2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2012
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
10 In this chapter, we introduce the basic concepts that underpin the
11 elegance and power of the Scheme language.
13 Readers who already possess a background knowledge of Scheme may happily
14 skip this chapter. For the reader who is new to the language, however,
15 the following discussions on data, procedures, expressions and closure
16 are designed to provide a minimum level of Scheme understanding that is
17 more or less assumed by the chapters that follow.
19 The style of this introductory material aims about halfway between the terse
20 precision of R5RS and the discursiveness of existing Scheme tutorials. For
21 pointers to useful Scheme resources on the web, please see @ref{Further
25 * About Data:: Latent typing, types, values and variables.
26 * About Procedures:: The representation and use of procedures.
27 * About Expressions:: All kinds of expressions and their meaning.
28 * About Closure:: Closure, scoping and environments.
29 * Further Reading:: Where to find out more about Scheme.
34 @section Data Types, Values and Variables
36 This section discusses the representation of data types and values, what
37 it means for Scheme to be a @dfn{latently typed} language, and the role
38 of variables. We conclude by introducing the Scheme syntaxes for
39 defining a new variable, and for changing the value of an existing
43 * Latent Typing:: Scheme as a "latently typed" language.
44 * Values and Variables:: About data types, values and variables.
45 * Definition:: Defining variables and setting their values.
50 @subsection Latent Typing
52 The term @dfn{latent typing} is used to describe a computer language,
53 such as Scheme, for which you cannot, @emph{in general}, simply look at
54 a program's source code and determine what type of data will be
55 associated with a particular variable, or with the result of a
56 particular expression.
58 Sometimes, of course, you @emph{can} tell from the code what the type of
59 an expression will be. If you have a line in your program that sets the
60 variable @code{x} to the numeric value 1, you can be certain that,
61 immediately after that line has executed (and in the absence of multiple
62 threads), @code{x} has the numeric value 1. Or if you write a procedure
63 that is designed to concatenate two strings, it is likely that the rest
64 of your application will always invoke this procedure with two string
65 parameters, and quite probable that the procedure would go wrong in some
66 way if it was ever invoked with parameters that were not both strings.
68 Nevertheless, the point is that there is nothing in Scheme which
69 requires the procedure parameters always to be strings, or @code{x}
70 always to hold a numeric value, and there is no way of declaring in your
71 program that such constraints should always be obeyed. In the same
72 vein, there is no way to declare the expected type of a procedure's
75 Instead, the types of variables and expressions are only known -- in
76 general -- at run time. If you @emph{need} to check at some point that
77 a value has the expected type, Scheme provides run time procedures that
78 you can invoke to do so. But equally, it can be perfectly valid for two
79 separate invocations of the same procedure to specify arguments with
80 different types, and to return values with different types.
82 The next subsection explains what this means in practice, for the ways
83 that Scheme programs use data types, values and variables.
86 @node Values and Variables
87 @subsection Values and Variables
89 Scheme provides many data types that you can use to represent your data.
90 Primitive types include characters, strings, numbers and procedures.
91 Compound types, which allow a group of primitive and compound values to
92 be stored together, include lists, pairs, vectors and multi-dimensional
93 arrays. In addition, Guile allows applications to define their own data
94 types, with the same status as the built-in standard Scheme types.
96 As a Scheme program runs, values of all types pop in and out of
97 existence. Sometimes values are stored in variables, but more commonly
98 they pass seamlessly from being the result of one computation to being
99 one of the parameters for the next.
101 Consider an example. A string value is created because the interpreter
102 reads in a literal string from your program's source code. Then a
103 numeric value is created as the result of calculating the length of the
104 string. A second numeric value is created by doubling the calculated
105 length. Finally the program creates a list with two elements -- the
106 doubled length and the original string itself -- and stores this list in
109 All of the values involved here -- in fact, all values in Scheme --
110 carry their type with them. In other words, every value ``knows,'' at
111 runtime, what kind of value it is. A number, a string, a list,
114 A variable, on the other hand, has no fixed type. A variable --
115 @code{x}, say -- is simply the name of a location -- a box -- in which
116 you can store any kind of Scheme value. So the same variable in a
117 program may hold a number at one moment, a list of procedures the next,
118 and later a pair of strings. The ``type'' of a variable -- insofar as
119 the idea is meaningful at all -- is simply the type of whatever value
120 the variable happens to be storing at a particular moment.
124 @subsection Defining and Setting Variables
126 To define a new variable, you use Scheme's @code{define} syntax like
130 (define @var{variable-name} @var{value})
133 This makes a new variable called @var{variable-name} and stores
134 @var{value} in it as the variable's initial value. For example:
137 ;; Make a variable `x' with initial numeric value 1.
140 ;; Make a variable `organization' with an initial string value.
141 (define organization "Free Software Foundation")
144 (In Scheme, a semicolon marks the beginning of a comment that continues
145 until the end of the line. So the lines beginning @code{;;} are
148 Changing the value of an already existing variable is very similar,
149 except that @code{define} is replaced by the Scheme syntax @code{set!},
153 (set! @var{variable-name} @var{new-value})
156 Remember that variables do not have fixed types, so @var{new-value} may
157 have a completely different type from whatever was previously stored in
158 the location named by @var{variable-name}. Both of the following
159 examples are therefore correct.
162 ;; Change the value of `x' to 5.
165 ;; Change the value of `organization' to the FSF's street number.
166 (set! organization 545)
169 In these examples, @var{value} and @var{new-value} are literal numeric
170 or string values. In general, however, @var{value} and @var{new-value}
171 can be any Scheme expression. Even though we have not yet covered the
172 forms that Scheme expressions can take (@pxref{About Expressions}), you
173 can probably guess what the following @code{set!} example does@dots{}
179 (Note: this is not a complete description of @code{define} and
180 @code{set!}, because we need to introduce some other aspects of Scheme
181 before the missing pieces can be filled in. If, however, you are
182 already familiar with the structure of Scheme, you may like to read
183 about those missing pieces immediately by jumping ahead to the following
188 @ref{Lambda Alternatives}, to read about an alternative form of the
189 @code{define} syntax that can be used when defining new procedures.
