(Fly Evaluation): Add scm_call_4, suggested by Bruce Korb.

[bpt/guile.git] / doc / ref / misc-modules.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@page
@node Pretty Printing
@section Pretty Printing

@c FIXME::martin: Review me!

@cindex pretty printing
The module @code{(ice-9 pretty-print)} provides the procedure
@code{pretty-print}, which provides nicely formatted output of Scheme
objects.  This is especially useful for deeply nested or complex data
structures, such as lists and vectors.

The module is loaded by simply saying.

@lisp
(use-modules (ice-9 pretty-print))
@end lisp

This makes the procedure @code{pretty-print} available.  As an example
how @code{pretty-print} will format the output, see the following:

@lisp
(pretty-print '(define (foo) (lambda (x)
(cond ((zero? x) #t) ((negative? x) -x) (else
(if (= x 1) 2 (* x x x)))))))
@print{}
(define (foo)
  (lambda (x)
    (cond ((zero? x) #t)
          ((negative? x) -x)
          (else (if (= x 1) 2 (* x x x))))))
@end lisp

@deffn {Scheme Procedure} pretty-print obj [port] [keyword-options]
Print the textual representation of the Scheme object @var{obj} to
@var{port}.  @var{port} defaults to the current output port, if not
given.

The further @var{keyword-options} are keywords and parameters as
follows,

@table @asis
@item @nicode{#:display?} @var{flag}
If @var{flag} is true then print using @code{display}.  The default is
@code{#f} which means use @code{write} style.  (@pxref{Writing})

@item @nicode{#:per-line-prefix} @var{string}
Print the given @var{string} as a prefix on each line.  The default is
no prefix.

@item @nicode{#:width} @var{columns}
Print within the given @var{columns}.  The default is 79.
@end table
@end deffn


@page
@node Formatted Output
@section Formatted Output
@cindex formatted output

@c  For reference, in this section escapes like ~a are given in
@c  @nicode, to give code font in TeX etc, but leave them unadorned in
@c  Info.
@c
@c  The idea is to reduce clutter around what's shown, and avoid any
@c  possible confusion over whether the ` ' quotes are part of what
@c  should be entered.  (In particular for instance of course ' is
@c  meaningful in a format string, introducing a char parameter).

The @code{format} function is a powerful way to print numbers, strings
and other objects together with literal text under the control of a
format string.  This function is available from

@example
(use-modules (ice-9 format))
@end example

A format string is generally more compact and easier than using just
the standard procedures like @code{display}, @code{write} and
@code{newline}.  Parameters in the output string allow various output
styles, and parameters can be taken from the arguments for runtime
flexibility.

@code{format} is similar to the Common Lisp procedure of the same
name, but it's not identical and doesn't have quite all the features
found in Common Lisp.

C programmers will note the similarity between @code{format} and
@code{printf}, though escape sequences are marked with @nicode{~}
instead of @nicode{%}, and are more powerful.

@sp 1
@deffn {Scheme Procedure} format dest fmt [args@dots{}]
Write output specified by the @var{fmt} string to @var{dest}.
@var{dest} can be an output port, @code{#t} for
@code{current-output-port} (@pxref{Default Ports}), a number for
@code{current-error-port}, or @code{#f} to return the output as a
string.

@var{fmt} can contain literal text to be output, and @nicode{~}
escapes.  Each escape has the form

@example
~ [param [, param@dots{}] [:] [@@] code
@end example

@nicode{code} is a character determining the escape sequence.  The
@nicode{:} and @nicode{@@} characters are optional modifiers, one or
both of which change the way various codes operate.  Optional
parameters are accepted by some codes too.  Parameters have the
following forms,

@table @asis
@item @nicode{[+/-]number}
An integer, with optional @nicode{+} or @nicode{-}.
@item @nicode{'}  (apostrophe)
The following character in the format string, for instance @nicode{'z}
for @nicode{z}.
@item @nicode{v}
The next function argument as the parameter.  @nicode{v} stands for
``variable'', a parameter can be calculated at runtime and included in
the arguments.  Upper case @nicode{V} can be used too.
@item @nicode{#}
The number of arguments remaining.  (See @nicode{~*} below for some
usages.)
@end table

Parameters are separated by commas (@nicode{,}).  A parameter can be
left empty to keep its default value when supplying later parameters.

