm_char_to_integer (chr)
1896 Return the number corresponding to ordinal position of @var{chr} in the
1897 @acronym{ASCII} sequence.
1900 @rnindex integer->char
1901 @deffn {Scheme Procedure} integer->char n
1902 @deffnx {C Function} scm_integer_to_char (n)
1903 Return the character at position @var{n} in the @acronym{ASCII} sequence.
1906 @rnindex char-upcase
1907 @deffn {Scheme Procedure} char-upcase chr
1908 @deffnx {C Function} scm_char_upcase (chr)
1909 Return the uppercase character version of @var{chr}.
1912 @rnindex char-downcase
1913 @deffn {Scheme Procedure} char-downcase chr
1914 @deffnx {C Function} scm_char_downcase (chr)
1915 Return the lowercase character version of @var{chr}.
1918 @node Character Sets
1919 @subsection Character Sets
1921 The features described in this section correspond directly to SRFI-14.
1923 The data type @dfn{charset} implements sets of characters
1924 (@pxref{Characters}). Because the internal representation of
1925 character sets is not visible to the user, a lot of procedures for
1926 handling them are provided.
1928 Character sets can be created, extended, tested for the membership of a
1929 characters and be compared to other character sets.
1931 The Guile implementation of character sets currently deals only with
1932 8-bit characters. In the future, when Guile gets support for
1933 international character sets, this will change, but the functions
1934 provided here will always then be able to efficiently cope with very
1935 large character sets.
1938 * Character Set Predicates/Comparison::
1939 * Iterating Over Character Sets:: Enumerate charset elements.
1940 * Creating Character Sets:: Making new charsets.
1941 * Querying Character Sets:: Test charsets for membership etc.
1942 * Character-Set Algebra:: Calculating new charsets.
1943 * Standard Character Sets:: Variables containing predefined charsets.
1946 @node Character Set Predicates/Comparison
1947 @subsubsection Character Set Predicates/Comparison
1949 Use these procedures for testing whether an object is a character set,
1950 or whether several character sets are equal or subsets of each other.
1951 @code{char-set-hash} can be used for calculating a hash value, maybe for
1952 usage in fast lookup procedures.
1954 @deffn {Scheme Procedure} char-set? obj
1955 @deffnx {C Function} scm_char_set_p (obj)
1956 Return @code{#t} if @var{obj} is a character set, @code{#f}
1960 @deffn {Scheme Procedure} char-set= . char_sets
1961 @deffnx {C Function} scm_char_set_eq (char_sets)
1962 Return @code{#t} if all given character sets are equal.
1965 @deffn {Scheme Procedure} char-set<= . char_sets
1966 @deffnx {C Function} scm_char_set_leq (char_sets)
1967 Return @code{#t} if every character set @var{cs}i is a subset
1968 of character set @var{cs}i+1.
1971 @deffn {Scheme Procedure} char-set-hash cs [bound]
1972 @deffnx {C Function} scm_char_set_hash (cs, bound)
1973 Compute a hash value for the character set @var{cs}. If
1974 @var{bound} is given and non-zero, it restricts the
1975 returned value to the range 0 @dots{} @var{bound - 1}.
1978 @c ===================================================================
1980 @node Iterating Over Character Sets
1981 @subsubsection Iterating Over Character Sets
1983 Character set cursors are a means for iterating over the members of a
1984 character sets. After creating a character set cursor with
1985 @code{char-set-cursor}, a cursor can be dereferenced with
1986 @code{char-set-ref}, advanced to the next member with
1987 @code{char-set-cursor-next}. Whether a cursor has passed past the last
1988 element of the set can be checked with @code{end-of-char-set?}.
1990 Additionally, mapping and (un-)folding procedures for character sets are
1993 @deffn {Scheme Procedure} char-set-cursor cs
1994 @deffnx {C Function} scm_char_set_cursor (cs)
1995 Return a cursor into the character set @var{cs}.
1998 @deffn {Scheme Procedure} char-set-ref cs cursor
1999 @deffnx {C Function} scm_char_set_ref (cs, cursor)
2000 Return the character at the current cursor position
2001 @var{cursor} in the character set @var{cs}. It is an error to
2002 pass a cursor for which @code{end-of-char-set?} returns true.
2005 @deffn {Scheme Procedure} char-set-cursor-next cs cursor
2006 @deffnx {C Function} scm_char_set_cursor_next (cs, cursor)
2007 Advance the character set cursor @var{cursor} to the next
2008 character in the character set @var{cs}. It is an error if the
2009 cursor given satisfies @code{end-of-char-set?}.
2012 @deffn {Scheme Procedure} end-of-char-set? cursor
2013 @deffnx {C Function} scm_end_of_char_set_p (cursor)
2014 Return @code{#t} if @var{cursor} has reached the end of a
2015 character set, @code{#f} otherwise.
2018 @deffn {Scheme Procedure} char-set-fold kons knil cs
2019 @deffnx {C Function} scm_char_set_fold (kons, knil, cs)
2020 Fold the procedure @var{kons} over the character set @var{cs},
2021 initializing it with @var{knil}.
2024 @deffn {Scheme Procedure} char-set-unfold p f g seed [base_cs]
2025 @deffnx {C Function} scm_char_set_unfold (p, f, g, seed, base_cs)
2026 This is a fundamental constructor for character sets.
2028 @item @var{g} is used to generate a series of ``seed'' values
2029 from the initial seed: @var{seed}, (@var{g} @var{seed}),
2030 (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
2031 @item @var{p} tells us when to stop -- when it returns true
2032 when applied to one of the seed values.
2033 @item @var{f} maps each seed value to a character. These
2034 characters are added to the base character set @var{base_cs} to
2035 form the result; @var{base_cs} defaults to the empty set.
2039 @deffn {Scheme Procedure} char-set-unfold! p f g seed base_cs
2040 @deffnx {C Function} scm_char_set_unfold_x (p, f, g, seed, base_cs)
2041 This is a fundamental constructor for character sets.
2043 @item @var{g} is used to generate a series of ``seed'' values
2044 from the initial seed: @var{seed}, (@var{g} @var{seed}),
2045 (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
2046 @item @var{p} tells us when to stop -- when it returns true
2047 when applied to one of the seed values.
2048 @item @var{f} maps each seed value to a character. These
2049 characters are added to the base character set @var{base_cs} to
2050 form the result; @var{base_cs} defaults to the empty set.
2054 @deffn {Scheme Procedure} char-set-for-each proc cs
2055 @deffnx {C Function} scm_char_set_for_each (proc, cs)
2056 Apply @var{proc} to every character in the character set
2057 @var{cs}. The return value is not specified.
2060 @deffn {Scheme Procedure} char-set-map proc cs
2061 @deffnx {C Function} scm_char_set_map (proc, cs)
2062 Map the procedure @var{proc} over every character in @var{cs}.
2063 @var{proc} must be a character -> character procedure.
2066 @c ===================================================================
2068 @node Creating Character Sets
2069 @subsubsection Creating Character Sets
2071 New character sets are produced with these procedures.
2073 @deffn {Scheme Procedure} char-set-copy cs
2074 @deffnx {C Function} scm_char_set_copy (cs)
2075 Return a newly allocated character set containing all
2076 characters in @var{cs}.
2079 @deffn {Scheme Procedure} char-set . rest
2080 @deffnx {C Function} scm_char_set (rest)
2081 Return a character set containing all given characters.
2084 @deffn {Scheme Procedure} list->char-set list [base_cs]
2085 @deffnx {C Function} scm_list_to_char_set (list, base_cs)
2086 Convert the character list @var{list} to a character set. If
2087 the character set @var{base_cs} is given, the character in this
2088 set are also included in the result.
2091 @deffn {Scheme Procedure} list->char-set! list base_cs
2092 @deffnx {C Function} scm_list_to_char_set_x (list, base_cs)
2093 Convert the character list @var{list} to a character set. The
2094 characters are added to @var{base_cs} and @var{base_cs} is
2098 @deffn {Scheme Procedure} string->char-set str [base_cs]
2099 @deffnx {C Function} scm_string_to_char_set (str, base_cs)
2100 Convert the string @var{str} to a character set. If the
2101 character set @var{base_cs} is given, the characters in this
2102 set are also included in the result.
2105 @deffn {Scheme Procedure} string->char-set! str base_cs
2106 @deffnx {C Function} scm_string_to_char_set_x (str, base_cs)
2107 Convert the string @var{str} to a character set. The
2108 characters from the string are added to @var{base_cs}, and
2109 @var{base_cs} is returned.
2112 @deffn {Scheme Procedure} char-set-filter pred cs [base_cs]
2113 @deffnx {C Function} scm_char_set_filter (pred, cs, base_cs)
2114 Return a character set containing every character from @var{cs}
2115 so that it satisfies @var{pred}. If provided, the characters
2116 from @var{base_cs} are added to the result.
2119 @deffn {Scheme Procedure} char-set-filter! pred cs base_cs
2120 @deffnx {C Function} scm_char_set_filter_x (pred, cs, base_cs)
2121 Return a character set containing every character from @var{cs}
2122 so that it satisfies @var{pred}. The characters are added to
2123 @var{base_cs} and @var{base_cs} is returned.
2126 @deffn {Scheme Procedure} ucs-range->char-set lower upper [error [base_cs]]
2127 @deffnx {C Function} scm_ucs_range_to_char_set (lower, upper, error, base_cs)
2128 Return a character set containing all characters whose
2129 character codes lie in the half-open range
2130 [@var{lower},@var{upper}).
2132 If @var{error} is a true value, an error is signalled if the
2133 specified range contains characters which are not contained in
2134 the implemented character range. If @var{error} is @code{#f},
2135 these characters are silently left out of the resultung
2138 The characters in @var{base_cs} are added to the result, if
2142 @deffn {Scheme Procedure} ucs-range->char-set! lower upper error base_cs
2143 @deffnx {C Function} scm_ucs_range_to_char_set_x (lower, upper, error, base_cs)
2144 Return a character set containing all characters whose
2145 character codes lie in the half-open range
2146 [@var{lower},@var{upper}).
2148 If @var{error} is a true value, an error is signalled if the
2149 specified range contains characters which are not contained in
2150 the implemented character range. If @var{error} is @code{#f},
2151 these characters are silently left out of the resultung
2154 The characters are added to @var{base_cs} and @var{base_cs} is
2158 @deffn {Scheme Procedure} ->char-set x
2159 @deffnx {C Function} scm_to_char_set (x)
2160 Coerces x into a char-set. @var{x} may be a string, character or char-set. A string is converted to the set of its constituent characters; a character is converted to a singleton set; a char-set is returned as-is.
2163 @c ===================================================================
2165 @node Querying Character Sets
2166 @subsubsection Querying Character Sets
2168 Access the elements and other information of a character set with these
2171 @deffn {Scheme Procedure} char-set-size cs
2172 @deffnx {C Function} scm_char_set_size (cs)
2173 Return the number of elements in character set @var{cs}.
2176 @deffn {Scheme Procedure} char-set-count pred cs
2177 @deffnx {C Function} scm_char_set_count (pred, cs)
2178 Return the number of the elements int the character set
2179 @var{cs} which satisfy the predicate @var{pred}.
2182 @deffn {Scheme Procedure} char-set->list cs
2183 @deffnx {C Function} scm_char_set_to_list (cs)
2184 Return a list containing the elements of the character set
2188 @deffn {Scheme Procedure} char-set->string cs
2189 @deffnx {C Function} scm_char_set_to_string (cs)
2190 Return a string containing the elements of the character set
2191 @var{cs}. The order in which the characters are placed in the
2192 string is not defined.
2195 @deffn {Scheme Procedure} char-set-contains? cs ch
2196 @deffnx {C Function} scm_char_set_contains_p (cs, ch)
2197 Return @code{#t} iff the character @var{ch} is contained in the
2198 character set @var{cs}.
2201 @deffn {Scheme Procedure} char-set-every pred cs
2202 @deffnx {C Function} scm_char_set_every (pred, cs)
2203 Return a true value if every character in the character set
2204 @var{cs} satisfies the predicate @var{pred}.
2207 @deffn {Scheme Procedure} char-set-any pred cs
2208 @deffnx {C Function} scm_char_set_any (pred, cs)
2209 Return a true value if any character in the character set
2210 @var{cs} satisfies the predicate @var{pred}.
2213 @c ===================================================================
2215 @node Character-Set Algebra
2216 @subsubsection Character-Set Algebra
2218 Character sets can be manipulated with the common set algebra operation,
2219 such as union, complement, intersection etc. All of these procedures
2220 provide side-effecting variants, which modify their character set
2223 @deffn {Scheme Procedure} char-set-adjoin cs . rest
2224 @deffnx {C Function} scm_char_set_adjoin (cs, rest)
2225 Add all character arguments to the first argument, which must
2229 @deffn {Scheme Procedure} char-set-delete cs . rest
2230 @deffnx {C Function} scm_char_set_delete (cs, rest)
2231 Delete all character arguments from the first argument, which
2232 must be a character set.
2235 @deffn {Scheme Procedure} char-set-adjoin! cs . rest
2236 @deffnx {C Function} scm_char_set_adjoin_x (cs, rest)
2237 Add all character arguments to the first argument, which must
2241 @deffn {Scheme Procedure} char-set-delete! cs . rest
2242 @deffnx {C Function} scm_char_set_delete_x (cs, rest)
2243 Delete all character arguments from the first argument, which
2244 must be a character set.