192 @ref{Procedures with Setters}, to read about an alternative form of the
193 @code{set!} syntax that helps with changing a single value in the depths
194 of a compound data structure.)
197 @xref{Internal Definitions}, to read about using @code{define} other
198 than at top level in a Scheme program, including a discussion of when it
199 works to use @code{define} rather than @code{set!} to change the value
200 of an existing variable.
204 @node About Procedures
205 @section The Representation and Use of Procedures
207 This section introduces the basics of using and creating Scheme
208 procedures. It discusses the representation of procedures as just
209 another kind of Scheme value, and shows how procedure invocation
210 expressions are constructed. We then explain how @code{lambda} is used
211 to create new procedures, and conclude by presenting the various
212 shorthand forms of @code{define} that can be used instead of writing an
213 explicit @code{lambda} expression.
216 * Procedures as Values:: Procedures are values like everything else.
217 * Simple Invocation:: How to write a simple procedure invocation.
218 * Creating a Procedure:: How to create your own procedures.
219 * Lambda Alternatives:: Other ways of writing procedure definitions.
223 @node Procedures as Values
224 @subsection Procedures as Values
226 One of the great simplifications of Scheme is that a procedure is just
227 another type of value, and that procedure values can be passed around
228 and stored in variables in exactly the same way as, for example, strings
229 and lists. When we talk about a built-in standard Scheme procedure such
230 as @code{open-input-file}, what we actually mean is that there is a
231 pre-defined top level variable called @code{open-input-file}, whose
232 value is a procedure that implements what R5RS says that
233 @code{open-input-file} should do.
235 Note that this is quite different from many dialects of Lisp ---
236 including Emacs Lisp --- in which a program can use the same name with
237 two quite separate meanings: one meaning identifies a Lisp function,
238 while the other meaning identifies a Lisp variable, whose value need
239 have nothing to do with the function that is associated with the first
240 meaning. In these dialects, functions and variables are said to live in
241 different @dfn{namespaces}.
243 In Scheme, on the other hand, all names belong to a single unified
244 namespace, and the variables that these names identify can hold any kind
245 of Scheme value, including procedure values.
247 One consequence of the ``procedures as values'' idea is that, if you
248 don't happen to like the standard name for a Scheme procedure, you can
251 For example, @code{call-with-current-continuation} is a very important
252 standard Scheme procedure, but it also has a very long name! So, many
253 programmers use the following definition to assign the same procedure
254 value to the more convenient name @code{call/cc}.
257 (define call/cc call-with-current-continuation)
260 Let's understand exactly how this works. The definition creates a new
261 variable @code{call/cc}, and then sets its value to the value of the
262 variable @code{call-with-current-continuation}; the latter value is a
263 procedure that implements the behaviour that R5RS specifies under the
264 name ``call-with-current-continuation''. So @code{call/cc} ends up
265 holding this value as well.
267 Now that @code{call/cc} holds the required procedure value, you could
268 choose to use @code{call-with-current-continuation} for a completely
269 different purpose, or just change its value so that you will get an
270 error if you accidentally use @code{call-with-current-continuation} as a
271 procedure in your program rather than @code{call/cc}. For example:
274 (set! call-with-current-continuation "Not a procedure any more!")
277 Or you could just leave @code{call-with-current-continuation} as it was.
278 It's perfectly fine for more than one variable to hold the same
282 @node Simple Invocation
283 @subsection Simple Procedure Invocation
285 A procedure invocation in Scheme is written like this:
288 (@var{procedure} [@var{arg1} [@var{arg2} @dots{}]])
291 In this expression, @var{procedure} can be any Scheme expression whose
292 value is a procedure. Most commonly, however, @var{procedure} is simply
293 the name of a variable whose value is a procedure.
295 For example, @code{string-append} is a standard Scheme procedure whose
296 behaviour is to concatenate together all the arguments, which are
297 expected to be strings, that it is given. So the expression
300 (string-append "/home" "/" "andrew")
304 is a procedure invocation whose result is the string value
305 @code{"/home/andrew"}.
307 Similarly, @code{string-length} is a standard Scheme procedure that
308 returns the length of a single string argument, so
311 (string-length "abc")
315 is a procedure invocation whose result is the numeric value 3.
317 Each of the parameters in a procedure invocation can itself be any
318 Scheme expression. Since a procedure invocation is itself a type of
319 expression, we can put these two examples together to get
322 (string-length (string-append "/home" "/" "andrew"))
326 --- a procedure invocation whose result is the numeric value 12.
328 (You may be wondering what happens if the two examples are combined the
329 other way round. If we do this, we can make a procedure invocation
330 expression that is @emph{syntactically} correct:
333 (string-append "/home" (string-length "abc"))
337 but when this expression is executed, it will cause an error, because
338 the result of @code{(string-length "abc")} is a numeric value, and
339 @code{string-append} is not designed to accept a numeric value as one of
343 @node Creating a Procedure
344 @subsection Creating and Using a New Procedure
346 Scheme has lots of standard procedures, and Guile provides all of these
347 via predefined top level variables. All of these standard procedures
348 are documented in the later chapters of this reference manual.
350 Before very long, though, you will want to create new procedures that
351 encapsulate aspects of your own applications' functionality. To do
352 this, you can use the famous @code{lambda} syntax.
354 For example, the value of the following Scheme expression
357 (lambda (name address) @var{expression} @dots{})
361 is a newly created procedure that takes two arguments:
362 @code{name} and @code{address}. The behaviour of the
363 new procedure is determined by the sequence of @var{expression}s in the
364 @dfn{body} of the procedure definition. (Typically, these
365 @var{expression}s would use the arguments in some way, or else there
366 wouldn't be any point in giving them to the procedure.) When invoked,
367 the new procedure returns a value that is the value of the last
368 @var{expression} in the procedure body.