@sp 1
The following escapes are available.  The code letters are not
case-sensitive, upper and lower case are the same.

@table @asis
@item @nicode{~a}
@itemx @nicode{~s}
Object output.  Parameters: @var{minwidth}, @var{padinc},
@var{minpad}, @var{padchar}.

@nicode{~a} outputs an argument like @code{display}, @nicode{~s}
outputs an argument like @code{write} (@pxref{Writing}).  

@example
(format #t "~a" "foo") @print{} foo
(format #t "~s" "foo") @print{} "foo"
@end example

@nicode{~:a} and @nicode{~:s} put objects that don't have an external
representation in quotes like a string.

@example
(format #t "~:a" car) @print{} "#<primitive-procedure car>"
@end example

If the output is less than @var{minwidth} characters (default 0), it's
padded on the right with @var{padchar} (default space).  @nicode{~@@a}
and @nicode{~@@s} put the padding on the left instead.

@example
(format #f "~5a" 'abc)       @result{} "abc  "
(format #f "~5,,,'-@@a" 'abc) @result{} "--abc"
@end example

@var{minpad} is a minimum for the padding then plus a multiple of
@var{padinc}.  Ie.@: the padding is @math{@var{minpad} + @var{N} *
@var{padinc}}, where @var{n} is the smallest integer making the total
object plus padding greater than or equal to @var{minwidth}.  The
default @var{minpad} is 0 and the default @var{padinc} is 1 (imposing
no minimum or multiple).

@example
(format #f "~5,1,4a" 'abc) @result{} "abc    "
@end example

@item @nicode{~c}
Character.  Parameter: @var{charnum}.

Output a character.  The default is to simply output, as per
@code{write-char} (@pxref{Writing}).  @nicode{~@@c} prints in
@code{write} style.  @nicode{~:c} prints control characters (ASCII 0
to 31) in @nicode{^X} form.

@example
(format #t "~c" #\z)        @print{} z
(format #t "~@@c" #\z)       @print{} #\z
(format #t "~:c" #\newline) @print{} ^J
@end example

If the @var{charnum} parameter is given then an argument is not taken
but instead the character is @code{(integer->char @var{charnum})}
(@pxref{Characters}).  This can be used for instance to output
characters given by their ASCII code.

@example
(format #t "~65c")  @print{} A
@end example

@item @nicode{~d}
@itemx @nicode{~x}
@itemx @nicode{~o}
@itemx @nicode{~b}
Integer.  Parameters: @var{minwidth}, @var{padchar}, @var{commachar},
@var{commawidth}.

Output an integer argument as a decimal, hexadecimal, octal or binary
integer (respectively).

@example
(format #t "~d" 123) @print{} 123
@end example

@nicode{~@@d} etc shows a @nicode{+} sign is shown on positive
numbers.

@c  FIXME: "+" is not shown on zero, unlike in Common Lisp.  Should
@c  that be changed in the code, or is it too late and should just be
@c  documented that way?

@example
(format #t "~@@b" 12) @print{} +1100
@end example

If the output is less than the @var{minwidth} parameter (default no
minimum), it's padded on the left with the @var{padchar} parameter
(default space).

@example
(format #t "~5,'*d" 12)   @print{} ***12
(format #t "~5,'0d" 12)   @print{} 00012
(format #t "~3d"    1234) @print{} 1234
@end example

@nicode{~:d} adds commas (or the @var{commachar} parameter) every
three digits (or the @var{commawidth} parameter many).

@example
(format #t "~:d" 1234567)         @print{} 1,234,567
(format #t "~10,'*,'/,2:d" 12345) @print{} ***1/23/45
@end example

Hexadecimal @nicode{~x} output is in lower case, but the @nicode{~(}
and @nicode{~)} case conversion directives described below can be used
to get upper case.

@example
(format #t "~x"       65261) @print{} feed
(format #t "~:@@(~x~)" 65261) @print{} FEED
@end example

@item @nicode{~r}
Integer in words, roman numerals, or a specified radix.  Parameters:
@var{radix}, @var{minwidth}, @var{padchar}, @var{commachar},
@var{commawidth}.