2247 @deffn {Scheme Procedure} char-set-complement cs
2248 @deffnx {C Function} scm_char_set_complement (cs)
2249 Return the complement of the character set @var{cs}.
2252 @deffn {Scheme Procedure} char-set-union . rest
2253 @deffnx {C Function} scm_char_set_union (rest)
2254 Return the union of all argument character sets.
2257 @deffn {Scheme Procedure} char-set-intersection . rest
2258 @deffnx {C Function} scm_char_set_intersection (rest)
2259 Return the intersection of all argument character sets.
2262 @deffn {Scheme Procedure} char-set-difference cs1 . rest
2263 @deffnx {C Function} scm_char_set_difference (cs1, rest)
2264 Return the difference of all argument character sets.
2267 @deffn {Scheme Procedure} char-set-xor . rest
2268 @deffnx {C Function} scm_char_set_xor (rest)
2269 Return the exclusive-or of all argument character sets.
2272 @deffn {Scheme Procedure} char-set-diff+intersection cs1 . rest
2273 @deffnx {C Function} scm_char_set_diff_plus_intersection (cs1, rest)
2274 Return the difference and the intersection of all argument
2278 @deffn {Scheme Procedure} char-set-complement! cs
2279 @deffnx {C Function} scm_char_set_complement_x (cs)
2280 Return the complement of the character set @var{cs}.
2283 @deffn {Scheme Procedure} char-set-union! cs1 . rest
2284 @deffnx {C Function} scm_char_set_union_x (cs1, rest)
2285 Return the union of all argument character sets.
2288 @deffn {Scheme Procedure} char-set-intersection! cs1 . rest
2289 @deffnx {C Function} scm_char_set_intersection_x (cs1, rest)
2290 Return the intersection of all argument character sets.
2293 @deffn {Scheme Procedure} char-set-difference! cs1 . rest
2294 @deffnx {C Function} scm_char_set_difference_x (cs1, rest)
2295 Return the difference of all argument character sets.
2298 @deffn {Scheme Procedure} char-set-xor! cs1 . rest
2299 @deffnx {C Function} scm_char_set_xor_x (cs1, rest)
2300 Return the exclusive-or of all argument character sets.
2303 @deffn {Scheme Procedure} char-set-diff+intersection! cs1 cs2 . rest
2304 @deffnx {C Function} scm_char_set_diff_plus_intersection_x (cs1, cs2, rest)
2305 Return the difference and the intersection of all argument
2309 @c ===================================================================
2311 @node Standard Character Sets
2312 @subsubsection Standard Character Sets
2314 In order to make the use of the character set data type and procedures
2315 useful, several predefined character set variables exist.
2321 Currently, the contents of these character sets are recomputed upon a
2322 successful @code{setlocale} call (@pxref{Locales}) in order to reflect
2323 the characters available in the current locale's codeset. For
2324 instance, @code{char-set:letter} contains 52 characters under an ASCII
2325 locale (e.g., the default @code{C} locale) and 117 characters under an
2326 ISO-8859-1 (``Latin-1'') locale.
2328 @defvr {Scheme Variable} char-set:lower-case
2329 @defvrx {C Variable} scm_char_set_lower_case
2330 All lower-case characters.
2333 @defvr {Scheme Variable} char-set:upper-case
2334 @defvrx {C Variable} scm_char_set_upper_case
2335 All upper-case characters.
2338 @defvr {Scheme Variable} char-set:title-case
2339 @defvrx {C Variable} scm_char_set_title_case
2340 This is empty, because ASCII has no titlecase characters.
2343 @defvr {Scheme Variable} char-set:letter
2344 @defvrx {C Variable} scm_char_set_letter
2345 All letters, e.g. the union of @code{char-set:lower-case} and
2346 @code{char-set:upper-case}.
2349 @defvr {Scheme Variable} char-set:digit
2350 @defvrx {C Variable} scm_char_set_digit
2354 @defvr {Scheme Variable} char-set:letter+digit
2355 @defvrx {C Variable} scm_char_set_letter_and_digit
2356 The union of @code{char-set:letter} and @code{char-set:digit}.
2359 @defvr {Scheme Variable} char-set:graphic
2360 @defvrx {C Variable} scm_char_set_graphic
2361 All characters which would put ink on the paper.
2364 @defvr {Scheme Variable} char-set:printing
2365 @defvrx {C Variable} scm_char_set_printing
2366 The union of @code{char-set:graphic} and @code{char-set:whitespace}.
2369 @defvr {Scheme Variable} char-set:whitespace
2370 @defvrx {C Variable} scm_char_set_whitespace
2371 All whitespace characters.
2374 @defvr {Scheme Variable} char-set:blank
2375 @defvrx {C Variable} scm_char_set_blank
2376 All horizontal whitespace characters, that is @code{#\space} and
2380 @defvr {Scheme Variable} char-set:iso-control
2381 @defvrx {C Variable} scm_char_set_iso_control
2382 The ISO control characters with the codes 0--31 and 127.
2385 @defvr {Scheme Variable} char-set:punctuation
2386 @defvrx {C Variable} scm_char_set_punctuation
2387 The characters @code{!"#%&'()*,-./:;?@@[\\]_@{@}}
2390 @defvr {Scheme Variable} char-set:symbol
2391 @defvrx {C Variable} scm_char_set_symbol
2392 The characters @code{$+<=>^`|~}.
2395 @defvr {Scheme Variable} char-set:hex-digit
2396 @defvrx {C Variable} scm_char_set_hex_digit
2397 The hexadecimal digits @code{0123456789abcdefABCDEF}.
2400 @defvr {Scheme Variable} char-set:ascii
2401 @defvrx {C Variable} scm_char_set_ascii
2402 All ASCII characters.
2405 @defvr {Scheme Variable} char-set:empty
2406 @defvrx {C Variable} scm_char_set_empty
2407 The empty character set.
2410 @defvr {Scheme Variable} char-set:full
2411 @defvrx {C Variable} scm_char_set_full
2412 This character set contains all possible characters.
2419 Strings are fixed-length sequences of characters. They can be created
2420 by calling constructor procedures, but they can also literally get
2421 entered at the @acronym{REPL} or in Scheme source files.
2423 @c Guile provides a rich set of string processing procedures, because text
2424 @c handling is very important when Guile is used as a scripting language.
2426 Strings always carry the information about how many characters they are
2427 composed of with them, so there is no special end-of-string character,
2428 like in C. That means that Scheme strings can contain any character,
2429 even the @samp{#\nul} character @samp{\0}.
2431 To use strings efficiently, you need to know a bit about how Guile
2432 implements them. In Guile, a string consists of two parts, a head and
2433 the actual memory where the characters are stored. When a string (or
2434 a substring of it) is copied, only a new head gets created, the memory
2435 is usually not copied. The two heads start out pointing to the same
2438 When one of these two strings is modified, as with @code{string-set!},
2439 their common memory does get copied so that each string has its own
2440 memory and modifying one does not accidently modify the other as well.
2441 Thus, Guile's strings are `copy on write'; the actual copying of their
2442 memory is delayed until one string is written to.
2444 This implementation makes functions like @code{substring} very
2445 efficient in the common case that no modifications are done to the
2448 If you do know that your strings are getting modified right away, you
2449 can use @code{substring/copy} instead of @code{substring}. This
2450 function performs the copy immediately at the time of creation. This
2451 is more efficient, especially in a multi-threaded program. Also,
2452 @code{substring/copy} can avoid the problem that a short substring
2453 holds on to the memory of a very large original string that could
2454 otherwise be recycled.
2456 If you want to avoid the copy altogether, so that modifications of one
2457 string show up in the other, you can use @code{substring/shared}. The
2458 strings created by this procedure are called @dfn{mutation sharing
2459 substrings} since the substring and the original string share
2460 modifications to each other.
2462 If you want to prevent modifications, use @code{substring/read-only}.
2464 Guile provides all procedures of SRFI-13 and a few more.
2467 * String Syntax:: Read syntax for strings.
2468 * String Predicates:: Testing strings for certain properties.
2469 * String Constructors:: Creating new string objects.
2470 * List/String Conversion:: Converting from/to lists of characters.
2471 * String Selection:: Select portions from strings.
2472 * String Modification:: Modify parts or whole strings.
2473 * String Comparison:: Lexicographic ordering predicates.
2474 * String Searching:: Searching in strings.
2475 * Alphabetic Case Mapping:: Convert the alphabetic case of strings.
2476 * Reversing and Appending Strings:: Appending strings to form a new string.
2477 * Mapping Folding and Unfolding:: Iterating over strings.
2478 * Miscellaneous String Operations:: Replicating, insertion, parsing, ...
2479 * Conversion to/from C::
2483 @subsubsection String Read Syntax
2485 @c In the following @code is used to get a good font in TeX etc, but
2486 @c is omitted for Info format, so as not to risk any confusion over
2487 @c whether surrounding ` ' quotes are part of the escape or are
2488 @c special in a string (they're not).
2490 The read syntax for strings is an arbitrarily long sequence of
2491 characters enclosed in double quotes (@nicode{"}).
2493 Backslash is an escape character and can be used to insert the
2494 following special characters. @nicode{\"} and @nicode{\\} are R5RS
2495 standard, the rest are Guile extensions, notice they follow C string
2500 Backslash character.
2503 Double quote character (an unescaped @nicode{"} is otherwise the end
2507 NUL character (ASCII 0).
2510 Bell character (ASCII 7).
2513 Formfeed character (ASCII 12).
2516 Newline character (ASCII 10).
2519 Carriage return character (ASCII 13).
2522 Tab character (ASCII 9).
2525 Vertical tab character (ASCII 11).
2528 Character code given by two hexadecimal digits. For example
2529 @nicode{\x7f} for an ASCII DEL (127).
2533 The following are examples of string literals:
2543 @node String Predicates
2544 @subsubsection String Predicates
2546 The following procedures can be used to check whether a given string
2547 fulfills some specified property.
2550 @deffn {Scheme Procedure} string? obj
2551 @deffnx {C Function} scm_string_p (obj)
2552 Return @code{#t} if @var{obj} is a string, else @code{#f}.
2555 @deftypefn {C Function} int scm_is_string (SCM obj)
2556 Returns @code{1} if @var{obj} is a string, @code{0} otherwise.
2559 @deffn {Scheme Procedure} string-null? str
2560 @deffnx {C Function} scm_string_null_p (str)
2561 Return @code{#t} if @var{str}'s length is zero, and
2562 @code{#f} otherwise.
2564 (string-null? "") @result{} #t
2566 (string-null? y) @result{} #f
2570 @deffn {Scheme Procedure} string-any char_pred s [start [end]]
2571 @deffnx {C Function} scm_string_any (char_pred, s, start, end)
2572 Check if @var{char_pred} is true for any character in string @var{s}.
2574 @var{char_pred} can be a character to check for any equal to that, or
2575 a character set (@pxref{Character Sets}) to check for any in that set,
2576 or a predicate procedure to call.
2578 For a procedure, calls @code{(@var{char_pred} c)} are made
2579 successively on the characters from @var{start} to @var{end}. If
2580 @var{char_pred} returns true (ie.@: non-@code{#f}), @code{string-any}
2581 stops and that return value is the return from @code{string-any}. The
2582 call on the last character (ie.@: at @math{@var{end}-1}), if that
2583 point is reached, is a tail call.
2585 If there are no characters in @var{s} (ie.@: @var{start} equals
2586 @var{end}) then the return is @code{#f}.
2589 @deffn {Scheme Procedure} string-every char_pred s [start [end]]
2590 @deffnx {C Function} scm_string_every (char_pred, s, start, end)
2591 Check if @var{char_pred} is true for every character in string
2594 @var{char_pred} can be a character to check for every character equal
2595 to that, or a character set (@pxref{Character Sets}) to check for
2596 every character being in that set, or a predicate procedure to call.
2598 For a procedure, calls @code{(@var{char_pred} c)} are made
2599 successively on the characters from @var{start} to @var{end}. If
2600 @var{char_pred} returns @code{#f}, @code{string-every} stops and
2601 returns @code{#f}. The call on the last character (ie.@: at
2602 @math{@var{end}-1}), if that point is reached, is a tail call and the
2603 return from that call is the return from @code{string-every}.
2605 If there are no characters in @var{s} (ie.@: @var{start} equals
2606 @var{end}) then the return is @code{#t}.
2609 @node String Constructors
2610 @subsubsection String Constructors
2612 The string constructor procedures create new string objects, possibly
2613 initializing them with some specified character data. See also
2614 @xref{String Selection}, for ways to create strings from existing
2617 @c FIXME::martin: list->string belongs into `List/String Conversion'
2619 @deffn {Scheme Procedure} string char@dots{}
2621 Return a newly allocated string made from the given character
2625 (string #\x #\y #\z) @result{} "xyz"
2626 (string) @result{} ""
2630 @deffn {Scheme Procedure} list->string lst
2631 @deffnx {C Function} scm_string (lst)
2632 @rnindex list->string
2633 Return a newly allocated string made from a list of characters.
2636 (list->string '(#\a #\b #\c)) @result{} "abc"
2640 @deffn {Scheme Procedure} reverse-list->string lst
2641 @deffnx {C Function} scm_reverse_list_to_string (lst)
2642 Return a newly allocated string made from a list of characters, in
2646 (reverse-list->string '(#\a #\B #\c)) @result{} "cBa"
2650 @rnindex make-string
2651 @deffn {Scheme Procedure} make-string k [chr]
2652 @deffnx {C Function} scm_make_string (k, chr)
2653 Return a newly allocated string of
2654 length @var{k}. If @var{chr} is given, then all elements of
2655 the string are initialized to @var{chr}, otherwise the contents
2656 of the @var{string} are unspecified.
2659 @deftypefn {C Function} SCM scm_c_make_string (size_t len, SCM chr)
2660 Like @code{scm_make_string}, but expects the length as a
2664 @deffn {Scheme Procedure} string-tabulate proc len
2665 @deffnx {C Function} scm_string_tabulate (proc, len)
2666 @var{proc} is an integer->char procedure. Construct a string
2667 of size @var{len} by applying @var{proc} to each index to
2668 produce the corresponding string element. The order in which
2669 @var{proc} is applied to the indices is not specified.
2672 @deffn {Scheme Procedure} string-join ls [delimiter [grammar]]
2673 @deffnx {C Function} scm_string_join (ls, delimiter, grammar)
2674 Append the string in the string list @var{ls}, using the string
2675 @var{delim} as a delimiter between the elements of @var{ls}.