370 To make things more concrete, let's suppose that the two arguments are
371 both strings, and that the purpose of this procedure is to form a
372 combined string that includes these arguments. Then the full lambda
373 expression might look like this:
376 (lambda (name address)
377 (string-append "Name=" name ":Address=" address))
380 We noted in the previous subsection that the @var{procedure} part of a
381 procedure invocation expression can be any Scheme expression whose value
382 is a procedure. But that's exactly what a lambda expression is! So we
383 can use a lambda expression directly in a procedure invocation, like
387 ((lambda (name address)
388 (string-append "Name=" name ":Address=" address))
394 This is a valid procedure invocation expression, and its result is the
398 "Name=FSF:Address=Cambridge"
401 It is more common, though, to store the procedure value in a variable ---
404 (define make-combined-string
405 (lambda (name address)
406 (string-append "Name=" name ":Address=" address)))
410 --- and then to use the variable name in the procedure invocation:
413 (make-combined-string "FSF" "Cambridge")
417 Which has exactly the same result.
419 It's important to note that procedures created using @code{lambda} have
420 exactly the same status as the standard built in Scheme procedures, and
421 can be invoked, passed around, and stored in variables in exactly the
425 @node Lambda Alternatives
426 @subsection Lambda Alternatives
428 Since it is so common in Scheme programs to want to create a procedure
429 and then store it in a variable, there is an alternative form of the
430 @code{define} syntax that allows you to do just that.
432 A @code{define} expression of the form
435 (define (@var{name} [@var{arg1} [@var{arg2} @dots{}]])
436 @var{expression} @dots{})
440 is exactly equivalent to the longer form
444 (lambda ([@var{arg1} [@var{arg2} @dots{}]])
445 @var{expression} @dots{}))
448 So, for example, the definition of @code{make-combined-string} in the
449 previous subsection could equally be written:
452 (define (make-combined-string name address)
453 (string-append "Name=" name ":Address=" address))
456 This kind of procedure definition creates a procedure that requires
457 exactly the expected number of arguments. There are two further forms
458 of the @code{lambda} expression, which create a procedure that can
459 accept a variable number of arguments:
462 (lambda (@var{arg1} @dots{} . @var{args}) @var{expression} @dots{})
464 (lambda @var{args} @var{expression} @dots{})
468 The corresponding forms of the alternative @code{define} syntax are:
471 (define (@var{name} @var{arg1} @dots{} . @var{args}) @var{expression} @dots{})
473 (define (@var{name} . @var{args}) @var{expression} @dots{})
477 For details on how these forms work, see @xref{Lambda}.
479 Prior to Guile 2.0, Guile provided an extension to @code{define} syntax
480 that allowed you to nest the previous extension up to an arbitrary
481 depth. These are no longer provided by default, and instead have been
482 moved to @ref{Curried Definitions}
484 (It could be argued that the alternative @code{define} forms are rather
485 confusing, especially for newcomers to the Scheme language, as they hide
486 both the role of @code{lambda} and the fact that procedures are values
487 that are stored in variables in the some way as any other kind of value.
488 On the other hand, they are very convenient, and they are also a good
489 example of another of Scheme's powerful features: the ability to specify
490 arbitrary syntactic transformations at run time, which can be applied to
491 subsequently read input.)
494 @node About Expressions
495 @section Expressions and Evaluation
497 So far, we have met expressions that @emph{do} things, such as the
498 @code{define} expressions that create and initialize new variables, and
499 we have also talked about expressions that have @emph{values}, for
500 example the value of the procedure invocation expression:
503 (string-append "/home" "/" "andrew")
507 but we haven't yet been precise about what causes an expression like
508 this procedure invocation to be reduced to its ``value'', or how the
509 processing of such expressions relates to the execution of a Scheme
512 This section clarifies what we mean by an expression's value, by
513 introducing the idea of @dfn{evaluation}. It discusses the side effects
514 that evaluation can have, explains how each of the various types of
515 Scheme expression is evaluated, and describes the behaviour and use of
516 the Guile REPL as a mechanism for exploring evaluation. The section
517 concludes with a very brief summary of Scheme's common syntactic
521 * Evaluating:: How a Scheme program is executed.
522 * Tail Calls:: Space-safe recursion.
523 * The REPL:: Interacting with the Guile interpreter.
524 * Syntax Summary:: Common syntactic expressions -- in brief.
529 @subsection Evaluating Expressions and Executing Programs
531 In Scheme, the process of executing an expression is known as
532 @dfn{evaluation}. Evaluation has two kinds of result:
536 the @dfn{value} of the evaluated expression
539 the @dfn{side effects} of the evaluation, which consist of any effects of
540 evaluating the expression that are not represented by the value.
543 Of the expressions that we have met so far, @code{define} and
544 @code{set!} expressions have side effects --- the creation or
545 modification of a variable --- but no value; @code{lambda} expressions
546 have values --- the newly constructed procedures --- but no side
547 effects; and procedure invocation expressions, in general, have either
548 values, or side effects, or both.
550 It is tempting to try to define more intuitively what we mean by
551 ``value'' and ``side effects'', and what the difference between them is.
552 In general, though, this is extremely difficult. It is also
553 unnecessary; instead, we can quite happily define the behaviour of a
554 Scheme program by specifying how Scheme executes a program as a whole,
555 and then by describing the value and side effects of evaluation for each
556 type of expression individually.
559 So, some@footnote{These definitions are approximate. For the whole
560 and detailed truth, see @ref{Formal syntax and semantics,R5RS
561 syntax,,r5rs,The Revised(5) Report on the Algorithmic Language
562 Scheme}.} definitions@dots{}
567 A Scheme program consists of a sequence of expressions.
570 A Scheme interpreter executes the program by evaluating these
571 expressions in order, one by one.