With no parameters output is in words as a cardinal like ``ten'', or
@nicode{~:r} prints an ordinal like ``tenth''.

@example
(format #t "~r" 9)  @print{} nine        ;; cardinal
(format #t "~r" -9) @print{} minus nine  ;; cardinal
(format #t "~:r" 9) @print{} ninth       ;; ordinal
@end example

And also with no parameters, @nicode{~@@r} gives roman numerals and
@nicode{~@@:r} gives old roman numerals.  In old roman numerals
there's no ``subtraction'', so 9 is @nicode{VIIII} instead of
@nicode{IX}.  In both cases only positive numbers can be output.

@example
(format #t "~@@r" 89)  @print{} LXXXIX     ;; roman
(format #t "~@@:r" 89) @print{} LXXXVIIII  ;; old roman
@end example

When a parameter is given it means numeric output in the specified
@var{radix}.  The modifiers and parameters following the radix are the
same as described for @nicode{~d} etc above.

@example
(format #f "~3r" 27)   @result{} "1000"    ;; base 3
(format #f "~3,5r" 26) @result{} "  222"   ;; base 3 width 5
@end example

@item @nicode{~f}
Fixed-point float.  Parameters: @var{width}, @var{decimals},
@var{scale}, @var{overflowchar}, @var{padchar}.

Output a number or number string in fixed-point format, ie.@: with a
decimal point.

@example
(format #t "~f" 5)      @print{} 5.0
(format #t "~f" "123")  @print{} 123.0
(format #t "~f" "1e-1") @print{} 0.1
@end example

@nicode{~@@f} prints a @nicode{+} sign on positive numbers (including
zero).

@example
(format #t "~@@f" 0) @print{} +0.0
@end example

If the output is less than @var{width} characters it's padded on the
left with @var{padchar} (space by default).  If the output equals or
exceeds @var{width} then there's no padding.  The default for
@var{width} is no padding.

@example
(format #f "~6f" -1.5)      @result{} "  -1.5"
(format #f "~6,,,,'*f" 23)  @result{} "**23.0"
(format #f "~6f" 1234567.0) @result{} "1234567.0"
@end example

@var{decimals} is how many digits to print after the decimal point,
with the value rounded or padded with zeros as necessary.  (The
default is to output as many decimals as required.)

@example
(format #t "~1,2f" 3.125) @print{} 3.13
(format #t "~1,2f" 1.5)   @print{} 1.50
@end example

@var{scale} is a power of 10 applied to the value, moving the decimal
point that many places.  A positive @var{scale} increases the value
shown, a negative decreases it.

@example
(format #t "~,,2f" 1234)  @print{} 123400.0
(format #t "~,,-2f" 1234) @print{} 12.34
@end example

If @var{overflowchar} and @var{width} are both given and if the output
would exceed @var{width}, then that many @var{overflowchar}s are
printed instead of the value.

@example
(format #t "~5,,,'xf" 12345) @print{} 12345
(format #t "~4,,,'xf" 12345) @print{} xxxx
@end example

@item @nicode{~e}
Exponential float.  Parameters: @var{width}, @var{mantdigits},
@var{expdigits}, @var{intdigits}, @var{overflowchar}, @var{padchar},
@var{expchar}.

Output a number or number string in exponential notation.

@example
(format #t "~e" 5000.25) @print{} 5.00025E+3
(format #t "~e" "123.4") @print{} 1.234E+2
(format #t "~e" "1e4")   @print{} 1.0E+4
@end example

@nicode{~@@e} prints a @nicode{+} sign on positive numbers (including
zero).  (This is for the mantissa, a @nicode{+} or @nicode{-} sign is
always shown on the exponent.)

@example
(format #t "~@@e" 5000.0) @print{} +5.0E+3
@end example

If the output is less than @var{width} characters it's padded on the
left with @var{padchar} (space by default).  The default for
@var{width} is to output with no padding.

@example
(format #f "~10e" 1234.0)     @result{} "  1.234E+3"
(format #f "~10,,,,,'*e" 0.5) @result{} "****5.0E-1"
@end example

@c  FIXME: Describe what happens when the number is bigger than WIDTH.
@c  There seems to be a bit of dodginess about this, or some deviation
@c  from Common Lisp.