2676 @var{grammar} is a symbol which specifies how the delimiter is
2677 placed between the strings, and defaults to the symbol
2682 Insert the separator between list elements. An empty string
2683 will produce an empty list.
2685 Like @code{infix}, but will raise an error if given the empty
2688 Insert the separator after every list element.
2690 Insert the separator before each list element.
2694 @node List/String Conversion
2695 @subsubsection List/String conversion
2697 When processing strings, it is often convenient to first convert them
2698 into a list representation by using the procedure @code{string->list},
2699 work with the resulting list, and then convert it back into a string.
2700 These procedures are useful for similar tasks.
2702 @rnindex string->list
2703 @deffn {Scheme Procedure} string->list str [start [end]]
2704 @deffnx {C Function} scm_substring_to_list (str, start, end)
2705 @deffnx {C Function} scm_string_to_list (str)
2706 Convert the string @var{str} into a list of characters.
2709 @deffn {Scheme Procedure} string-split str chr
2710 @deffnx {C Function} scm_string_split (str, chr)
2711 Split the string @var{str} into the a list of the substrings delimited
2712 by appearances of the character @var{chr}. Note that an empty substring
2713 between separator characters will result in an empty string in the
2717 (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
2719 ("root" "x" "0" "0" "root" "/root" "/bin/bash")
2721 (string-split "::" #\:)
2725 (string-split "" #\:)
2732 @node String Selection
2733 @subsubsection String Selection
2735 Portions of strings can be extracted by these procedures.
2736 @code{string-ref} delivers individual characters whereas
2737 @code{substring} can be used to extract substrings from longer strings.
2739 @rnindex string-length
2740 @deffn {Scheme Procedure} string-length string
2741 @deffnx {C Function} scm_string_length (string)
2742 Return the number of characters in @var{string}.
2745 @deftypefn {C Function} size_t scm_c_string_length (SCM str)
2746 Return the number of characters in @var{str} as a @code{size_t}.
2750 @deffn {Scheme Procedure} string-ref str k
2751 @deffnx {C Function} scm_string_ref (str, k)
2752 Return character @var{k} of @var{str} using zero-origin
2753 indexing. @var{k} must be a valid index of @var{str}.
2756 @deftypefn {C Function} SCM scm_c_string_ref (SCM str, size_t k)
2757 Return character @var{k} of @var{str} using zero-origin
2758 indexing. @var{k} must be a valid index of @var{str}.
2761 @rnindex string-copy
2762 @deffn {Scheme Procedure} string-copy str [start [end]]
2763 @deffnx {C Function} scm_substring_copy (str, start, end)
2764 @deffnx {C Function} scm_string_copy (str)
2765 Return a copy of the given string @var{str}.
2767 The returned string shares storage with @var{str} initially, but it is
2768 copied as soon as one of the two strings is modified.
2772 @deffn {Scheme Procedure} substring str start [end]
2773 @deffnx {C Function} scm_substring (str, start, end)
2774 Return a new string formed from the characters
2775 of @var{str} beginning with index @var{start} (inclusive) and
2776 ending with index @var{end} (exclusive).
2777 @var{str} must be a string, @var{start} and @var{end} must be
2778 exact integers satisfying:
2780 0 <= @var{start} <= @var{end} <= @code{(string-length @var{str})}.
2782 The returned string shares storage with @var{str} initially, but it is
2783 copied as soon as one of the two strings is modified.
2786 @deffn {Scheme Procedure} substring/shared str start [end]
2787 @deffnx {C Function} scm_substring_shared (str, start, end)
2788 Like @code{substring}, but the strings continue to share their storage
2789 even if they are modified. Thus, modifications to @var{str} show up
2790 in the new string, and vice versa.
2793 @deffn {Scheme Procedure} substring/copy str start [end]
2794 @deffnx {C Function} scm_substring_copy (str, start, end)
2795 Like @code{substring}, but the storage for the new string is copied
2799 @deffn {Scheme Procedure} substring/read-only str start [end]
2800 @deffnx {C Function} scm_substring_read_only (str, start, end)
2801 Like @code{substring}, but the resulting string can not be modified.
2804 @deftypefn {C Function} SCM scm_c_substring (SCM str, size_t start, size_t end)
2805 @deftypefnx {C Function} SCM scm_c_substring_shared (SCM str, size_t start, size_t end)
2806 @deftypefnx {C Function} SCM scm_c_substring_copy (SCM str, size_t start, size_t end)
2807 @deftypefnx {C Function} SCM scm_c_substring_read_only (SCM str, size_t start, size_t end)
2808 Like @code{scm_substring}, etc. but the bounds are given as a @code{size_t}.
2811 @deffn {Scheme Procedure} string-take s n
2812 @deffnx {C Function} scm_string_take (s, n)
2813 Return the @var{n} first characters of @var{s}.
2816 @deffn {Scheme Procedure} string-drop s n
2817 @deffnx {C Function} scm_string_drop (s, n)
2818 Return all but the first @var{n} characters of @var{s}.
2821 @deffn {Scheme Procedure} string-take-right s n
2822 @deffnx {C Function} scm_string_take_right (s, n)
2823 Return the @var{n} last characters of @var{s}.
2826 @deffn {Scheme Procedure} string-drop-right s n
2827 @deffnx {C Function} scm_string_drop_right (s, n)
2828 Return all but the last @var{n} characters of @var{s}.
2831 @deffn {Scheme Procedure} string-pad s len [chr [start [end]]]
2832 @deffnx {Scheme Procedure} string-pad-right s len [chr [start [end]]]
2833 @deffnx {C Function} scm_string_pad (s, len, chr, start, end)
2834 @deffnx {C Function} scm_string_pad_right (s, len, chr, start, end)
2835 Take characters @var{start} to @var{end} from the string @var{s} and
2836 either pad with @var{char} or truncate them to give @var{len}
2839 @code{string-pad} pads or truncates on the left, so for example
2842 (string-pad "x" 3) @result{} " x"
2843 (string-pad "abcde" 3) @result{} "cde"
2846 @code{string-pad-right} pads or truncates on the right, so for example
2849 (string-pad-right "x" 3) @result{} "x "
2850 (string-pad-right "abcde" 3) @result{} "abc"
2854 @deffn {Scheme Procedure} string-trim s [char_pred [start [end]]]
2855 @deffnx {Scheme Procedure} string-trim-right s [char_pred [start [end]]]
2856 @deffnx {Scheme Procedure} string-trim-both s [char_pred [start [end]]]
2857 @deffnx {C Function} scm_string_trim (s, char_pred, start, end)
2858 @deffnx {C Function} scm_string_trim_right (s, char_pred, start, end)
2859 @deffnx {C Function} scm_string_trim_both (s, char_pred, start, end)
2860 Trim occurrances of @var{char_pred} from the ends of @var{s}.
2862 @code{string-trim} trims @var{char_pred} characters from the left
2863 (start) of the string, @code{string-trim-right} trims them from the
2864 right (end) of the string, @code{string-trim-both} trims from both
2867 @var{char_pred} can be a character, a character set, or a predicate
2868 procedure to call on each character. If @var{char_pred} is not given
2869 the default is whitespace as per @code{char-set:whitespace}
2870 (@pxref{Standard Character Sets}).
2873 (string-trim " x ") @result{} "x "
2874 (string-trim-right "banana" #\a) @result{} "banan"
2875 (string-trim-both ".,xy:;" char-set:punctuation)
2877 (string-trim-both "xyzzy" (lambda (c)
2884 @node String Modification
2885 @subsubsection String Modification
2887 These procedures are for modifying strings in-place. This means that the
2888 result of the operation is not a new string; instead, the original string's
2889 memory representation is modified.
2891 @rnindex string-set!
2892 @deffn {Scheme Procedure} string-set! str k chr
2893 @deffnx {C Function} scm_string_set_x (str, k, chr)
2894 Store @var{chr} in element @var{k} of @var{str} and return
2895 an unspecified value. @var{k} must be a valid index of
2899 @deftypefn {C Function} void scm_c_string_set_x (SCM str, size_t k, SCM chr)
2900 Like @code{scm_string_set_x}, but the index is given as a @code{size_t}.
2903 @rnindex string-fill!
2904 @deffn {Scheme Procedure} string-fill! str chr [start [end]]
2905 @deffnx {C Function} scm_substring_fill_x (str, chr, start, end)
2906 @deffnx {C Function} scm_string_fill_x (str, chr)
2907 Stores @var{chr} in every element of the given @var{str} and
2908 returns an unspecified value.
2911 @deffn {Scheme Procedure} substring-fill! str start end fill
2912 @deffnx {C Function} scm_substring_fill_x (str, start, end, fill)
2913 Change every character in @var{str} between @var{start} and
2914 @var{end} to @var{fill}.
2917 (define y "abcdefg")
2918 (substring-fill! y 1 3 #\r)
2924 @deffn {Scheme Procedure} substring-move! str1 start1 end1 str2 start2
2925 @deffnx {C Function} scm_substring_move_x (str1, start1, end1, str2, start2)
2926 Copy the substring of @var{str1} bounded by @var{start1} and @var{end1}
2927 into @var{str2} beginning at position @var{start2}.
2928 @var{str1} and @var{str2} can be the same string.
2931 @deffn {Scheme Procedure} string-copy! target tstart s [start [end]]
2932 @deffnx {C Function} scm_string_copy_x (target, tstart, s, start, end)
2933 Copy the sequence of characters from index range [@var{start},
2934 @var{end}) in string @var{s} to string @var{target}, beginning
2935 at index @var{tstart}. The characters are copied left-to-right
2936 or right-to-left as needed -- the copy is guaranteed to work,
2937 even if @var{target} and @var{s} are the same string. It is an
2938 error if the copy operation runs off the end of the target
2943 @node String Comparison
2944 @subsubsection String Comparison
2946 The procedures in this section are similar to the character ordering
2947 predicates (@pxref{Characters}), but are defined on character sequences.
2949 The first set is specified in R5RS and has names that end in @code{?}.
2950 The second set is specified in SRFI-13 and the names have no ending
2951 @code{?}. The predicates ending in @code{-ci} ignore the character case
2952 when comparing strings. @xref{The ice-9 i18n Module, the @code{(ice-9
2953 i18n)} module}, for locale-dependent string comparison.
2956 @deffn {Scheme Procedure} string=? s1 s2
2957 Lexicographic equality predicate; return @code{#t} if the two
2958 strings are the same length and contain the same characters in
2959 the same positions, otherwise return @code{#f}.
2961 The procedure @code{string-ci=?} treats upper and lower case
2962 letters as though they were the same character, but
2963 @code{string=?} treats upper and lower case as distinct
2968 @deffn {Scheme Procedure} string<? s1 s2
2969 Lexicographic ordering predicate; return @code{#t} if @var{s1}
2970 is lexicographically less than @var{s2}.
2974 @deffn {Scheme Procedure} string<=? s1 s2
2975 Lexicographic ordering predicate; return @code{#t} if @var{s1}
2976 is lexicographically less than or equal to @var{s2}.
2980 @deffn {Scheme Procedure} string>? s1 s2
2981 Lexicographic ordering predicate; return @code{#t} if @var{s1}
2982 is lexicographically greater than @var{s2}.
2986 @deffn {Scheme Procedure} string>=? s1 s2
2987 Lexicographic ordering predicate; return @code{#t} if @var{s1}
2988 is lexicographically greater than or equal to @var{s2}.
2991 @rnindex string-ci=?
2992 @deffn {Scheme Procedure} string-ci=? s1 s2
2993 Case-insensitive string equality predicate; return @code{#t} if
2994 the two strings are the same length and their component
2995 characters match (ignoring case) at each position; otherwise
2999 @rnindex string-ci<?
3000 @deffn {Scheme Procedure} string-ci<? s1 s2
3001 Case insensitive lexicographic ordering predicate; return
3002 @code{#t} if @var{s1} is lexicographically less than @var{s2}
3007 @deffn {Scheme Procedure} string-ci<=? s1 s2
3008 Case insensitive lexicographic ordering predicate; return
3009 @code{#t} if @var{s1} is lexicographically less than or equal
3010 to @var{s2} regardless of case.
3013 @rnindex string-ci>?
3014 @deffn {Scheme Procedure} string-ci>? s1 s2
3015 Case insensitive lexicographic ordering predicate; return
3016 @code{#t} if @var{s1} is lexicographically greater than
3017 @var{s2} regardless of case.
3020 @rnindex string-ci>=?
3021 @deffn {Scheme Procedure} string-ci>=? s1 s2
3022 Case insensitive lexicographic ordering predicate; return
3023 @code{#t} if @var{s1} is lexicographically greater than or
3024 equal to @var{s2} regardless of case.
3027 @deffn {Scheme Procedure} string-compare s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
3028 @deffnx {C Function} scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
3029 Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
3030 mismatch index, depending upon whether @var{s1} is less than,
3031 equal to, or greater than @var{s2}. The mismatch index is the
3032 largest index @var{i} such that for every 0 <= @var{j} <
3033 @var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
3034 @var{i} is the first position that does not match.
3037 @deffn {Scheme Procedure} string-compare-ci s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
3038 @deffnx {C Function} scm_string_compare_ci (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
3039 Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
3040 mismatch index, depending upon whether @var{s1} is less than,
3041 equal to, or greater than @var{s2}. The mismatch index is the
3042 largest index @var{i} such that for every 0 <= @var{j} <
3043 @var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
3044 @var{i} is the first position that does not match. The
3045 character comparison is done case-insensitively.