578 a piece of literal data, such as a number @code{2.3} or a string
579 @code{"Hello world!"}
583 a procedure invocation expression
585 one of Scheme's special syntactic expressions.
590 The following subsections describe how each of these types of expression
594 * Eval Literal:: Evaluating literal data.
595 * Eval Variable:: Evaluating variable references.
596 * Eval Procedure:: Evaluating procedure invocation expressions.
597 * Eval Special:: Evaluating special syntactic expressions.
601 @subsubsection Evaluating Literal Data
603 When a literal data expression is evaluated, the value of the expression
604 is simply the value that the expression describes. The evaluation of a
605 literal data expression has no side effects.
612 the value of the expression @code{"abc"} is the string value
616 the value of the expression @code{3+4i} is the complex number 3 + 4i
619 the value of the expression @code{#(1 2 3)} is a three-element vector
620 containing the numeric values 1, 2 and 3.
623 For any data type which can be expressed literally like this, the syntax
624 of the literal data expression for that data type --- in other words,
625 what you need to write in your code to indicate a literal value of that
626 type --- is known as the data type's @dfn{read syntax}. This manual
627 specifies the read syntax for each such data type in the section that
628 describes that data type.
630 Some data types do not have a read syntax. Procedures, for example,
631 cannot be expressed as literal data; they must be created using a
632 @code{lambda} expression (@pxref{Creating a Procedure}) or implicitly
633 using the shorthand form of @code{define} (@pxref{Lambda Alternatives}).
637 @subsubsection Evaluating a Variable Reference
639 When an expression that consists simply of a variable name is evaluated,
640 the value of the expression is the value of the named variable. The
641 evaluation of a variable reference expression has no side effects.
646 (define key "Paul Evans")
650 the value of the expression @code{key} is the string value @code{"Paul
651 Evans"}. If @var{key} is then modified by
658 the value of the expression @code{key} is the numeric value 3.74.
660 If there is no variable with the specified name, evaluation of the
661 variable reference expression signals an error.
665 @subsubsection Evaluating a Procedure Invocation Expression
667 This is where evaluation starts getting interesting! As already noted,
668 a procedure invocation expression has the form
671 (@var{procedure} [@var{arg1} [@var{arg2} @dots{}]])
675 where @var{procedure} must be an expression whose value, when evaluated,
678 The evaluation of a procedure invocation expression like this proceeds
683 evaluating individually the expressions @var{procedure}, @var{arg1},
684 @var{arg2}, and so on
687 calling the procedure that is the value of the @var{procedure}
688 expression with the list of values obtained from the evaluations of
689 @var{arg1}, @var{arg2} etc. as its parameters.
692 For a procedure defined in Scheme, ``calling the procedure with the list
693 of values as its parameters'' means binding the values to the
694 procedure's formal parameters and then evaluating the sequence of
695 expressions that make up the body of the procedure definition. The
696 value of the procedure invocation expression is the value of the last
697 evaluated expression in the procedure body. The side effects of calling
698 the procedure are the combination of the side effects of the sequence of
699 evaluations of expressions in the procedure body.
701 For a built-in procedure, the value and side-effects of calling the
702 procedure are best described by that procedure's documentation.
704 Note that the complete side effects of evaluating a procedure invocation
705 expression consist not only of the side effects of the procedure call,
706 but also of any side effects of the preceding evaluation of the
707 expressions @var{procedure}, @var{arg1}, @var{arg2}, and so on.
709 To illustrate this, let's look again at the procedure invocation
713 (string-length (string-append "/home" "/" "andrew"))
716 In the outermost expression, @var{procedure} is @code{string-length} and
717 @var{arg1} is @code{(string-append "/home" "/" "andrew")}.
721 Evaluation of @code{string-length}, which is a variable, gives a
722 procedure value that implements the expected behaviour for
726 Evaluation of @code{(string-append "/home" "/" "andrew")}, which is
727 another procedure invocation expression, means evaluating each of
731 @code{string-append}, which gives a procedure value that implements the
732 expected behaviour for ``string-append''
735 @code{"/home"}, which gives the string value @code{"/home"}
738 @code{"/"}, which gives the string value @code{"/"}
741 @code{"andrew"}, which gives the string value @code{"andrew"}
744 and then invoking the procedure value with this list of string values as
745 its arguments. The resulting value is a single string value that is the
746 concatenation of all the arguments, namely @code{"/home/andrew"}.
749 In the evaluation of the outermost expression, the interpreter can now
750 invoke the procedure value obtained from @var{procedure} with the value
751 obtained from @var{arg1} as its arguments. The resulting value is a
752 numeric value that is the length of the argument string, which is 12.
756 @subsubsection Evaluating Special Syntactic Expressions
758 When a procedure invocation expression is evaluated, the procedure and
759 @emph{all} the argument expressions must be evaluated before the
760 procedure can be invoked. Special syntactic expressions are special
761 because they are able to manipulate their arguments in an unevaluated
762 form, and can choose whether to evaluate any or all of the argument
765 Why is this needed? Consider a program fragment that asks the user
766 whether or not to delete a file, and then deletes the file if the user
770 (if (string=? (read-answer "Should I delete this file?")
775 If the outermost @code{(if @dots{})} expression here was a procedure
776 invocation expression, the expression @code{(delete-file file)}, whose
777 side effect is to actually delete a file, would already have been
778 evaluated before the @code{if} procedure even got invoked! Clearly this
779 is no use --- the whole point of an @code{if} expression is that the
780 @dfn{consequent} expression is only evaluated if the condition of the
781 @code{if} expression is ``true''.