@var{mantdigits} is the number of digits shown in the mantissa after
the decimal point.  The value is rounded or trailing zeros are added
as necessary.  The default @var{mantdigits} is to show as much as
needed by the value.

@example
(format #f "~,3e" 11111.0) @result{} "1.111E+4"
(format #f "~,8e" 123.0)   @result{} "1.23000000E+2"
@end example

@var{expdigits} is the minimum number of digits shown for the
exponent, with leading zeros added if necessary.  The default for
@var{expdigits} is to show only as many digits as required.  At least
1 digit is always shown.

@example
(format #f "~,,1e" 1.0e99) @result{} "1.0E+99"
(format #f "~,,6e" 1.0e99) @result{} "1.0E+000099"
@end example

@var{intdigits} (default 1) is the number of digits to show before the
decimal point in the mantissa.  @var{intdigits} can be zero, in which
case the integer part is a single @nicode{0}, or it can be negative,
in which case leading zeros are shown after the decimal point.

@c  FIXME: When INTDIGITS is 0, Common Lisp format apparently only
@c  shows the single 0 digit if it fits in WIDTH.  format.scm seems to
@c  show it always.  Is it meant to?

@example
(format #t "~,,,3e" 12345.0)  @print{} 123.45E+2
(format #t "~,,,0e" 12345.0)  @print{} 0.12345E+5
(format #t "~,,,-3e" 12345.0) @print{} 0.00012345E+8
@end example

@c  FIXME: MANTDIGITS with negative INTDIGITS doesn't match CL spec,
@c  believe the spec says it ought to still show mantdigits+1 sig
@c  figures, ie. leading zeros don't count towards MANTDIGITS, but it
@c  seems to just treat MANTDIGITS as how many digits after the
@c  decimal point.

If @var{overflowchar} is given then @var{width} is a hard limit.  If
the output would exceed @var{width} then instead that many
@var{overflowchar}s are printed.

@example
(format #f "~6,,,,'xe" 100.0) @result{} "1.0E+2"
(format #f "~3,,,,'xe" 100.0) @result{} "xxx"
@end example

@var{expchar} is the exponent marker character (default @nicode{E}).

@example
(format #t "~,,,,,,'ee" 100.0) @print{} 1.0e+2
@end example

@item @nicode{~g}
General float.  Parameters: @var{width}, @var{mantdigits},
@var{expdigits}, @var{intdigits}, @var{overflowchar}, @var{padchar},
@var{expchar}.

Output a number or number string in either exponential format the same
as @nicode{~e}, or fixed-point format like @nicode{~f} but aligned
where the mantissa would have been and followed by padding where the
exponent would have been.

@c  FIXME: The default MANTDIGITS is apparently max(needed,min(n,7))
@c  where 10^(n-1)<=abs(x)<=10^n.  But the Common Lisp spec seems to
@c  ask for "needed" to be without leading or trailing zeros, whereas
@c  format.scm seems to include trailing zeros, ending up with it
@c  using fixed format for bigger values than it should.

Fixed-point is used when the absolute value is 0.1 or more and it
takes no more space than the mantissa in exponential format, ie.@:
basically up to @var{mantdigits} digits.

@example
(format #f "~12,4,2g" 999.0)    @result{} "   999.0    "
(format #f "~12,4,2g" "100000") @result{} "  1.0000E+05"
@end example

The parameters are interpreted as per @nicode{~e} above.  When
fixed-point is used, the @var{decimals} parameter to @nicode{~f} is
established from @var{mantdigits}, so as to give a total
@math{@var{mantdigits}+1} figures.

@item @nicode{~$}
Monetary style fixed-point float.  Parameters: @var{decimals},
@var{intdigits}, @var{width}, @var{padchar}.

@c  For reference, fmtdoc.txi from past versions of slib showed the
@c  INTDIGITS parameter as SCALE.  That looks like a typo, in the code
@c  and in the Common Lisp spec it's a minimum digits for the integer
@c  part, it isn't a power of 10 like in ~f.

Output a number or number string in fixed-point format, ie.@: with a
decimal point.  @var{decimals} is the number of decimal places to
show, default 2.