3048 @deffn {Scheme Procedure} string= s1 s2 [start1 [end1 [start2 [end2]]]]
3049 @deffnx {C Function} scm_string_eq (s1, s2, start1, end1, start2, end2)
3050 Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
3054 @deffn {Scheme Procedure} string<> s1 s2 [start1 [end1 [start2 [end2]]]]
3055 @deffnx {C Function} scm_string_neq (s1, s2, start1, end1, start2, end2)
3056 Return @code{#f} if @var{s1} and @var{s2} are equal, a true
3060 @deffn {Scheme Procedure} string< s1 s2 [start1 [end1 [start2 [end2]]]]
3061 @deffnx {C Function} scm_string_lt (s1, s2, start1, end1, start2, end2)
3062 Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
3063 true value otherwise.
3066 @deffn {Scheme Procedure} string> s1 s2 [start1 [end1 [start2 [end2]]]]
3067 @deffnx {C Function} scm_string_gt (s1, s2, start1, end1, start2, end2)
3068 Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
3069 true value otherwise.
3072 @deffn {Scheme Procedure} string<= s1 s2 [start1 [end1 [start2 [end2]]]]
3073 @deffnx {C Function} scm_string_le (s1, s2, start1, end1, start2, end2)
3074 Return @code{#f} if @var{s1} is greater to @var{s2}, a true
3078 @deffn {Scheme Procedure} string>= s1 s2 [start1 [end1 [start2 [end2]]]]
3079 @deffnx {C Function} scm_string_ge (s1, s2, start1, end1, start2, end2)
3080 Return @code{#f} if @var{s1} is less to @var{s2}, a true value
3084 @deffn {Scheme Procedure} string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
3085 @deffnx {C Function} scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
3086 Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
3087 value otherwise. The character comparison is done
3091 @deffn {Scheme Procedure} string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
3092 @deffnx {C Function} scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
3093 Return @code{#f} if @var{s1} and @var{s2} are equal, a true
3094 value otherwise. The character comparison is done
3098 @deffn {Scheme Procedure} string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
3099 @deffnx {C Function} scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
3100 Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
3101 true value otherwise. The character comparison is done
3105 @deffn {Scheme Procedure} string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
3106 @deffnx {C Function} scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
3107 Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
3108 true value otherwise. The character comparison is done
3112 @deffn {Scheme Procedure} string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
3113 @deffnx {C Function} scm_string_ci_le (s1, s2, start1, end1, start2, end2)
3114 Return @code{#f} if @var{s1} is greater to @var{s2}, a true
3115 value otherwise. The character comparison is done
3119 @deffn {Scheme Procedure} string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
3120 @deffnx {C Function} scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
3121 Return @code{#f} if @var{s1} is less to @var{s2}, a true value
3122 otherwise. The character comparison is done
3126 @deffn {Scheme Procedure} string-hash s [bound [start [end]]]
3127 @deffnx {C Function} scm_substring_hash (s, bound, start, end)
3128 Compute a hash value for @var{S}. the optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
3131 @deffn {Scheme Procedure} string-hash-ci s [bound [start [end]]]
3132 @deffnx {C Function} scm_substring_hash_ci (s, bound, start, end)
3133 Compute a hash value for @var{S}. the optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
3136 @node String Searching
3137 @subsubsection String Searching
3139 @deffn {Scheme Procedure} string-index s char_pred [start [end]]
3140 @deffnx {C Function} scm_string_index (s, char_pred, start, end)
3141 Search through the string @var{s} from left to right, returning
3142 the index of the first occurence of a character which
3146 equals @var{char_pred}, if it is character,
3149 satisifies the predicate @var{char_pred}, if it is a procedure,
3152 is in the set @var{char_pred}, if it is a character set.
3156 @deffn {Scheme Procedure} string-rindex s char_pred [start [end]]
3157 @deffnx {C Function} scm_string_rindex (s, char_pred, start, end)
3158 Search through the string @var{s} from right to left, returning
3159 the index of the last occurence of a character which
3163 equals @var{char_pred}, if it is character,
3166 satisifies the predicate @var{char_pred}, if it is a procedure,
3169 is in the set if @var{char_pred} is a character set.
3173 @deffn {Scheme Procedure} string-prefix-length s1 s2 [start1 [end1 [start2 [end2]]]]
3174 @deffnx {C Function} scm_string_prefix_length (s1, s2, start1, end1, start2, end2)
3175 Return the length of the longest common prefix of the two
3179 @deffn {Scheme Procedure} string-prefix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3180 @deffnx {C Function} scm_string_prefix_length_ci (s1, s2, start1, end1, start2, end2)
3181 Return the length of the longest common prefix of the two
3182 strings, ignoring character case.
3185 @deffn {Scheme Procedure} string-suffix-length s1 s2 [start1 [end1 [start2 [end2]]]]
3186 @deffnx {C Function} scm_string_suffix_length (s1, s2, start1, end1, start2, end2)
3187 Return the length of the longest common suffix of the two
3191 @deffn {Scheme Procedure} string-suffix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3192 @deffnx {C Function} scm_string_suffix_length_ci (s1, s2, start1, end1, start2, end2)
3193 Return the length of the longest common suffix of the two
3194 strings, ignoring character case.
3197 @deffn {Scheme Procedure} string-prefix? s1 s2 [start1 [end1 [start2 [end2]]]]
3198 @deffnx {C Function} scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
3199 Is @var{s1} a prefix of @var{s2}?
3202 @deffn {Scheme Procedure} string-prefix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
3203 @deffnx {C Function} scm_string_prefix_ci_p (s1, s2, start1, end1, start2, end2)
3204 Is @var{s1} a prefix of @var{s2}, ignoring character case?
3207 @deffn {Scheme Procedure} string-suffix? s1 s2 [start1 [end1 [start2 [end2]]]]
3208 @deffnx {C Function} scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
3209 Is @var{s1} a suffix of @var{s2}?
3212 @deffn {Scheme Procedure} string-suffix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
3213 @deffnx {C Function} scm_string_suffix_ci_p (s1, s2, start1, end1, start2, end2)
3214 Is @var{s1} a suffix of @var{s2}, ignoring character case?
3217 @deffn {Scheme Procedure} string-index-right s char_pred [start [end]]
3218 @deffnx {C Function} scm_string_index_right (s, char_pred, start, end)
3219 Search through the string @var{s} from right to left, returning
3220 the index of the last occurence of a character which
3224 equals @var{char_pred}, if it is character,
3227 satisifies the predicate @var{char_pred}, if it is a procedure,
3230 is in the set if @var{char_pred} is a character set.
3234 @deffn {Scheme Procedure} string-skip s char_pred [start [end]]
3235 @deffnx {C Function} scm_string_skip (s, char_pred, start, end)
3236 Search through the string @var{s} from left to right, returning
3237 the index of the first occurence of a character which
3241 does not equal @var{char_pred}, if it is character,
3244 does not satisify the predicate @var{char_pred}, if it is a
3248 is not in the set if @var{char_pred} is a character set.
3252 @deffn {Scheme Procedure} string-skip-right s char_pred [start [end]]
3253 @deffnx {C Function} scm_string_skip_right (s, char_pred, start, end)
3254 Search through the string @var{s} from right to left, returning
3255 the index of the last occurence of a character which
3259 does not equal @var{char_pred}, if it is character,
3262 does not satisfy the predicate @var{char_pred}, if it is a
3266 is not in the set if @var{char_pred} is a character set.
3270 @deffn {Scheme Procedure} string-count s char_pred [start [end]]
3271 @deffnx {C Function} scm_string_count (s, char_pred, start, end)
3272 Return the count of the number of characters in the string
3277 equals @var{char_pred}, if it is character,
3280 satisifies the predicate @var{char_pred}, if it is a procedure.
3283 is in the set @var{char_pred}, if it is a character set.
3287 @deffn {Scheme Procedure} string-contains s1 s2 [start1 [end1 [start2 [end2]]]]
3288 @deffnx {C Function} scm_string_contains (s1, s2, start1, end1, start2, end2)
3289 Does string @var{s1} contain string @var{s2}? Return the index
3290 in @var{s1} where @var{s2} occurs as a substring, or false.
3291 The optional start/end indices restrict the operation to the
3292 indicated substrings.
3295 @deffn {Scheme Procedure} string-contains-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3296 @deffnx {C Function} scm_string_contains_ci (s1, s2, start1, end1, start2, end2)
3297 Does string @var{s1} contain string @var{s2}? Return the index
3298 in @var{s1} where @var{s2} occurs as a substring, or false.
3299 The optional start/end indices restrict the operation to the
3300 indicated substrings. Character comparison is done
3304 @node Alphabetic Case Mapping
3305 @subsubsection Alphabetic Case Mapping
3307 These are procedures for mapping strings to their upper- or lower-case
3308 equivalents, respectively, or for capitalizing strings.
3310 @deffn {Scheme Procedure} string-upcase str [start [end]]
3311 @deffnx {C Function} scm_substring_upcase (str, start, end)
3312 @deffnx {C Function} scm_string_upcase (str)
3313 Upcase every character in @code{str}.
3316 @deffn {Scheme Procedure} string-upcase! str [start [end]]
3317 @deffnx {C Function} scm_substring_upcase_x (str, start, end)
3318 @deffnx {C Function} scm_string_upcase_x (str)
3319 Destructively upcase every character in @code{str}.
3329 @deffn {Scheme Procedure} string-downcase str [start [end]]
3330 @deffnx {C Function} scm_substring_downcase (str, start, end)
3331 @deffnx {C Function} scm_string_downcase (str)
3332 Downcase every character in @var{str}.
3335 @deffn {Scheme Procedure} string-downcase! str [start [end]]
3336 @deffnx {C Function} scm_substring_downcase_x (str, start, end)
3337 @deffnx {C Function} scm_string_downcase_x (str)
3338 Destructively downcase every character in @var{str}.
3343 (string-downcase! y)
3350 @deffn {Scheme Procedure} string-capitalize str
3351 @deffnx {C Function} scm_string_capitalize (str)
3352 Return a freshly allocated string with the characters in
3353 @var{str}, where the first character of every word is
3357 @deffn {Scheme Procedure} string-capitalize! str
3358 @deffnx {C Function} scm_string_capitalize_x (str)
3359 Upcase the first character of every word in @var{str}
3360 destructively and return @var{str}.
3363 y @result{} "hello world"
3364 (string-capitalize! y) @result{} "Hello World"
3365 y @result{} "Hello World"
3369 @deffn {Scheme Procedure} string-titlecase str [start [end]]
3370 @deffnx {C Function} scm_string_titlecase (str, start, end)
3371 Titlecase every first character in a word in @var{str}.
3374 @deffn {Scheme Procedure} string-titlecase! str [start [end]]
3375 @deffnx {C Function} scm_string_titlecase_x (str, start, end)
3376 Destructively titlecase every first character in a word in
3380 @node Reversing and Appending Strings
3381 @subsubsection Reversing and Appending Strings
3383 @deffn {Scheme Procedure} string-reverse str [start [end]]
3384 @deffnx {C Function} scm_string_reverse (str, start, end)
3385 Reverse the string @var{str}. The optional arguments
3386 @var{start} and @var{end} delimit the region of @var{str} to
3390 @deffn {Scheme Procedure} string-reverse! str [start [end]]
3391 @deffnx {C Function} scm_string_reverse_x (str, start, end)
3392 Reverse the string @var{str} in-place. The optional arguments
3393 @var{start} and @var{end} delimit the region of @var{str} to
3394 operate on. The return value is unspecified.
3397 @rnindex string-append
3398 @deffn {Scheme Procedure} string-append . args
3399 @deffnx {C Function} scm_string_append (args)
3400 Return a newly allocated string whose characters form the
3401 concatenation of the given strings, @var{args}.
3405 (string-append h "world"))
3406 @result{} "hello world"
3410 @deffn {Scheme Procedure} string-append/shared . ls
3411 @deffnx {C Function} scm_string_append_shared (ls)
3412 Like @code{string-append}, but the result may share memory
3413 with the argument strings.
3416 @deffn {Scheme Procedure} string-concatenate ls
3417 @deffnx {C Function} scm_string_concatenate (ls)
3418 Append the elements of @var{ls} (which must be strings)
3419 together into a single string. Guaranteed to return a freshly
3423 @deffn {Scheme Procedure} string-concatenate-reverse ls [final_string [end]]
3424 @deffnx {C Function} scm_string_concatenate_reverse (ls, final_string, end)
3425 Without optional arguments, this procedure is equivalent to
3428 (string-concatenate (reverse ls))
3431 If the optional argument @var{final_string} is specified, it is
3432 consed onto the beginning to @var{ls} before performing the
3433 list-reverse and string-concatenate operations. If @var{end}
3434 is given, only the characters of @var{final_string} up to index
3437 Guaranteed to return a freshly allocated string.
3440 @deffn {Scheme Procedure} string-concatenate/shared ls
3441 @deffnx {C Function} scm_string_concatenate_shared (ls)
3442 Like @code{string-concatenate}, but the result may share memory
3443 with the strings in the list @var{ls}.
3446 @deffn {Scheme Procedure} string-concatenate-reverse/shared ls [final_string [end]]
3447 @deffnx {C Function} scm_string_concatenate_reverse_shared (ls, final_string, end)
3448 Like @code{string-concatenate-reverse}, but the result may
3449 share memory with the the strings in the @var{ls} arguments.
3452 @node Mapping Folding and Unfolding
3453 @subsubsection Mapping, Folding, and Unfolding
3455 @deffn {Scheme Procedure} string-map proc s [start [end]]
3456 @deffnx {C Function} scm_string_map (proc, s, start, end)
3457 @var{proc} is a char->char procedure, it is mapped over
3458 @var{s}. The order in which the procedure is applied to the
3459 string elements is not specified.
3462 @deffn {Scheme Procedure} string-map! proc s [start [end]]
3463 @deffnx {C Function} scm_string_map_x (proc, s, start, end)
3464 @var{proc} is a char->char procedure, it is mapped over
3465 @var{s}. The order in which the procedure is applied to the
3466 string elements is not specified. The string @var{s} is
3467 modified in-place, the return value is not specified.