783 Therefore @code{if} must be special syntax, not a procedure. Other
784 special syntaxes that we have already met are @code{define}, @code{set!}
785 and @code{lambda}. @code{define} and @code{set!} are syntax because
786 they need to know the variable @emph{name} that is given as the first
787 argument in a @code{define} or @code{set!} expression, not that
788 variable's value. @code{lambda} is syntax because it does not
789 immediately evaluate the expressions that define the procedure body;
790 instead it creates a procedure object that incorporates these
791 expressions so that they can be evaluated in the future, when that
792 procedure is invoked.
794 The rules for evaluating each special syntactic expression are specified
795 individually for each special syntax. For a summary of standard special
796 syntax, see @xref{Syntax Summary}.
800 @subsection Tail calls
804 Scheme is ``properly tail recursive'', meaning that tail calls or
805 recursions from certain contexts do not consume stack space or other
806 resources and can therefore be used on arbitrarily large data or for
807 an arbitrarily long calculation. Consider for example,
823 @code{foo} prints numbers infinitely, starting from the given @var{n}.
824 It's implemented by printing @var{n} then recursing to itself to print
825 @math{@var{n}+1} and so on. This recursion is a tail call, it's the
826 last thing done, and in Scheme such tail calls can be made without
829 Or consider a case where a value is returned, a version of the SRFI-1
830 @code{last} function (@pxref{SRFI-1 Selectors}) returning the last
834 (define (my-last lst)
835 (if (null? (cdr lst))
837 (my-last (cdr lst))))
839 (my-last '(1 2 3)) @result{} 3
842 If the list has more than one element, @code{my-last} applies itself
843 to the @code{cdr}. This recursion is a tail call, there's no code
844 after it, and the return value is the return value from that call. In
845 Scheme this can be used on an arbitrarily long list argument.
848 A proper tail call is only available from certain contexts, namely the
849 following special form positions,
853 @code{and} --- last expression
856 @code{begin} --- last expression
859 @code{case} --- last expression in each clause
862 @code{cond} --- last expression in each clause, and the call to a
863 @code{=>} procedure is a tail call
866 @code{do} --- last result expression
869 @code{if} --- ``true'' and ``false'' leg expressions
872 @code{lambda} --- last expression in body
875 @code{let}, @code{let*}, @code{letrec}, @code{let-syntax},
876 @code{letrec-syntax} --- last expression in body
879 @code{or} --- last expression
883 The following core functions make tail calls,
887 @code{apply} --- tail call to given procedure
890 @code{call-with-current-continuation} --- tail call to the procedure
891 receiving the new continuation
894 @code{call-with-values} --- tail call to the values-receiving
898 @code{eval} --- tail call to evaluate the form
901 @code{string-any}, @code{string-every} --- tail call to predicate on
902 the last character (if that point is reached)
906 The above are just core functions and special forms. Tail calls in
907 other modules are described with the relevant documentation, for
908 example SRFI-1 @code{any} and @code{every} (@pxref{SRFI-1 Searching}).
910 It will be noted there are a lot of places which could potentially be
911 tail calls, for instance the last call in a @code{for-each}, but only
912 those explicitly described are guaranteed.
916 @subsection Using the Guile REPL
918 If you start Guile without specifying a particular program for it to
919 execute, Guile enters its standard Read Evaluate Print Loop --- or
920 @dfn{REPL} for short. In this mode, Guile repeatedly reads in the next
921 Scheme expression that the user types, evaluates it, and prints the
924 The REPL is a useful mechanism for exploring the evaluation behaviour
925 described in the previous subsection. If you type @code{string-append},
926 for example, the REPL replies @code{#<primitive-procedure
927 string-append>}, illustrating the relationship between the variable
928 @code{string-append} and the procedure value stored in that variable.
930 In this manual, the notation @result{} is used to mean ``evaluates
931 to''. Wherever you see an example of the form
940 feel free to try it out yourself by typing @var{expression} into the
941 REPL and checking that it gives the expected @var{result}.
945 @subsection Summary of Common Syntax
947 This subsection lists the most commonly used Scheme syntactic
948 expressions, simply so that you will recognize common special syntax
949 when you see it. For a full description of each of these syntaxes,
950 follow the appropriate reference.
952 @code{lambda} (@pxref{Lambda}) is used to construct procedure objects.
954 @code{define} (@pxref{Top Level}) is used to create a new variable and
955 set its initial value.
957 @code{set!} (@pxref{Top Level}) is used to modify an existing variable's
960 @code{let}, @code{let*} and @code{letrec} (@pxref{Local Bindings})
961 create an inner lexical environment for the evaluation of a sequence of
962 expressions, in which a specified set of local variables is bound to the
963 values of a corresponding set of expressions. For an introduction to
964 environments, see @xref{About Closure}.
966 @code{begin} (@pxref{begin}) executes a sequence of expressions in order
967 and returns the value of the last expression. Note that this is not the
968 same as a procedure which returns its last argument, because the
969 evaluation of a procedure invocation expression does not guarantee to
970 evaluate the arguments in order.
972 @code{if} and @code{cond} (@pxref{Conditionals}) provide conditional
973 evaluation of argument expressions depending on whether one or more
974 conditions evaluate to ``true'' or ``false''.
976 @code{case} (@pxref{Conditionals}) provides conditional evaluation of
977 argument expressions depending on whether a variable has one of a
978 specified group of values.
980 @code{and} (@pxref{and or}) executes a sequence of expressions in order
981 until either there are no expressions left, or one of them evaluates to
984 @code{or} (@pxref{and or}) executes a sequence of expressions in order
985 until either there are no expressions left, or one of them evaluates to
990 @section The Concept of Closure
994 The concept of @dfn{closure} is the idea that a lambda expression
995 ``captures'' the variable bindings that are in lexical scope at the
996 point where the lambda expression occurs. The procedure created by the
997 lambda expression can refer to and mutate the captured bindings, and the
998 values of those bindings persist between procedure calls.
1000 This section explains and explores the various parts of this idea in
1004 * About Environments:: Names, locations, values and environments.