@example
(format #t "~$" 5)       @print{} 5.00
(format #t "~4$" "2.25") @print{} 2.2500
(format #t "~4$" "1e-2") @print{} 0.0100
@end example

@nicode{~@@$} prints a @nicode{+} sign on positive numbers (including
zero).

@example
(format #t "~@@$" 0) @print{} +0.00
@end example

@var{intdigits} is a minimum number of digits to show in the integer
part of the value (default 1).

@example
(format #t "~,3$" 9.5)   @print{} 009.50
(format #t "~,0$" 0.125) @print{} .13
@end example

If the output is less than @var{width} characters (default 0), it's
padded on the left with @var{padchar} (default space).  @nicode{~:$}
puts the padding after the sign.

@example
(format #f "~,,8$" -1.5)   @result{} "   -1.50"
(format #f "~,,8:$" -1.5)  @result{} "-   1.50"
(format #f "~,,8,'.@@:$" 3) @result{} "+...3.00"
@end example

Note that floating point for dollar amounts is generally not a good
idea, because a cent @math{0.01} cannot be represented exactly in the
binary floating point Guile uses, which leads to slowly accumulating
rounding errors.  Keeping values as cents (or fractions of a cent) in
integers then printing with the scale option in @nicode{~f} may be a
better approach.

@c  For reference, fractions don't work with ~$ (or any of the float
@c  conversions) currently.  If they did work then we could perhaps
@c  suggest keeping dollar amounts as rationals, which would of course
@c  give exact cents.  An integer as cents is probably still a better
@c  recommendation though, since it forces one to think about where
@c  and when rounding can or should occur.

@item @nicode{~i}
Complex fixed-point float.  Parameters: @var{width}, @var{decimals},
@var{scale}, @var{overflowchar}, @var{padchar}.

@c  For reference, in Common Lisp ~i is an indent, but slib fmtdoc.txi
@c  described it as complex number output, so we keep that.

Output the argument as a complex number, with both real and imaginary
part shown (even if one or both are zero).

The parameters and modifiers are the same as for fixed-point
@nicode{~f} described above.  The real and imaginary parts are both
output with the same given parameters and modifiers, except that for
the imaginary part the @nicode{@@} modifier is always enabled, so as
to print a @nicode{+} sign between the real and imaginary parts.

@example
(format #t "~i" 1)  @print{} 1.0+0.0i
@end example

@item @nicode{~p}
Plural.  No parameters.

Output nothing if the argument is 1, or @samp{s} for any other
value.

@example
(format #t "enter name~p" 1) @print{} enter name
(format #t "enter name~p" 2) @print{} enter names
@end example

@nicode{~@@p} prints @samp{y} for 1 or @samp{ies} otherwise.

@example
(format #t "pupp~@@p" 1) @print{} puppy
(format #t "pupp~@@p" 2) @print{} puppies
@end example

@nicode{~:p} re-uses the preceding argument instead of taking a new
one, which can be convenient when printing some sort of count.

@example
(format #t "~d cat~:p" 9)   @print{} 9 cats
(format #t "~d pupp~@@:p" 5) @print{} 5 puppies
@end example

@nicode{~p} is designed for English plurals and there's no attempt to
support other languages.  @nicode{~[} conditionals (below) may be able
to help.  When using @code{gettext} to translate messages
@code{ngettext} is probably best though
(@pxref{Internationalization}).

@item @nicode{~y}
Pretty print.  No parameters.

Output an argument with @code{pretty-print} (@pxref{Pretty Printing}).

@item @nicode{~?}
@itemx @nicode{~k}
Sub-format.  No parameters.

Take a format string argument and a second argument which is a list of
arguments for that string, and output the result.

@example
(format #t "~?" "~d ~d" '(1 2))    @print{} 1 2
@end example

@nicode{~@@?} takes arguments for the sub-format directly rather than
in a list.

@example
(format #t "~@@? ~s" "~d ~d" 1 2 "foo") @print{} 1 2 "foo"
@end example

@nicode{~?} and @nicode{~k} are the same, @nicode{~k} is provided for
T-Scheme compatibility.

@item @nicode{~*}
Argument jumping.  Parameter: @var{N}.

Move forward @var{N} arguments (default 1) in the argument list.
@nicode{~:*} moves backwards.  (@var{N} cannot be negative.)