3470 @deffn {Scheme Procedure} string-for-each proc s [start [end]]
3471 @deffnx {C Function} scm_string_for_each (proc, s, start, end)
3472 @var{proc} is mapped over @var{s} in left-to-right order. The
3473 return value is not specified.
3476 @deffn {Scheme Procedure} string-for-each-index proc s [start [end]]
3477 @deffnx {C Function} scm_string_for_each_index (proc, s, start, end)
3478 Call @code{(@var{proc} i)} for each index i in @var{s}, from left to
3481 For example, to change characters to alternately upper and lower case,
3484 (define str (string-copy "studly"))
3485 (string-for-each-index (lambda (i)
3487 ((if (even? i) char-upcase char-downcase)
3488 (string-ref str i))))
3490 str @result{} "StUdLy"
3494 @deffn {Scheme Procedure} string-fold kons knil s [start [end]]
3495 @deffnx {C Function} scm_string_fold (kons, knil, s, start, end)
3496 Fold @var{kons} over the characters of @var{s}, with @var{knil}
3497 as the terminating element, from left to right. @var{kons}
3498 must expect two arguments: The actual character and the last
3499 result of @var{kons}' application.
3502 @deffn {Scheme Procedure} string-fold-right kons knil s [start [end]]
3503 @deffnx {C Function} scm_string_fold_right (kons, knil, s, start, end)
3504 Fold @var{kons} over the characters of @var{s}, with @var{knil}
3505 as the terminating element, from right to left. @var{kons}
3506 must expect two arguments: The actual character and the last
3507 result of @var{kons}' application.
3510 @deffn {Scheme Procedure} string-unfold p f g seed [base [make_final]]
3511 @deffnx {C Function} scm_string_unfold (p, f, g, seed, base, make_final)
3513 @item @var{g} is used to generate a series of @emph{seed}
3514 values from the initial @var{seed}: @var{seed}, (@var{g}
3515 @var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
3517 @item @var{p} tells us when to stop -- when it returns true
3518 when applied to one of these seed values.
3519 @item @var{f} maps each seed value to the corresponding
3520 character in the result string. These chars are assembled
3521 into the string in a left-to-right order.
3522 @item @var{base} is the optional initial/leftmost portion
3523 of the constructed string; it default to the empty
3525 @item @var{make_final} is applied to the terminal seed
3526 value (on which @var{p} returns true) to produce
3527 the final/rightmost portion of the constructed string.
3528 The default is nothing extra.
3532 @deffn {Scheme Procedure} string-unfold-right p f g seed [base [make_final]]
3533 @deffnx {C Function} scm_string_unfold_right (p, f, g, seed, base, make_final)
3535 @item @var{g} is used to generate a series of @emph{seed}
3536 values from the initial @var{seed}: @var{seed}, (@var{g}
3537 @var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
3539 @item @var{p} tells us when to stop -- when it returns true
3540 when applied to one of these seed values.
3541 @item @var{f} maps each seed value to the corresponding
3542 character in the result string. These chars are assembled
3543 into the string in a right-to-left order.
3544 @item @var{base} is the optional initial/rightmost portion
3545 of the constructed string; it default to the empty
3547 @item @var{make_final} is applied to the terminal seed
3548 value (on which @var{p} returns true) to produce
3549 the final/leftmost portion of the constructed string.
3550 It defaults to @code{(lambda (x) )}.
3554 @node Miscellaneous String Operations
3555 @subsubsection Miscellaneous String Operations
3557 @deffn {Scheme Procedure} xsubstring s from [to [start [end]]]
3558 @deffnx {C Function} scm_xsubstring (s, from, to, start, end)
3559 This is the @emph{extended substring} procedure that implements
3560 replicated copying of a substring of some string.
3562 @var{s} is a string, @var{start} and @var{end} are optional
3563 arguments that demarcate a substring of @var{s}, defaulting to
3564 0 and the length of @var{s}. Replicate this substring up and
3565 down index space, in both the positive and negative directions.
3566 @code{xsubstring} returns the substring of this string
3567 beginning at index @var{from}, and ending at @var{to}, which
3568 defaults to @var{from} + (@var{end} - @var{start}).
3571 @deffn {Scheme Procedure} string-xcopy! target tstart s sfrom [sto [start [end]]]
3572 @deffnx {C Function} scm_string_xcopy_x (target, tstart, s, sfrom, sto, start, end)
3573 Exactly the same as @code{xsubstring}, but the extracted text
3574 is written into the string @var{target} starting at index
3575 @var{tstart}. The operation is not defined if @code{(eq?
3576 @var{target} @var{s})} or these arguments share storage -- you
3577 cannot copy a string on top of itself.
3580 @deffn {Scheme Procedure} string-replace s1 s2 [start1 [end1 [start2 [end2]]]]
3581 @deffnx {C Function} scm_string_replace (s1, s2, start1, end1, start2, end2)
3582 Return the string @var{s1}, but with the characters
3583 @var{start1} @dots{} @var{end1} replaced by the characters
3584 @var{start2} @dots{} @var{end2} from @var{s2}.
3587 @deffn {Scheme Procedure} string-tokenize s [token_set [start [end]]]
3588 @deffnx {C Function} scm_string_tokenize (s, token_set, start, end)
3589 Split the string @var{s} into a list of substrings, where each
3590 substring is a maximal non-empty contiguous sequence of
3591 characters from the character set @var{token_set}, which
3592 defaults to @code{char-set:graphic}.
3593 If @var{start} or @var{end} indices are provided, they restrict
3594 @code{string-tokenize} to operating on the indicated substring
3598 @deffn {Scheme Procedure} string-filter s char_pred [start [end]]
3599 @deffnx {C Function} scm_string_filter (s, char_pred, start, end)
3600 Filter the string @var{s}, retaining only those characters which
3601 satisfy @var{char_pred}.
3603 If @var{char_pred} is a procedure, it is applied to each character as
3604 a predicate, if it is a character, it is tested for equality and if it
3605 is a character set, it is tested for membership.
3608 @deffn {Scheme Procedure} string-delete s char_pred [start [end]]
3609 @deffnx {C Function} scm_string_delete (s, char_pred, start, end)
3610 Delete characters satisfying @var{char_pred} from @var{s}.
3612 If @var{char_pred} is a procedure, it is applied to each character as
3613 a predicate, if it is a character, it is tested for equality and if it
3614 is a character set, it is tested for membership.
3617 @node Conversion to/from C
3618 @subsubsection Conversion to/from C
3620 When creating a Scheme string from a C string or when converting a
3621 Scheme string to a C string, the concept of character encoding becomes
3624 In C, a string is just a sequence of bytes, and the character encoding
3625 describes the relation between these bytes and the actual characters
3626 that make up the string. For Scheme strings, character encoding is
3627 not an issue (most of the time), since in Scheme you never get to see
3628 the bytes, only the characters.
3630 Well, ideally, anyway. Right now, Guile simply equates Scheme
3631 characters and bytes, ignoring the possibility of multi-byte encodings
3632 completely. This will change in the future, where Guile will use
3633 Unicode codepoints as its characters and UTF-8 or some other encoding
3634 as its internal encoding. When you exclusively use the functions
3635 listed in this section, you are `future-proof'.
3637 Converting a Scheme string to a C string will often allocate fresh
3638 memory to hold the result. You must take care that this memory is
3639 properly freed eventually. In many cases, this can be achieved by
3640 using @code{scm_dynwind_free} inside an appropriate dynwind context,
3641 @xref{Dynamic Wind}.
3643 @deftypefn {C Function} SCM scm_from_locale_string (const char *str)
3644 @deftypefnx {C Function} SCM scm_from_locale_stringn (const char *str, size_t len)
3645 Creates a new Scheme string that has the same contents as @var{str}
3646 when interpreted in the current locale character encoding.
3648 For @code{scm_from_locale_string}, @var{str} must be null-terminated.
3650 For @code{scm_from_locale_stringn}, @var{len} specifies the length of
3651 @var{str} in bytes, and @var{str} does not need to be null-terminated.
3652 If @var{len} is @code{(size_t)-1}, then @var{str} does need to be
3653 null-terminated and the real length will be found with @code{strlen}.
3656 @deftypefn {C Function} SCM scm_take_locale_string (char *str)
3657 @deftypefnx {C Function} SCM scm_take_locale_stringn (char *str, size_t len)
3658 Like @code{scm_from_locale_string} and @code{scm_from_locale_stringn},
3659 respectively, but also frees @var{str} with @code{free} eventually.
3660 Thus, you can use this function when you would free @var{str} anyway
3661 immediately after creating the Scheme string. In certain cases, Guile
3662 can then use @var{str} directly as its internal representation.
3665 @deftypefn {C Function} {char *} scm_to_locale_string (SCM str)
3666 @deftypefnx {C Function} {char *} scm_to_locale_stringn (SCM str, size_t *lenp)
3667 Returns a C string in the current locale encoding with the same
3668 contents as @var{str}. The C string must be freed with @code{free}
3669 eventually, maybe by using @code{scm_dynwind_free}, @xref{Dynamic
3672 For @code{scm_to_locale_string}, the returned string is
3673 null-terminated and an error is signalled when @var{str} contains
3674 @code{#\nul} characters.
3676 For @code{scm_to_locale_stringn} and @var{lenp} not @code{NULL},
3677 @var{str} might contain @code{#\nul} characters and the length of the
3678 returned string in bytes is stored in @code{*@var{lenp}}. The
3679 returned string will not be null-terminated in this case. If
3680 @var{lenp} is @code{NULL}, @code{scm_to_locale_stringn} behaves like
3681 @code{scm_to_locale_string}.
3684 @deftypefn {C Function} size_t scm_to_locale_stringbuf (SCM str, char *buf, size_t max_len)
3685 Puts @var{str} as a C string in the current locale encoding into the
3686 memory pointed to by @var{buf}. The buffer at @var{buf} has room for
3687 @var{max_len} bytes and @code{scm_to_local_stringbuf} will never store
3688 more than that. No terminating @code{'\0'} will be stored.
3690 The return value of @code{scm_to_locale_stringbuf} is the number of
3691 bytes that are needed for all of @var{str}, regardless of whether
3692 @var{buf} was large enough to hold them. Thus, when the return value
3693 is larger than @var{max_len}, only @var{max_len} bytes have been
3694 stored and you probably need to try again with a larger buffer.
3697 @node Regular Expressions
3698 @subsection Regular Expressions
3699 @tpindex Regular expressions
3701 @cindex regular expressions
3703 @cindex emacs regexp
3705 A @dfn{regular expression} (or @dfn{regexp}) is a pattern that
3706 describes a whole class of strings. A full description of regular
3707 expressions and their syntax is beyond the scope of this manual;
3708 an introduction can be found in the Emacs manual (@pxref{Regexps,
3709 , Syntax of Regular Expressions, emacs, The GNU Emacs Manual}), or
3710 in many general Unix reference books.
3712 If your system does not include a POSIX regular expression library,
3713 and you have not linked Guile with a third-party regexp library such
3714 as Rx, these functions will not be available. You can tell whether
3715 your Guile installation includes regular expression support by
3716 checking whether @code{(provided? 'regex)} returns true.
3718 The following regexp and string matching features are provided by the
3719 @code{(ice-9 regex)} module. Before using the described functions,
3720 you should load this module by executing @code{(use-modules (ice-9
3724 * Regexp Functions:: Functions that create and match regexps.
3725 * Match Structures:: Finding what was matched by a regexp.
3726 * Backslash Escapes:: Removing the special meaning of regexp
3731 @node Regexp Functions
3732 @subsubsection Regexp Functions
3734 By default, Guile supports POSIX extended regular expressions.
3735 That means that the characters @samp{(}, @samp{)}, @samp{+} and
3736 @samp{?} are special, and must be escaped if you wish to match the
3739 This regular expression interface was modeled after that
3740 implemented by SCSH, the Scheme Shell. It is intended to be
3741 upwardly compatible with SCSH regular expressions.
3743 Zero bytes (@code{#\nul}) cannot be used in regex patterns or input
3744 strings, since the underlying C functions treat that as the end of
3745 string. If there's a zero byte an error is thrown.
3747 Patterns and input strings are treated as being in the locale
3748 character set if @code{setlocale} has been called (@pxref{Locales}),
3749 and in a multibyte locale this includes treating multi-byte sequences
3750 as a single character. (Guile strings are currently merely bytes,
3751 though this may change in the future, @xref{Conversion to/from C}.)
3753 @deffn {Scheme Procedure} string-match pattern str [start]
3754 Compile the string @var{pattern} into a regular expression and compare
3755 it with @var{str}. The optional numeric argument @var{start} specifies
3756 the position of @var{str} at which to begin matching.
3758 @code{string-match} returns a @dfn{match structure} which
3759 describes what, if anything, was matched by the regular
3760 expression. @xref{Match Structures}. If @var{str} does not match
3761 @var{pattern} at all, @code{string-match} returns @code{#f}.
3764 Two examples of a match follow. In the first example, the pattern
3765 matches the four digits in the match string. In the second, the pattern
3769 (string-match "[0-9][0-9][0-9][0-9]" "blah2002")
3770 @result{} #("blah2002" (4 . 8))
3772 (string-match "[A-Za-z]" "123456")
3776 Each time @code{string-match} is called, it must compile its
3777 @var{pattern} argument into a regular expression structure. This
3778 operation is expensive, which makes @code{string-match} inefficient if
3779 the same regular expression is used several times (for example, in a
3780 loop). For better performance, you can compile a regular expression in
3781 advance and then match strings against the compiled regexp.
3783 @deffn {Scheme Procedure} make-regexp pat flag@dots{}
3784 @deffnx {C Function} scm_make_regexp (pat, flaglst)
3785 Compile the regular expression described by @var{pat}, and
3786 return the compiled regexp structure. If @var{pat} does not
3787 describe a legal regular expression, @code{make-regexp} throws
3788 a @code{regular-expression-syntax} error.