1005 * Local Variables:: Local variables and local environments.
1006 * Chaining:: Environment chaining.
1007 * Lexical Scope:: The meaning of lexical scoping.
1008 * Closure:: Explaining the concept of closure.
1009 * Serial Number:: Example 1: a serial number generator.
1010 * Shared Variable:: Example 2: a shared persistent variable.
1011 * Callback Closure:: Example 3: the callback closure problem.
1012 * OO Closure:: Example 4: object orientation.
1015 @node About Environments
1016 @subsection Names, Locations, Values and Environments
1021 @cindex top level environment
1022 @cindex environment, top level
1024 We said earlier that a variable name in a Scheme program is associated
1025 with a location in which any kind of Scheme value may be stored.
1026 (Incidentally, the term ``vcell'' is often used in Lisp and Scheme
1027 circles as an alternative to ``location''.) Thus part of what we mean
1028 when we talk about ``creating a variable'' is in fact establishing an
1029 association between a name, or identifier, that is used by the Scheme
1030 program code, and the variable location to which that name refers.
1031 Although the value that is stored in that location may change, the
1032 location to which a given name refers is always the same.
1034 We can illustrate this by breaking down the operation of the
1035 @code{define} syntax into three parts: @code{define}
1039 creates a new location
1042 establishes an association between that location and the name specified
1043 as the first argument of the @code{define} expression
1046 stores in that location the value obtained by evaluating the second
1047 argument of the @code{define} expression.
1050 A collection of associations between names and locations is called an
1051 @dfn{environment}. When you create a top level variable in a program
1052 using @code{define}, the name-location association for that variable is
1053 added to the ``top level'' environment. The ``top level'' environment
1054 also includes name-location associations for all the procedures that are
1055 supplied by standard Scheme.
1057 It is also possible to create environments other than the top level one,
1058 and to create variable bindings, or name-location associations, in those
1059 environments. This ability is a key ingredient in the concept of
1060 closure; the next subsection shows how it is done.
1063 @node Local Variables
1064 @subsection Local Variables and Environments
1066 @cindex local variable
1067 @cindex variable, local
1068 @cindex local environment
1069 @cindex environment, local
1071 We have seen how to create top level variables using the @code{define}
1072 syntax (@pxref{Definition}). It is often useful to create variables
1073 that are more limited in their scope, typically as part of a procedure
1074 body. In Scheme, this is done using the @code{let} syntax, or one of
1075 its modified forms @code{let*} and @code{letrec}. These syntaxes are
1076 described in full later in the manual (@pxref{Local Bindings}). Here
1077 our purpose is to illustrate their use just enough that we can see how
1078 local variables work.
1080 For example, the following code uses a local variable @code{s} to
1081 simplify the computation of the area of a triangle given the lengths of
1090 (let ((s (/ (+ a b c) 2)))
1091 (sqrt (* s (- s a) (- s b) (- s c)))))
1094 The effect of the @code{let} expression is to create a new environment
1095 and, within this environment, an association between the name @code{s}
1096 and a new location whose initial value is obtained by evaluating
1097 @code{(/ (+ a b c) 2)}. The expressions in the body of the @code{let},
1098 namely @code{(sqrt (* s (- s a) (- s b) (- s c)))}, are then evaluated
1099 in the context of the new environment, and the value of the last
1100 expression evaluated becomes the value of the whole @code{let}
1101 expression, and therefore the value of the variable @code{area}.
1105 @subsection Environment Chaining
1107 @cindex shadowing an imported variable binding
1108 @cindex chaining environments
1110 In the example of the previous subsection, we glossed over an important
1111 point. The body of the @code{let} expression in that example refers not
1112 only to the local variable @code{s}, but also to the top level variables
1113 @code{a}, @code{b}, @code{c} and @code{sqrt}. (@code{sqrt} is the
1114 standard Scheme procedure for calculating a square root.) If the body
1115 of the @code{let} expression is evaluated in the context of the
1116 @emph{local} @code{let} environment, how does the evaluation get at the
1117 values of these top level variables?
1119 The answer is that the local environment created by a @code{let}
1120 expression automatically has a reference to its containing environment
1121 --- in this case the top level environment --- and that the Scheme
1122 interpreter automatically looks for a variable binding in the containing
1123 environment if it doesn't find one in the local environment. More
1124 generally, every environment except for the top level one has a
1125 reference to its containing environment, and the interpreter keeps
1126 searching back up the chain of environments --- from most local to top
1127 level --- until it either finds a variable binding for the required
1128 identifier or exhausts the chain.
1130 This description also determines what happens when there is more than
1131 one variable binding with the same name. Suppose, continuing the
1132 example of the previous subsection, that there was also a pre-existing
1133 top level variable @code{s} created by the expression:
1136 (define s "Some beans, my lord!")
1139 Then both the top level environment and the local @code{let} environment
1140 would contain bindings for the name @code{s}. When evaluating code
1141 within the @code{let} body, the interpreter looks first in the local
1142 @code{let} environment, and so finds the binding for @code{s} created by
1143 the @code{let} syntax. Even though this environment has a reference to
1144 the top level environment, which also has a binding for @code{s}, the
1145 interpreter doesn't get as far as looking there. When evaluating code
1146 outside the @code{let} body, the interpreter looks up variable names in
1147 the top level environment, so the name @code{s} refers to the top level
1150 Within the @code{let} body, the binding for @code{s} in the local
1151 environment is said to @dfn{shadow} the binding for @code{s} in the top
1156 @subsection Lexical Scope
1158 The rules that we have just been describing are the details of how
1159 Scheme implements ``lexical scoping''. This subsection takes a brief
1160 diversion to explain what lexical scope means in general and to present
1161 an example of non-lexical scoping.