@example
(format #f "~d ~2*~d" 1 2 3 4) @result{} "1 4"
(format #f "~d ~:*~d" 6)       @result{} "6 6"
@end example

@nicode{~@@*} moves to argument number @var{N}.  The first argument is
number 0 (and that's the default for @var{N}).

@example
(format #f "~d~d again ~@@*~d~d" 1 2) @result{} "12 again 12"
(format #f "~d~d~d ~1@@*~d~d" 1 2 3)  @result{} "123 23"
@end example

A @nicode{#} move to the end followed by a @nicode{:} modifier move
back can be used for an absolute position relative to the end of the
argument list, a reverse of what the @nicode{@@} modifier does.

@example
(format #t "~#*~2:*~a" 'a 'b 'c 'd)   @print{} c
@end example

At the end of the format string the current argument postion doesn't
matter, any further arguments are ignored.

@item @nicode{~t}
Advance to a column position.  Parameters: @var{colnum}, @var{colinc},
@var{padchar}.

Output @var{padchar} (space by default) to move to the given
@var{colnum} column.  The start of the line is column 0, the default
for @var{colnum} is 1.

@example
(format #f "~tX")  @result{} " X"
(format #f "~3tX") @result{} "   X"
@end example

If the current column is already past @var{colnum}, then the move is
to there plus a multiple of @var{colinc}, ie.@: column
@math{@var{colnum} + @var{N} * @var{colinc}} for the smallest @var{N}
which makes that value greater than or equal to the current column.
The default @var{colinc} is 1 (which means no further move).

@example
(format #f "abcd~2,5,'.tx") @result{} "abcd...x"
@end example

@nicode{~@@t} takes @var{colnum} as an offset from the current column.
@var{colnum} many pad characters are output, then further padding to
make the current column a multiple of @var{colinc}, if it isn't
already so.

@example
(format #f "a~3,5'*@@tx") @result{} "a****x"
@end example

@nicode{~t} is implemented using @code{port-column} (@pxref{Reading}),
so it works even there has been other output before @code{format}.

@item @nicode{~~}
Tilde character.  Parameter: @var{n}.

Output a tilde character @nicode{~}, or @var{n} many if a parameter is
given.  Normally @nicode{~} introduces an escape sequence, @nicode{~~}
is the way to output a literal tilde.

@item @nicode{~%}
Newline.  Parameter: @var{n}.

Output a newline character, or @var{n} many if a parameter is given.
A newline (or a few newlines) can of course be output just by
including them in the format string.

@item @nicode{~&}
Start a new line.  Parameter: @var{n}.

Output a newline if not already at the start of a line.  With a
parameter, output that many newlines, but with the first only if not
already at the start of a line.  So for instance 3 would be a newline
if not already at the start of a line, and 2 further newlines.

@item @nicode{~_}
Space character.  Parameter: @var{n}.

@c  For reference, in Common Lisp ~_ is a conditional newline, but
@c  slib fmtdoc.txi described it as a space, so we keep that.

Output a space character, or @var{n} many if a parameter is given.

With a variable parameter this is one way to insert runtime calculated
padding (@nicode{~t} or the various field widths can do similar
things).

@example
(format #f "~v_foo" 4) @result{} "    foo"
@end example

@item @nicode{~/}
Tab character.  Parameter: @var{n}.

Output a tab character, or @var{n} many if a parameter is given.

@item @nicode{~|}
Formfeed character.  Parameter: @var{n}.

Output a formfeed character, or @var{n} many if a parameter is given.

@item @nicode{~!}
Force output.  No parameters.

At the end of output, call @code{force-output} to flush any buffers on
the destination (@pxref{Writing}).  @nicode{~!} can occur anywhere in
the format string, but the force is done at the end of output.

When output is to a string (destination @code{#f}), @nicode{~!} does
nothing.

@item @nicode{~newline}  (ie.@: newline character)
Continuation line.  No parameters.

Skip this newline and any following whitespace in the format string,
ie.@: don't send it to the output.  This can be used to break up a
long format string for readability, but not print the extra
whitespace.