3790 The @var{flag} arguments change the behavior of the compiled
3791 regular expression. The following values may be supplied:
3793 @defvar regexp/icase
3794 Consider uppercase and lowercase letters to be the same when
3798 @defvar regexp/newline
3799 If a newline appears in the target string, then permit the
3800 @samp{^} and @samp{$} operators to match immediately after or
3801 immediately before the newline, respectively. Also, the
3802 @samp{.} and @samp{[^...]} operators will never match a newline
3803 character. The intent of this flag is to treat the target
3804 string as a buffer containing many lines of text, and the
3805 regular expression as a pattern that may match a single one of
3809 @defvar regexp/basic
3810 Compile a basic (``obsolete'') regexp instead of the extended
3811 (``modern'') regexps that are the default. Basic regexps do
3812 not consider @samp{|}, @samp{+} or @samp{?} to be special
3813 characters, and require the @samp{@{...@}} and @samp{(...)}
3814 metacharacters to be backslash-escaped (@pxref{Backslash
3815 Escapes}). There are several other differences between basic
3816 and extended regular expressions, but these are the most
3820 @defvar regexp/extended
3821 Compile an extended regular expression rather than a basic
3822 regexp. This is the default behavior; this flag will not
3823 usually be needed. If a call to @code{make-regexp} includes
3824 both @code{regexp/basic} and @code{regexp/extended} flags, the
3825 one which comes last will override the earlier one.
3829 @deffn {Scheme Procedure} regexp-exec rx str [start [flags]]
3830 @deffnx {C Function} scm_regexp_exec (rx, str, start, flags)
3831 Match the compiled regular expression @var{rx} against
3832 @code{str}. If the optional integer @var{start} argument is
3833 provided, begin matching from that position in the string.
3834 Return a match structure describing the results of the match,
3835 or @code{#f} if no match could be found.
3837 The @var{flags} argument changes the matching behavior. The following
3838 flag values may be supplied, use @code{logior} (@pxref{Bitwise
3839 Operations}) to combine them,
3841 @defvar regexp/notbol
3842 Consider that the @var{start} offset into @var{str} is not the
3843 beginning of a line and should not match operator @samp{^}.
3845 If @var{rx} was created with the @code{regexp/newline} option above,
3846 @samp{^} will still match after a newline in @var{str}.
3849 @defvar regexp/noteol
3850 Consider that the end of @var{str} is not the end of a line and should
3851 not match operator @samp{$}.
3853 If @var{rx} was created with the @code{regexp/newline} option above,
3854 @samp{$} will still match before a newline in @var{str}.
3859 ;; Regexp to match uppercase letters
3860 (define r (make-regexp "[A-Z]*"))
3862 ;; Regexp to match letters, ignoring case
3863 (define ri (make-regexp "[A-Z]*" regexp/icase))
3865 ;; Search for bob using regexp r
3866 (match:substring (regexp-exec r "bob"))
3867 @result{} "" ; no match
3869 ;; Search for bob using regexp ri
3870 (match:substring (regexp-exec ri "Bob"))
3871 @result{} "Bob" ; matched case insensitive
3874 @deffn {Scheme Procedure} regexp? obj
3875 @deffnx {C Function} scm_regexp_p (obj)
3876 Return @code{#t} if @var{obj} is a compiled regular expression,
3877 or @code{#f} otherwise.
3881 @deffn {Scheme Procedure} list-matches regexp str [flags]
3882 Return a list of match structures which are the non-overlapping
3883 matches of @var{regexp} in @var{str}. @var{regexp} can be either a
3884 pattern string or a compiled regexp. The @var{flags} argument is as
3885 per @code{regexp-exec} above.
3888 (map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
3889 @result{} ("abc" "def")
3893 @deffn {Scheme Procedure} fold-matches regexp str init proc [flags]
3894 Apply @var{proc} to the non-overlapping matches of @var{regexp} in
3895 @var{str}, to build a result. @var{regexp} can be either a pattern
3896 string or a compiled regexp. The @var{flags} argument is as per
3897 @code{regexp-exec} above.
3899 @var{proc} is called as @code{(@var{proc} match prev)} where
3900 @var{match} is a match structure and @var{prev} is the previous return
3901 from @var{proc}. For the first call @var{prev} is the given
3902 @var{init} parameter. @code{fold-matches} returns the final value
3905 For example to count matches,
3908 (fold-matches "[a-z][0-9]" "abc x1 def y2" 0
3909 (lambda (match count)
3916 Regular expressions are commonly used to find patterns in one string
3917 and replace them with the contents of another string. The following
3918 functions are convenient ways to do this.
3920 @c begin (scm-doc-string "regex.scm" "regexp-substitute")
3921 @deffn {Scheme Procedure} regexp-substitute port match [item@dots{}]
3922 Write to @var{port} selected parts of the match structure @var{match}.
3923 Or if @var{port} is @code{#f} then form a string from those parts and
3926 Each @var{item} specifies a part to be written, and may be one of the
3931 A string. String arguments are written out verbatim.
3934 An integer. The submatch with that number is written
3935 (@code{match:substring}). Zero is the entire match.
3938 The symbol @samp{pre}. The portion of the matched string preceding
3939 the regexp match is written (@code{match:prefix}).
3942 The symbol @samp{post}. The portion of the matched string following
3943 the regexp match is written (@code{match:suffix}).
3946 For example, changing a match and retaining the text before and after,
3949 (regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
3951 @result{} "number 37 is good"
3954 Or matching a @sc{yyyymmdd} format date such as @samp{20020828} and
3955 re-ordering and hyphenating the fields.
3958 (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
3959 (define s "Date 20020429 12am.")
3960 (regexp-substitute #f (string-match date-regex s)
3961 'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
3962 @result{} "Date 04-29-2002 12am. (20020429)"
3967 @c begin (scm-doc-string "regex.scm" "regexp-substitute")
3968 @deffn {Scheme Procedure} regexp-substitute/global port regexp target [item@dots{}]
3969 @cindex search and replace
3970 Write to @var{port} selected parts of matches of @var{regexp} in
3971 @var{target}. If @var{port} is @code{#f} then form a string from
3972 those parts and return that. @var{regexp} can be a string or a
3975 This is similar to @code{regexp-substitute}, but allows global
3976 substitutions on @var{target}. Each @var{item} behaves as per
3977 @code{regexp-substitute}, with the following differences,
3981 A function. Called as @code{(@var{item} match)} with the match
3982 structure for the @var{regexp} match, it should return a string to be
3983 written to @var{port}.
3986 The symbol @samp{post}. This doesn't output anything, but instead
3987 causes @code{regexp-substitute/global} to recurse on the unmatched
3988 portion of @var{target}.
3990 This @emph{must} be supplied to perform a global search and replace on
3991 @var{target}; without it @code{regexp-substitute/global} returns after
3992 a single match and output.
3995 For example, to collapse runs of tabs and spaces to a single hyphen
3999 (regexp-substitute/global #f "[ \t]+" "this is the text"
4001 @result{} "this-is-the-text"
4004 Or using a function to reverse the letters in each word,
4007 (regexp-substitute/global #f "[a-z]+" "to do and not-do"
4008 'pre (lambda (m) (string-reverse (match:substring m))) 'post)
4009 @result{} "ot od dna ton-od"
4012 Without the @code{post} symbol, just one regexp match is made. For
4013 example the following is the date example from
4014 @code{regexp-substitute} above, without the need for the separate
4015 @code{string-match} call.
4018 (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
4019 (define s "Date 20020429 12am.")
4020 (regexp-substitute/global #f date-regex s
4021 'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
4023 @result{} "Date 04-29-2002 12am. (20020429)"
4028 @node Match Structures
4029 @subsubsection Match Structures
4031 @cindex match structures
4033 A @dfn{match structure} is the object returned by @code{string-match} and
4034 @code{regexp-exec}. It describes which portion of a string, if any,
4035 matched the given regular expression. Match structures include: a
4036 reference to the string that was checked for matches; the starting and
4037 ending positions of the regexp match; and, if the regexp included any
4038 parenthesized subexpressions, the starting and ending positions of each
4041 In each of the regexp match functions described below, the @code{match}
4042 argument must be a match structure returned by a previous call to
4043 @code{string-match} or @code{regexp-exec}. Most of these functions
4044 return some information about the original target string that was
4045 matched against a regular expression; we will call that string
4046 @var{target} for easy reference.
4048 @c begin (scm-doc-string "regex.scm" "regexp-match?")
4049 @deffn {Scheme Procedure} regexp-match? obj
4050 Return @code{#t} if @var{obj} is a match structure returned by a
4051 previous call to @code{regexp-exec}, or @code{#f} otherwise.
4054 @c begin (scm-doc-string "regex.scm" "match:substring")
4055 @deffn {Scheme Procedure} match:substring match [n]
4056 Return the portion of @var{target} matched by subexpression number
4057 @var{n}. Submatch 0 (the default) represents the entire regexp match.
4058 If the regular expression as a whole matched, but the subexpression
4059 number @var{n} did not match, return @code{#f}.
4063 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4067 ;; match starting at offset 6 in the string
4069 (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
4073 @c begin (scm-doc-string "regex.scm" "match:start")
4074 @deffn {Scheme Procedure} match:start match [n]
4075 Return the starting position of submatch number @var{n}.
4078 In the following example, the result is 4, since the match starts at
4082 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4087 @c begin (scm-doc-string "regex.scm" "match:end")
4088 @deffn {Scheme Procedure} match:end match [n]
4089 Return the ending position of submatch number @var{n}.
4092 In the following example, the result is 8, since the match runs between
4093 characters 4 and 8 (i.e. the ``2002'').
4096 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4101 @c begin (scm-doc-string "regex.scm" "match:prefix")
4102 @deffn {Scheme Procedure} match:prefix match
4103 Return the unmatched portion of @var{target} preceding the regexp match.
4106 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4112 @c begin (scm-doc-string "regex.scm" "match:suffix")
4113 @deffn {Scheme Procedure} match:suffix match
4114 Return the unmatched portion of @var{target} following the regexp match.
4118 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4123 @c begin (scm-doc-string "regex.scm" "match:count")
4124 @deffn {Scheme Procedure} match:count match
4125 Return the number of parenthesized subexpressions from @var{match}.
4126 Note that the entire regular expression match itself counts as a
4127 subexpression, and failed submatches are included in the count.
4130 @c begin (scm-doc-string "regex.scm" "match:string")
4131 @deffn {Scheme Procedure} match:string match
4132 Return the original @var{target} string.
4136 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4138 @result{} "blah2002foo"
4142 @node Backslash Escapes
4143 @subsubsection Backslash Escapes
4145 Sometimes you will want a regexp to match characters like @samp{*} or
4146 @samp{$} exactly. For example, to check whether a particular string
4147 represents a menu entry from an Info node, it would be useful to match
4148 it against a regexp like @samp{^* [^:]*::}. However, this won't work;
4149 because the asterisk is a metacharacter, it won't match the @samp{*} at
4150 the beginning of the string. In this case, we want to make the first
4153 You can do this by preceding the metacharacter with a backslash
4154 character @samp{\}. (This is also called @dfn{quoting} the
4155 metacharacter, and is known as a @dfn{backslash escape}.) When Guile
4156 sees a backslash in a regular expression, it considers the following
4157 glyph to be an ordinary character, no matter what special meaning it
4158 would ordinarily have. Therefore, we can make the above example work by
4159 changing the regexp to @samp{^\* [^:]*::}. The @samp{\*} sequence tells
4160 the regular expression engine to match only a single asterisk in the
4163 Since the backslash is itself a metacharacter, you may force a regexp to
4164 match a backslash in the target string by preceding the backslash with
4165 itself. For example, to find variable references in a @TeX{} program,
4166 you might want to find occurrences of the string @samp{\let\} followed
4167 by any number of alphabetic characters. The regular expression
4168 @samp{\\let\\[A-Za-z]*} would do this: the double backslashes in the
4169 regexp each match a single backslash in the target string.
4171 @c begin (scm-doc-string "regex.scm" "regexp-quote")
4172 @deffn {Scheme Procedure} regexp-quote str
4173 Quote each special character found in @var{str} with a backslash, and
4174 return the resulting string.
4177 @strong{Very important:} Using backslash escapes in Guile source code
4178 (as in Emacs Lisp or C) can be tricky, because the backslash character
4179 has special meaning for the Guile reader. For example, if Guile
4180 encounters the character sequence @samp{\n} in the middle of a string
4181 while processing Scheme code, it replaces those characters with a
4182 newline character. Similarly, the character sequence @samp{\t} is
4183 replaced by a horizontal tab. Several of these @dfn{escape sequences}
4184 are processed by the Guile reader before your code is executed.
4185 Unrecognized escape sequences are ignored: if the characters @samp{\*}
4186 appear in a string, they will be translated to the single character
4189 This translation is obviously undesirable for regular expressions, since
4190 we want to be able to include backslashes in a string in order to
4191 escape regexp metacharacters. Therefore, to make sure that a backslash
4192 is preserved in a string in your Guile program, you must use @emph{two}
4193 consecutive backslashes:
4196 (define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))
4199 The string in this example is preprocessed by the Guile reader before
4200 any code is executed. The resulting argument to @code{make-regexp} is
4201 the string @samp{^\* [^:]*}, which is what we really want.
4203 This also means that in order to write a regular expression that matches
4204 a single backslash character, the regular expression string in the
4205 source code must include @emph{four} backslashes. Each consecutive pair
4206 of backslashes gets translated by the Guile reader to a single
4207 backslash, and the resulting double-backslash is interpreted by the
4208 regexp engine as matching a single backslash character. Hence:
4211 (define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))
4214 The reason for the unwieldiness of this syntax is historical. Both
4215 regular expression pattern matchers and Unix string processing systems
4216 have traditionally used backslashes with the special meanings
4217 described above. The POSIX regular expression specification and ANSI C
4218 standard both require these semantics. Attempting to abandon either
4219 convention would cause other kinds of compatibility problems, possibly
4220 more severe ones. Therefore, without extending the Scheme reader to
4221 support strings with different quoting conventions (an ungainly and
4222 confusing extension when implemented in other languages), we must adhere
4223 to this cumbersome escape syntax.