1163 ``Lexical scope'' in general is the idea that
1167 an identifier at a particular place in a program always refers to the
1168 same variable location --- where ``always'' means ``every time that the
1169 containing expression is executed'', and that
1172 the variable location to which it refers can be determined by static
1173 examination of the source code context in which that identifier appears,
1174 without having to consider the flow of execution through the program as
1178 In practice, lexical scoping is the norm for most programming languages,
1179 and probably corresponds to what you would intuitively consider to be
1180 ``normal''. You may even be wondering how the situation could possibly
1181 --- and usefully --- be otherwise. To demonstrate that another kind of
1182 scoping is possible, therefore, and to compare it against lexical
1183 scoping, the following subsection presents an example of non-lexical
1184 scoping and examines in detail how its behavior differs from the
1185 corresponding lexically scoped code.
1188 * Scoping Example:: An example of non-lexical scoping.
1192 @node Scoping Example
1193 @subsubsection An Example of Non-Lexical Scoping
1195 To demonstrate that non-lexical scoping does exist and can be useful, we
1196 present the following example from Emacs Lisp, which is a ``dynamically
1200 (defvar currency-abbreviation "USD")
1202 (defun currency-string (units hundredths)
1203 (concat currency-abbreviation
1204 (number-to-string units)
1206 (number-to-string hundredths)))
1208 (defun french-currency-string (units hundredths)
1209 (let ((currency-abbreviation "FRF"))
1210 (currency-string units hundredths)))
1213 The question to focus on here is: what does the identifier
1214 @code{currency-abbreviation} refer to in the @code{currency-string}
1215 function? The answer, in Emacs Lisp, is that all variable bindings go
1216 onto a single stack, and that @code{currency-abbreviation} refers to the
1217 topmost binding from that stack which has the name
1218 ``currency-abbreviation''. The binding that is created by the
1219 @code{defvar} form, to the value @code{"USD"}, is only relevant if none
1220 of the code that calls @code{currency-string} rebinds the name
1221 ``currency-abbreviation'' in the meanwhile.
1223 The second function @code{french-currency-string} works precisely by
1224 taking advantage of this behaviour. It creates a new binding for the
1225 name ``currency-abbreviation'' which overrides the one established by
1226 the @code{defvar} form.
1229 ;; Note! This is Emacs Lisp evaluation, not Scheme!
1230 (french-currency-string 33 44)
1235 Now let's look at the corresponding, @emph{lexically scoped} Scheme
1239 (define currency-abbreviation "USD")
1241 (define (currency-string units hundredths)
1242 (string-append currency-abbreviation
1243 (number->string units)
1245 (number->string hundredths)))
1247 (define (french-currency-string units hundredths)
1248 (let ((currency-abbreviation "FRF"))
1249 (currency-string units hundredths)))
1252 According to the rules of lexical scoping, the
1253 @code{currency-abbreviation} in @code{currency-string} refers to the
1254 variable location in the innermost environment at that point in the code
1255 which has a binding for @code{currency-abbreviation}, which is the
1256 variable location in the top level environment created by the preceding
1257 @code{(define currency-abbreviation @dots{})} expression.
1259 In Scheme, therefore, the @code{french-currency-string} procedure does
1260 not work as intended. The variable binding that it creates for
1261 ``currency-abbreviation'' is purely local to the code that forms the
1262 body of the @code{let} expression. Since this code doesn't directly use
1263 the name ``currency-abbreviation'' at all, the binding is pointless.
1266 (french-currency-string 33 44)
1271 This begs the question of how the Emacs Lisp behaviour can be
1272 implemented in Scheme. In general, this is a design question whose
1273 answer depends upon the problem that is being addressed. In this case,
1274 the best answer may be that @code{currency-string} should be
1275 redesigned so that it can take an optional third argument. This third
1276 argument, if supplied, is interpreted as a currency abbreviation that
1277 overrides the default.
1279 It is possible to change @code{french-currency-string} so that it mostly
1280 works without changing @code{currency-string}, but the fix is inelegant,
1281 and susceptible to interrupts that could leave the
1282 @code{currency-abbreviation} variable in the wrong state:
1285 (define (french-currency-string units hundredths)
1286 (set! currency-abbreviation "FRF")
1287 (let ((result (currency-string units hundredths)))
1288 (set! currency-abbreviation "USD")
1292 The key point here is that the code does not create any local binding
1293 for the identifier @code{currency-abbreviation}, so all occurrences of
1294 this identifier refer to the top level variable.
1300 Consider a @code{let} expression that doesn't contain any
1304 (let ((s (/ (+ a b c) 2)))
1305 (sqrt (* s (- s a) (- s b) (- s c))))
1309 When the Scheme interpreter evaluates this, it
1313 creates a new environment with a reference to the environment that was
1314 current when it encountered the @code{let}
1317 creates a variable binding for @code{s} in the new environment, with
1318 value given by @code{(/ (+ a b c) 2)}
1321 evaluates the expression in the body of the @code{let} in the context of
1322 the new local environment, and remembers the value @code{V}
1325 forgets the local environment
1328 continues evaluating the expression that contained the @code{let}, using
1329 the value @code{V} as the value of the @code{let} expression, in the
1330 context of the containing environment.
1333 After the @code{let} expression has been evaluated, the local
1334 environment that was created is simply forgotten, and there is no longer
1335 any way to access the binding that was created in this environment. If
1336 the same code is evaluated again, it will follow the same steps again,
1337 creating a second new local environment that has no connection with the
1338 first, and then forgetting this one as well.
1340 If the @code{let} body contains a @code{lambda} expression, however, the
1341 local environment is @emph{not} forgotten. Instead, it becomes
1342 associated with the procedure that is created by the @code{lambda}
1343 expression, and is reinstated every time that that procedure is called.
1344 In detail, this works as follows.
1348 When the Scheme interpreter evaluates a @code{lambda} expression, to
1349 create a procedure object, it stores the current environment as part of
1350 the procedure definition.