@example
(format #f "abc~
            ~d def~
            ~d" 1 2) @result{} "abc1 def2"
@end example

@nicode{~:newline} skips the newline but leaves any further whitespace
to be printed normally.

@nicode{~@@newline} prints the newline then skips following
whitespace.

@item @nicode{~(} @nicode{~)}
Case conversion.  No parameters.

Between @nicode{~(} and @nicode{~)} the case of all output is changed.
The modifiers on @nicode{~(} control the conversion.

@itemize @w{}
@item
@nicode{~(} --- lower case.
@c
@c  FIXME: The : and @ modifiers are not yet documented because the
@c  code applies string-capitalize and string-capitalize-first to each
@c  separate format:out-str call, which has various subtly doubtful
@c  effects.  And worse they're applied to individual characters,
@c  including literal characters in the format string, which has the
@c  silly effect of being always an upcase.
@c
@c  The Common Lisp spec is apparently for the capitalization to be
@c  applied in one hit to the whole of the output between ~( and ~).
@c  (This can no doubt be implemented without accumulating all that
@c  text, just by keeping a state or the previous char to tell whether
@c  within a word.)
@c
@c  @item
@c  @nicode{:} --- first letter of each word upper case, the rest lower
@c  case, as per the @code{string-capitalize} function (@pxref{Alphabetic
@c  Case Mapping}).
@c  @item
@c  @nicode{@@} --- first letter of just the first word upper case, the
@c  rest lower case.
@c
@item
@nicode{~@@:(} --- upper case.
@end itemize

For example,

@example
(format #t "~(Hello~)")   @print{} hello
(format #t "~@@:(Hello~)") @print{} HELLO
@end example

In the future it's intended the modifiers @nicode{:} and @nicode{@@}
alone will capitalize the first letters of words, as per Common Lisp
@code{format}, but the current implementation of this is flawed and
not recommended for use.

Case conversions do not nest, currently.  This might change in the
future, but if it does then it will be to Common Lisp style where the
outermost conversion has priority, overriding inner ones (making those
fairly pointless).

@item @nicode{~@{} @nicode{~@}}
Iteration.  Parameter: @var{maxreps} (for @nicode{~@{}).

The format between @nicode{~@{} and @nicode{~@}} is iterated.  The
modifiers to @nicode{~@{} determine how arguments are taken.  The
default is a list argument with each iteration successively consuming
elements from it.  This is a convenient way to output a whole list.

@example
(format #t "~@{~d~@}"     '(1 2 3))       @print{} 123
(format #t "~@{~s=~d ~@}" '("x" 1 "y" 2)) @print{} "x"=1 "y"=2
@end example

@nicode{~:@{} takes a single argument which is a list of lists, each
of those contained lists gives the arguments for the iterated format.

@example
(format #t "~:@{~dx~d ~@}" '((1 2) (3 4) (5 6))) @print{} 1x2 3x4 5x6
@end example

@nicode{~@@@{} takes arguments directly, with each iteration
successively consuming arguments.

@example
(format #t "~@@@{~d~@}"     1 2 3)       @print{} 123
(format #t "~@@@{~s=~d ~@}" "x" 1 "y" 2) @print{} "x"=1 "y"=2
@end example

@nicode{~@@:@{} takes list arguments, one argument for each iteration,
using that list for the format.

@example
(format #t "~@@:@{~dx~d ~@}" '(1 2) '(3 4) '(5 6)) @print{} 1x2 3x4 5x6
@end example

Iterating stops when there are no more arguments or when the
@var{maxreps} parameter to @nicode{~@{} is reached (default no
maximum).

@example
(format #t "~2@{~d~@}" '(1 2 3 4)) @print{} 12
@end example

If the format between @nicode{~@{} and @nicode{~@}} is empty, then a
format string argument is taken (before iteration argument(s)) and
used instead.  This allows a sub-format (like @nicode{~?} above) to be
iterated.

@example
(format #t "~@{~@}" "~d" '(1 2 3)) @print{} 123
@end example

@c  FIXME: What is the @nicode{:} modifier to ~} meant to do?  The
@c  Common Lisp spec says it's a minimum of 1 iteration, but the
@c  format.scm code seems to merely make it have MAXREPS default to 1.