4230 Symbols in Scheme are widely used in three ways: as items of discrete
4231 data, as lookup keys for alists and hash tables, and to denote variable
4234 A @dfn{symbol} is similar to a string in that it is defined by a
4235 sequence of characters. The sequence of characters is known as the
4236 symbol's @dfn{name}. In the usual case --- that is, where the symbol's
4237 name doesn't include any characters that could be confused with other
4238 elements of Scheme syntax --- a symbol is written in a Scheme program by
4239 writing the sequence of characters that make up the name, @emph{without}
4240 any quotation marks or other special syntax. For example, the symbol
4241 whose name is ``multiply-by-2'' is written, simply:
4247 Notice how this differs from a @emph{string} with contents
4248 ``multiply-by-2'', which is written with double quotation marks, like
4255 Looking beyond how they are written, symbols are different from strings
4256 in two important respects.
4258 The first important difference is uniqueness. If the same-looking
4259 string is read twice from two different places in a program, the result
4260 is two @emph{different} string objects whose contents just happen to be
4261 the same. If, on the other hand, the same-looking symbol is read twice
4262 from two different places in a program, the result is the @emph{same}
4263 symbol object both times.
4265 Given two read symbols, you can use @code{eq?} to test whether they are
4266 the same (that is, have the same name). @code{eq?} is the most
4267 efficient comparison operator in Scheme, and comparing two symbols like
4268 this is as fast as comparing, for example, two numbers. Given two
4269 strings, on the other hand, you must use @code{equal?} or
4270 @code{string=?}, which are much slower comparison operators, to
4271 determine whether the strings have the same contents.
4274 (define sym1 (quote hello))
4275 (define sym2 (quote hello))
4276 (eq? sym1 sym2) @result{} #t
4278 (define str1 "hello")
4279 (define str2 "hello")
4280 (eq? str1 str2) @result{} #f
4281 (equal? str1 str2) @result{} #t
4284 The second important difference is that symbols, unlike strings, are not
4285 self-evaluating. This is why we need the @code{(quote @dots{})}s in the
4286 example above: @code{(quote hello)} evaluates to the symbol named
4287 "hello" itself, whereas an unquoted @code{hello} is @emph{read} as the
4288 symbol named "hello" and evaluated as a variable reference @dots{} about
4289 which more below (@pxref{Symbol Variables}).
4292 * Symbol Data:: Symbols as discrete data.
4293 * Symbol Keys:: Symbols as lookup keys.
4294 * Symbol Variables:: Symbols as denoting variables.
4295 * Symbol Primitives:: Operations related to symbols.
4296 * Symbol Props:: Function slots and property lists.
4297 * Symbol Read Syntax:: Extended read syntax for symbols.
4298 * Symbol Uninterned:: Uninterned symbols.
4303 @subsubsection Symbols as Discrete Data
4305 Numbers and symbols are similar to the extent that they both lend
4306 themselves to @code{eq?} comparison. But symbols are more descriptive
4307 than numbers, because a symbol's name can be used directly to describe
4308 the concept for which that symbol stands.
4310 For example, imagine that you need to represent some colours in a
4311 computer program. Using numbers, you would have to choose arbitrarily
4312 some mapping between numbers and colours, and then take care to use that
4313 mapping consistently:
4316 ;; 1=red, 2=green, 3=purple
4318 (if (eq? (colour-of car) 1)
4323 You can make the mapping more explicit and the code more readable by
4331 (if (eq? (colour-of car) red)
4336 But the simplest and clearest approach is not to use numbers at all, but
4337 symbols whose names specify the colours that they refer to:
4340 (if (eq? (colour-of car) 'red)
4344 The descriptive advantages of symbols over numbers increase as the set
4345 of concepts that you want to describe grows. Suppose that a car object
4346 can have other properties as well, such as whether it has or uses:
4350 automatic or manual transmission
4352 leaded or unleaded fuel
4354 power steering (or not).
4358 Then a car's combined property set could be naturally represented and
4359 manipulated as a list of symbols:
4362 (properties-of car1)
4364 (red manual unleaded power-steering)
4366 (if (memq 'power-steering (properties-of car1))
4367 (display "Unfit people can drive this car.\n")
4368 (display "You'll need strong arms to drive this car!\n"))
4370 Unfit people can drive this car.
4373 Remember, the fundamental property of symbols that we are relying on
4374 here is that an occurrence of @code{'red} in one part of a program is an
4375 @emph{indistinguishable} symbol from an occurrence of @code{'red} in
4376 another part of a program; this means that symbols can usefully be
4377 compared using @code{eq?}. At the same time, symbols have naturally
4378 descriptive names. This combination of efficiency and descriptive power
4379 makes them ideal for use as discrete data.
4383 @subsubsection Symbols as Lookup Keys
4385 Given their efficiency and descriptive power, it is natural to use
4386 symbols as the keys in an association list or hash table.
4388 To illustrate this, consider a more structured representation of the car
4389 properties example from the preceding subsection. Rather than
4390 mixing all the properties up together in a flat list, we could use an
4391 association list like this:
4394 (define car1-properties '((colour . red)
4395 (transmission . manual)
4397 (steering . power-assisted)))
4400 Notice how this structure is more explicit and extensible than the flat
4401 list. For example it makes clear that @code{manual} refers to the
4402 transmission rather than, say, the windows or the locking of the car.
4403 It also allows further properties to use the same symbols among their
4404 possible values without becoming ambiguous:
4407 (define car1-properties '((colour . red)
4408 (transmission . manual)
4410 (steering . power-assisted)
4412 (locking . manual)))
4415 With a representation like this, it is easy to use the efficient
4416 @code{assq-XXX} family of procedures (@pxref{Association Lists}) to
4417 extract or change individual pieces of information:
4420 (assq-ref car1-properties 'fuel) @result{} unleaded
4421 (assq-ref car1-properties 'transmission) @result{} manual
4423 (assq-set! car1-properties 'seat-colour 'black)
4426 (transmission . manual)
4428 (steering . power-assisted)
4429 (seat-colour . black)
4430 (locking . manual)))
4433 Hash tables also have keys, and exactly the same arguments apply to the
4434 use of symbols in hash tables as in association lists. The hash value
4435 that Guile uses to decide where to add a symbol-keyed entry to a hash
4436 table can be obtained by calling the @code{symbol-hash} procedure:
4438 @deffn {Scheme Procedure} symbol-hash symbol
4439 @deffnx {C Function} scm_symbol_hash (symbol)
4440 Return a hash value for @var{symbol}.
4443 See @ref{Hash Tables} for information about hash tables in general, and
4444 for why you might choose to use a hash table rather than an association
4448 @node Symbol Variables
4449 @subsubsection Symbols as Denoting Variables
4451 When an unquoted symbol in a Scheme program is evaluated, it is
4452 interpreted as a variable reference, and the result of the evaluation is
4453 the appropriate variable's value.
4455 For example, when the expression @code{(string-length "abcd")} is read
4456 and evaluated, the sequence of characters @code{string-length} is read
4457 as the symbol whose name is "string-length". This symbol is associated
4458 with a variable whose value is the procedure that implements string
4459 length calculation. Therefore evaluation of the @code{string-length}
4460 symbol results in that procedure.
4462 The details of the connection between an unquoted symbol and the
4463 variable to which it refers are explained elsewhere. See @ref{Binding
4464 Constructs}, for how associations between symbols and variables are
4465 created, and @ref{Modules}, for how those associations are affected by
4466 Guile's module system.
4469 @node Symbol Primitives
4470 @subsubsection Operations Related to Symbols
4472 Given any Scheme value, you can determine whether it is a symbol using
4473 the @code{symbol?} primitive:
4476 @deffn {Scheme Procedure} symbol? obj
4477 @deffnx {C Function} scm_symbol_p (obj)
4478 Return @code{#t} if @var{obj} is a symbol, otherwise return
4482 @deftypefn {C Function} int scm_is_symbol (SCM val)
4483 Equivalent to @code{scm_is_true (scm_symbol_p (val))}.
4486 Once you know that you have a symbol, you can obtain its name as a
4487 string by calling @code{symbol->string}. Note that Guile differs by
4488 default from R5RS on the details of @code{symbol->string} as regards
4491 @rnindex symbol->string
4492 @deffn {Scheme Procedure} symbol->string s
4493 @deffnx {C Function} scm_symbol_to_string (s)
4494 Return the name of symbol @var{s} as a string. By default, Guile reads
4495 symbols case-sensitively, so the string returned will have the same case
4496 variation as the sequence of characters that caused @var{s} to be
4499 If Guile is set to read symbols case-insensitively (as specified by
4500 R5RS), and @var{s} comes into being as part of a literal expression
4501 (@pxref{Literal expressions,,,r5rs, The Revised^5 Report on Scheme}) or
4502 by a call to the @code{read} or @code{string-ci->symbol} procedures,
4503 Guile converts any alphabetic characters in the symbol's name to
4504 lower case before creating the symbol object, so the string returned
4505 here will be in lower case.
4507 If @var{s} was created by @code{string->symbol}, the case of characters
4508 in the string returned will be the same as that in the string that was
4509 passed to @code{string->symbol}, regardless of Guile's case-sensitivity
4510 setting at the time @var{s} was created.
4512 It is an error to apply mutation procedures like @code{string-set!} to
4513 strings returned by this procedure.
4516 Most symbols are created by writing them literally in code. However it
4517 is also possible to create symbols programmatically using the following
4518 @code{string->symbol} and @code{string-ci->symbol} procedures:
4520 @rnindex string->symbol
4521 @deffn {Scheme Procedure} string->symbol string
4522 @deffnx {C Function} scm_string_to_symbol (string)
4523 Return the symbol whose name is @var{string}. This procedure can create
4524 symbols with names containing special characters or letters in the
4525 non-standard case, but it is usually a bad idea to create such symbols
4526 because in some implementations of Scheme they cannot be read as
4530 @deffn {Scheme Procedure} string-ci->symbol str
4531 @deffnx {C Function} scm_string_ci_to_symbol (str)
4532 Return the symbol whose name is @var{str}. If Guile is currently
4533 reading symbols case-insensitively, @var{str} is converted to lowercase
4534 before the returned symbol is looked up or created.
4537 The following examples illustrate Guile's detailed behaviour as regards
4538 the case-sensitivity of symbols:
4541 (read-enable 'case-insensitive) ; R5RS compliant behaviour
4543 (symbol->string 'flying-fish) @result{} "flying-fish"
4544 (symbol->string 'Martin) @result{} "martin"
4546 (string->symbol "Malvina")) @result{} "Malvina"
4548 (eq? 'mISSISSIppi 'mississippi) @result{} #t
4549 (string->symbol "mISSISSIppi") @result{} mISSISSIppi
4550 (eq? 'bitBlt (string->symbol "bitBlt")) @result{} #f
4552 (string->symbol (symbol->string 'LolliPop))) @result{} #t
4553 (string=? "K. Harper, M.D."
4555 (string->symbol "K. Harper, M.D."))) @result{} #t
4557 (read-disable 'case-insensitive) ; Guile default behaviour
4559 (symbol->string 'flying-fish) @result{} "flying-fish"
4560 (symbol->string 'Martin) @result{} "Martin"
4562 (string->symbol "Malvina")) @result{} "Malvina"
4564 (eq? 'mISSISSIppi 'mississippi) @result{} #f
4565 (string->symbol "mISSISSIppi") @result{} mISSISSIppi
4566 (eq? 'bitBlt (string->symbol "bitBlt")) @result{} #t
4568 (string->symbol (symbol->string 'LolliPop))) @result{} #t
4569 (string=? "K. Harper, M.D."
4571 (string->symbol "K. Harper, M.D."))) @result{} #t
4574 From C, there are lower level functions that construct a Scheme symbol
4575 from a C string in the current locale encoding.
4577 When you want to do more from C, you should convert between symbols
4578 and strings using @code{scm_symbol_to_string} and
4579 @code{scm_string_to_symbol} and work with the strings.
4581 @deffn {C Function} scm_from_locale_symbol (const char *name)
4582 @deffnx {C Function} scm_from_locale_symboln (const char *name, size_t len)
4583 Construct and return a Scheme symbol whose name is specified by
4584 @var{name}. For @code{scm_from_locale_symbol}, @var{name} must be null
4585 terminated; for @code{scm_from_locale_symboln} the length of @var{name} is
4586 specified explicitly by @var{len}.
4589 @deftypefn {C Function} SCM scm_take_locale_symbol (char *str)
4590 @deftypefnx {C Function} SCM scm_take_locale_symboln (char *str, size_t len)
4591 Like @code{scm_from_locale_symbol} and @code{scm_from_locale_symboln},
4592 respectively, but also frees @var{str} with @code{free} eventually.
4593 Thus, you can use this function when you would free @var{str} anyway
4594 immediately after creating the Scheme string. In certain cases, Guile
4595 can then use @var{str} directly as its internal representation.
4599 Finally, some applications, especially those that generate new Scheme
4600 code dynamically, need to generate symbols for use in the generated
4601 code. The @code{gensym} primitive meets this need:
4603 @deffn {Scheme Procedure} gensym [prefix]
4604 @deffnx {C Function} scm_gensym (prefix)
4605 Create a new symbol with a name constructed from a prefix and a counter
4606 value. The string @var{prefix} can be specified as an optional
4607 argument. Default prefix is @samp{@w{ g}}. The counter is increased by 1
4608 at each call. There is no provision for resetting the counter.