1353 Then, whenever that procedure is called, the interpreter reinstates the
1354 environment that is stored in the procedure definition and evaluates the
1355 procedure body within the context of that environment.
1358 The result is that the procedure body is always evaluated in the context
1359 of the environment that was current when the procedure was created.
1361 This is what is meant by @dfn{closure}. The next few subsections
1362 present examples that explore the usefulness of this concept.
1366 @subsection Example 1: A Serial Number Generator
1368 This example uses closure to create a procedure with a variable binding
1369 that is private to the procedure, like a local variable, but whose value
1370 persists between procedure calls.
1373 (define (make-serial-number-generator)
1374 (let ((current-serial-number 0))
1376 (set! current-serial-number (+ current-serial-number 1))
1377 current-serial-number)))
1379 (define entry-sn-generator (make-serial-number-generator))
1381 (entry-sn-generator)
1385 (entry-sn-generator)
1390 When @code{make-serial-number-generator} is called, it creates a local
1391 environment with a binding for @code{current-serial-number} whose
1392 initial value is 0, then, within this environment, creates a procedure.
1393 The local environment is stored within the created procedure object and
1394 so persists for the lifetime of the created procedure.
1396 Every time the created procedure is invoked, it increments the value of
1397 the @code{current-serial-number} binding in the captured environment and
1398 then returns the current value.
1400 Note that @code{make-serial-number-generator} can be called again to
1401 create a second serial number generator that is independent of the
1402 first. Every new invocation of @code{make-serial-number-generator}
1403 creates a new local @code{let} environment and returns a new procedure
1404 object with an association to this environment.
1407 @node Shared Variable
1408 @subsection Example 2: A Shared Persistent Variable
1410 This example uses closure to create two procedures, @code{get-balance}
1411 and @code{deposit}, that both refer to the same captured local
1412 environment so that they can both access the @code{balance} variable
1413 binding inside that environment. The value of this variable binding
1414 persists between calls to either procedure.
1416 Note that the captured @code{balance} variable binding is private to
1417 these two procedures: it is not directly accessible to any other code.
1418 It can only be accessed indirectly via @code{get-balance} or
1419 @code{deposit}, as illustrated by the @code{withdraw} procedure.
1422 (define get-balance #f)
1431 (set! balance (+ balance amount))
1434 (define (withdraw amount)
1435 (deposit (- amount)))
1450 An important detail here is that the @code{get-balance} and
1451 @code{deposit} variables must be set up by @code{define}ing them at top
1452 level and then @code{set!}ing their values inside the @code{let} body.
1453 Using @code{define} within the @code{let} body would not work: this
1454 would create variable bindings within the local @code{let} environment
1455 that would not be accessible at top level.
1458 @node Callback Closure
1459 @subsection Example 3: The Callback Closure Problem
1461 A frequently used programming model for library code is to allow an
1462 application to register a callback function for the library to call when
1463 some particular event occurs. It is often useful for the application to
1464 make several such registrations using the same callback function, for
1465 example if several similar library events can be handled using the same
1466 application code, but the need then arises to distinguish the callback
1467 function calls that are associated with one callback registration from
1468 those that are associated with different callback registrations.
1470 In languages without the ability to create functions dynamically, this
1471 problem is usually solved by passing a @code{user_data} parameter on the
1472 registration call, and including the value of this parameter as one of
1473 the parameters on the callback function. Here is an example of
1474 declarations using this solution in C:
1477 typedef void (event_handler_t) (int event_type,
1480 void register_callback (int event_type,
1481 event_handler_t *handler,
1485 In Scheme, closure can be used to achieve the same functionality without
1486 requiring the library code to store a @code{user-data} for each callback
1492 (define (register-callback event-type handler-proc)
1495 ;; In the application:
1497 (define (make-handler event-type user-data)
1500 <code referencing event-type and user-data>
1503 (register-callback event-type
1504 (make-handler event-type @dots{}))
1507 As far as the library is concerned, @code{handler-proc} is a procedure
1508 with no arguments, and all the library has to do is call it when the
1509 appropriate event occurs. From the application's point of view, though,
1510 the handler procedure has used closure to capture an environment that
1511 includes all the context that the handler code needs ---
1512 @code{event-type} and @code{user-data} --- to handle the event
1517 @subsection Example 4: Object Orientation
1519 Closure is the capture of an environment, containing persistent variable
1520 bindings, within the definition of a procedure or a set of related
1521 procedures. This is rather similar to the idea in some object oriented
1522 languages of encapsulating a set of related data variables inside an
1523 ``object'', together with a set of ``methods'' that operate on the
1524 encapsulated data. The following example shows how closure can be used
1525 to emulate the ideas of objects, methods and encapsulation in Scheme.
1528 (define (make-account)
1530 (define (get-balance)
1532 (define (deposit amount)
1533 (set! balance (+ balance amount))
1535 (define (withdraw amount)
1536 (deposit (- amount)))
1541 ((get-balance) get-balance)
1543 ((withdraw) withdraw)
1544 (else (error "Invalid method!")))
1548 Each call to @code{make-account} creates and returns a new procedure,
1549 created by the expression in the example code that begins ``(lambda
1553 (define my-account (make-account))
1560 This procedure acts as an account object with methods
1561 @code{get-balance}, @code{deposit} and @code{withdraw}. To apply one of
1562 the methods to the account, you call the procedure with a symbol
1563 indicating the required method as the first parameter, followed by any
1564 other parameters that are required by that method.
1567 (my-account 'get-balance)
1571 (my-account 'withdraw 5)
1575 (my-account 'deposit 396)
1579 (my-account 'get-balance)
1584 Note how, in this example, both the current balance and the helper
1585 procedures @code{get-balance}, @code{deposit} and @code{withdraw}, used
1586 to implement the guts of the account object's methods, are all stored in
1587 variable bindings within the private local environment captured by the
1588 @code{lambda} expression that creates the account object procedure.
1592 @c TeX-master: "guile.texi"