4611 The symbols generated by @code{gensym} are @emph{likely} to be unique,
4612 since their names begin with a space and it is only otherwise possible
4613 to generate such symbols if a programmer goes out of their way to do
4614 so. Uniqueness can be guaranteed by instead using uninterned symbols
4615 (@pxref{Symbol Uninterned}), though they can't be usefully written out
4620 @subsubsection Function Slots and Property Lists
4622 In traditional Lisp dialects, symbols are often understood as having
4623 three kinds of value at once:
4627 a @dfn{variable} value, which is used when the symbol appears in
4628 code in a variable reference context
4631 a @dfn{function} value, which is used when the symbol appears in
4632 code in a function name position (i.e. as the first element in an
4636 a @dfn{property list} value, which is used when the symbol is given as
4637 the first argument to Lisp's @code{put} or @code{get} functions.
4640 Although Scheme (as one of its simplifications with respect to Lisp)
4641 does away with the distinction between variable and function namespaces,
4642 Guile currently retains some elements of the traditional structure in
4643 case they turn out to be useful when implementing translators for other
4644 languages, in particular Emacs Lisp.
4646 Specifically, Guile symbols have two extra slots. for a symbol's
4647 property list, and for its ``function value.'' The following procedures
4648 are provided to access these slots.
4650 @deffn {Scheme Procedure} symbol-fref symbol
4651 @deffnx {C Function} scm_symbol_fref (symbol)
4652 Return the contents of @var{symbol}'s @dfn{function slot}.
4655 @deffn {Scheme Procedure} symbol-fset! symbol value
4656 @deffnx {C Function} scm_symbol_fset_x (symbol, value)
4657 Set the contents of @var{symbol}'s function slot to @var{value}.
4660 @deffn {Scheme Procedure} symbol-pref symbol
4661 @deffnx {C Function} scm_symbol_pref (symbol)
4662 Return the @dfn{property list} currently associated with @var{symbol}.
4665 @deffn {Scheme Procedure} symbol-pset! symbol value
4666 @deffnx {C Function} scm_symbol_pset_x (symbol, value)
4667 Set @var{symbol}'s property list to @var{value}.
4670 @deffn {Scheme Procedure} symbol-property sym prop
4671 From @var{sym}'s property list, return the value for property
4672 @var{prop}. The assumption is that @var{sym}'s property list is an
4673 association list whose keys are distinguished from each other using
4674 @code{equal?}; @var{prop} should be one of the keys in that list. If
4675 the property list has no entry for @var{prop}, @code{symbol-property}
4679 @deffn {Scheme Procedure} set-symbol-property! sym prop val
4680 In @var{sym}'s property list, set the value for property @var{prop} to
4681 @var{val}, or add a new entry for @var{prop}, with value @var{val}, if
4682 none already exists. For the structure of the property list, see
4683 @code{symbol-property}.
4686 @deffn {Scheme Procedure} symbol-property-remove! sym prop
4687 From @var{sym}'s property list, remove the entry for property
4688 @var{prop}, if there is one. For the structure of the property list,
4689 see @code{symbol-property}.
4692 Support for these extra slots may be removed in a future release, and it
4693 is probably better to avoid using them. For a more modern and Schemely
4694 approach to properties, see @ref{Object Properties}.
4697 @node Symbol Read Syntax
4698 @subsubsection Extended Read Syntax for Symbols
4700 The read syntax for a symbol is a sequence of letters, digits, and
4701 @dfn{extended alphabetic characters}, beginning with a character that
4702 cannot begin a number. In addition, the special cases of @code{+},
4703 @code{-}, and @code{...} are read as symbols even though numbers can
4704 begin with @code{+}, @code{-} or @code{.}.
4706 Extended alphabetic characters may be used within identifiers as if
4707 they were letters. The set of extended alphabetic characters is:
4710 ! $ % & * + - . / : < = > ? @@ ^ _ ~
4713 In addition to the standard read syntax defined above (which is taken
4714 from R5RS (@pxref{Formal syntax,,,r5rs,The Revised^5 Report on
4715 Scheme})), Guile provides an extended symbol read syntax that allows the
4716 inclusion of unusual characters such as space characters, newlines and
4717 parentheses. If (for whatever reason) you need to write a symbol
4718 containing characters not mentioned above, you can do so as follows.
4722 Begin the symbol with the characters @code{#@{},
4725 write the characters of the symbol and
4728 finish the symbol with the characters @code{@}#}.
4731 Here are a few examples of this form of read syntax. The first symbol
4732 needs to use extended syntax because it contains a space character, the
4733 second because it contains a line break, and the last because it looks
4745 Although Guile provides this extended read syntax for symbols,
4746 widespread usage of it is discouraged because it is not portable and not
4750 @node Symbol Uninterned
4751 @subsubsection Uninterned Symbols
4753 What makes symbols useful is that they are automatically kept unique.
4754 There are no two symbols that are distinct objects but have the same
4755 name. But of course, there is no rule without exception. In addition
4756 to the normal symbols that have been discussed up to now, you can also
4757 create special @dfn{uninterned} symbols that behave slightly
4760 To understand what is different about them and why they might be useful,
4761 we look at how normal symbols are actually kept unique.
4763 Whenever Guile wants to find the symbol with a specific name, for
4764 example during @code{read} or when executing @code{string->symbol}, it
4765 first looks into a table of all existing symbols to find out whether a
4766 symbol with the given name already exists. When this is the case, Guile
4767 just returns that symbol. When not, a new symbol with the name is
4768 created and entered into the table so that it can be found later.
4770 Sometimes you might want to create a symbol that is guaranteed `fresh',
4771 i.e. a symbol that did not exist previously. You might also want to
4772 somehow guarantee that no one else will ever unintentionally stumble
4773 across your symbol in the future. These properties of a symbol are
4774 often needed when generating code during macro expansion. When
4775 introducing new temporary variables, you want to guarantee that they
4776 don't conflict with variables in other people's code.
4778 The simplest way to arrange for this is to create a new symbol but
4779 not enter it into the global table of all symbols. That way, no one
4780 will ever get access to your symbol by chance. Symbols that are not in
4781 the table are called @dfn{uninterned}. Of course, symbols that
4782 @emph{are} in the table are called @dfn{interned}.
4784 You create new uninterned symbols with the function @code{make-symbol}.
4785 You can test whether a symbol is interned or not with
4786 @code{symbol-interned?}.
4788 Uninterned symbols break the rule that the name of a symbol uniquely
4789 identifies the symbol object. Because of this, they can not be written
4790 out and read back in like interned symbols. Currently, Guile has no
4791 support for reading uninterned symbols. Note that the function
4792 @code{gensym} does not return uninterned symbols for this reason.
4794 @deffn {Scheme Procedure} make-symbol name
4795 @deffnx {C Function} scm_make_symbol (name)
4796 Return a new uninterned symbol with the name @var{name}. The returned
4797 symbol is guaranteed to be unique and future calls to
4798 @code{string->symbol} will not return it.
4801 @deffn {Scheme Procedure} symbol-interned? symbol
4802 @deffnx {C Function} scm_symbol_interned_p (symbol)
4803 Return @code{#t} if @var{symbol} is interned, otherwise return
4810 (define foo-1 (string->symbol "foo"))
4811 (define foo-2 (string->symbol "foo"))
4812 (define foo-3 (make-symbol "foo"))
4813 (define foo-4 (make-symbol "foo"))
4817 ; Two interned symbols with the same name are the same object,
4821 ; but a call to make-symbol with the same name returns a
4826 ; A call to make-symbol always returns a new object, even for
4830 @result{} #<uninterned-symbol foo 8085290>
4831 ; Uninterned symbols print differently from interned symbols,
4835 ; but they are still symbols,
4837 (symbol-interned? foo-3)
4839 ; just not interned.
4844 @subsection Keywords
4847 Keywords are self-evaluating objects with a convenient read syntax that
4848 makes them easy to type.
4850 Guile's keyword support conforms to R5RS, and adds a (switchable) read
4851 syntax extension to permit keywords to begin with @code{:} as well as
4855 * Why Use Keywords?:: Motivation for keyword usage.
4856 * Coding With Keywords:: How to use keywords.
4857 * Keyword Read Syntax:: Read syntax for keywords.
4858 * Keyword Procedures:: Procedures for dealing with keywords.
4861 @node Why Use Keywords?
4862 @subsubsection Why Use Keywords?
4864 Keywords are useful in contexts where a program or procedure wants to be
4865 able to accept a large number of optional arguments without making its
4866 interface unmanageable.
4868 To illustrate this, consider a hypothetical @code{make-window}
4869 procedure, which creates a new window on the screen for drawing into
4870 using some graphical toolkit. There are many parameters that the caller
4871 might like to specify, but which could also be sensibly defaulted, for
4876 color depth -- Default: the color depth for the screen
4879 background color -- Default: white
4882 width -- Default: 600
4885 height -- Default: 400
4888 If @code{make-window} did not use keywords, the caller would have to
4889 pass in a value for each possible argument, remembering the correct
4890 argument order and using a special value to indicate the default value
4894 (make-window 'default ;; Color depth
4895 'default ;; Background color
4898 @dots{}) ;; More make-window arguments
4901 With keywords, on the other hand, defaulted arguments are omitted, and
4902 non-default arguments are clearly tagged by the appropriate keyword. As
4903 a result, the invocation becomes much clearer:
4906 (make-window #:width 800 #:height 100)
4909 On the other hand, for a simpler procedure with few arguments, the use
4910 of keywords would be a hindrance rather than a help. The primitive
4911 procedure @code{cons}, for example, would not be improved if it had to
4915 (cons #:car x #:cdr y)
4918 So the decision whether to use keywords or not is purely pragmatic: use
4919 them if they will clarify the procedure invocation at point of call.
4921 @node Coding With Keywords
4922 @subsubsection Coding With Keywords
4924 If a procedure wants to support keywords, it should take a rest argument
4925 and then use whatever means is convenient to extract keywords and their
4926 corresponding arguments from the contents of that rest argument.
4928 The following example illustrates the principle: the code for
4929 @code{make-window} uses a helper procedure called
4930 @code{get-keyword-value} to extract individual keyword arguments from
4934 (define (get-keyword-value args keyword default)
4935 (let ((kv (memq keyword args)))
4936 (if (and kv (>= (length kv) 2))
4940 (define (make-window . args)
4941 (let ((depth (get-keyword-value args #:depth screen-depth))
4942 (bg (get-keyword-value args #:bg "white"))
4943 (width (get-keyword-value args #:width 800))
4944 (height (get-keyword-value args #:height 100))
4949 But you don't need to write @code{get-keyword-value}. The @code{(ice-9
4950 optargs)} module provides a set of powerful macros that you can use to
4951 implement keyword-supporting procedures like this:
4954 (use-modules (ice-9 optargs))
4956 (define (make-window . args)
4957 (let-keywords args #f ((depth screen-depth)
4965 Or, even more economically, like this:
4968 (use-modules (ice-9 optargs))
4970 (define* (make-window #:key (depth screen-depth)
4977 For further details on @code{let-keywords}, @code{define*} and other
4978 facilities provided by the @code{(ice-9 optargs)} module, see
4979 @ref{Optional Arguments}.
4982 @node Keyword Read Syntax
4983 @subsubsection Keyword Read Syntax
4985 Guile, by default, only recognizes a keyword syntax that is compatible
4986 with R5RS. A token of the form @code{#:NAME}, where @code{NAME} has the
4987 same syntax as a Scheme symbol (@pxref{Symbol Read Syntax}), is the
4988 external representation of the keyword named @code{NAME}. Keyword
4989 objects print using this syntax as well, so values containing keyword
4990 objects can be read back into Guile. When used in an expression,
4991 keywords are self-quoting objects.
4993 If the @code{keyword} read option is set to @code{'prefix}, Guile also
4994 recognizes the alternative read syntax @code{:NAME}. Otherwise, tokens
4995 of the form @code{:NAME} are read as symbols, as required by R5RS.
4997 To enable and disable the alternative non-R5RS keyword syntax, you use
4998 the @code{read-set!} procedure documented in @ref{User level options
4999 interfaces} and @ref{Reader options}.
5002 (read-set! keywords 'prefix)
5012 (read-set! keywords #f)
5020 ERROR: In expression :type:
5021 ERROR: Unbound variable: :type
5022 ABORT: (unbound-variable)
5025 @node Keyword Procedures
5026 @subsubsection Keyword Procedures
5028 @deffn {Scheme Procedure} keyword? obj
5029 @deffnx {C Function} scm_keyword_p (obj)
5030 Return @code{#t} if the argument @var{obj} is a keyword, else
5034 @deffn {Scheme Procedure} keyword->symbol keyword
5035 @deffnx {C Function} scm_keyword_to_symbol (keyword)
5036 Return the symbol with the same name as @var{keyword}.
5039 @deffn {Scheme Procedure} symbol->keyword symbol
5040 @deffnx {C Function} scm_symbol_to_keyword (symbol)
5041 Return the keyword with the same name as @var{symbol}.
5044 @deftypefn {C Function} int scm_is_keyword (SCM obj)
5045 Equivalent to @code{scm_is_true (scm_keyword_p (@var{obj}))}.
5048 @deftypefn {C Function} SCM scm_from_locale_keyword (const char *str)
5049 @deftypefnx {C Function} SCM scm_from_locale_keywordn (const char *str, size_t len)
5050 Equivalent to @code{scm_symbol_to_keyword (scm_from_locale_symbol
5051 (@var{str}))} and @code{scm_symbol_to_keyword (scm_from_locale_symboln
5052 (@var{str}, @var{len}))}, respectively.
5056 @subsection ``Functionality-Centric'' Data Types
5058 Procedures and macros are documented in their own chapter: see
5059 @ref{Procedures and Macros}.
5061 Variable objects are documented as part of the description of Guile's
5062 module system: see @ref{Variables}.
5064 Asyncs, dynamic roots and fluids are described in the chapter on
5065 scheduling: see @ref{Scheduling}.
5067 Hooks are documented in the chapter on general utility functions: see
5070 Ports are described in the chapter on I/O: see @ref{Input and Output}.
5074 @c TeX-master: "guile.texi"