Iterations can be nested, an inner iteration operates in the same way
as described, but of course on the arguments the outer iteration
provides it.  This can be used to work into nested list structures.
For example in the following the inner @nicode{~@{~d~@}x} is applied
to @code{(1 2)} then @code{(3 4 5)} etc.

@example
(format #t "~@{~@{~d~@}x~@}" '((1 2) (3 4 5))) @print{} 12x345x
@end example

See also @nicode{~^} below for escaping from iteration.

@item @nicode{~[} @nicode{~;} @nicode{~]}
Conditional.  Parameter: @var{selector}.

A conditional block is delimited by @nicode{~[} and @nicode{~]}, and
@nicode{~;} separates clauses within the block.  @nicode{~[} takes an
integer argument and that number clause is used.  The first clause is
number 0.

@example
(format #f "~[peach~;banana~;mango~]" 1)  @result{} "banana"
@end example

The @var{selector} parameter can be used for the clause number,
instead of taking an argument.

@example
(format #f "~2[peach~;banana~;mango~]") @result{} "mango"
@end example

If the clause number is out of range then nothing is output.  Or the
last clause can be @nicode{~:;} to use that for a number out of range.

@example
(format #f "~[banana~;mango~]"         99) @result{} ""
(format #f "~[banana~;mango~:;fruit~]" 99) @result{} "fruit"
@end example

@nicode{~:[} treats the argument as a flag, and expects two clauses.
The first is used if the argument is @code{#f} or the second
otherwise.

@example
(format #f "~:[false~;not false~]" #f)   @result{} "false"
(format #f "~:[false~;not false~]" 'abc) @result{} "not false"

(let ((n 3))
  (format #t "~d gnu~:[s are~; is~] here" n (= 1 n)))
@print{} 3 gnus are here
@end example

@nicode{~@@[} also treats the argument as a flag, and expects one
clause.  If the argument is @code{#f} then no output is produced and
the argument is consumed, otherwise the clause is used and the
argument is not consumed, it's left for the clause.  This can be used
for instance to suppress output if @code{#f} means something not
available.

@example
(format #f "~@@[temperature=~d~]" 27) @result{} "temperature=27"
(format #f "~@@[temperature=~d~]" #f) @result{} ""
@end example

@item @nicode{~^}
Escape.  Parameters: @var{val1}, @var{val2}, @var{val3}.

Stop formatting if there are no more arguments.  This can be used for
instance to have a format string adapt to a variable number of
arguments.

@example
(format #t "~d~^ ~d" 1)   @print{} 1
(format #t "~d~^ ~d" 1 2) @print{} 1 2
@end example

Within a @nicode{~@{} @nicode{~@}} iteration, @nicode{~^} stops the
current iteration step if there are no more arguments to that step,
but continuing with possible further steps and the rest of the format.
This can be used for instance to avoid a separator on the last
iteration, or to adapt to variable length argument lists.

@example
(format #f "~@{~d~^/~@} go"    '(1 2 3))     @result{} "1/2/3 go"
(format #f "~:@{ ~d~^~d~@} go" '((1) (2 3))) @result{} " 1 23 go"
@end example

@c  For reference, format.scm doesn't implement that Common Lisp ~:^
@c  modifier which stops the entire iterating of ~:{ or ~@:{.

@c  FIXME: Believe the Common Lisp spec is for ~^ within ~[ ~]
@c  conditional to terminate the whole format (or iteration step if in
@c  an iteration).  But format.scm seems to terminate just the
@c  conditional form.
@c
@c      (format #f "~[abc~^def~;ghi~] blah" 0)
@c      @result{} "abc blah"                    ;; looks wrong

@c  FIXME: Believe the Common Lisp spec is for ~^ within ~( ~) to end
@c  that case conversion and then also terminate the whole format (or
@c  iteration step if in an iteration).  But format.scm doesn't seem
@c  to do that quite right.
@c
@c      (format #f "~d ~^ ~d" 1)      @result{} "1 "
@c      (format #f "~(~d ~^ ~d~)" 1)  @result{} ERROR

Within a @nicode{~?} sub-format, @nicode{~^} operates just on that
sub-format.  If it terminates the sub-format then the originating
format will still continue.

@example
(format #t "~? items" "~d~^ ~d" '(1))   @print{} 1 items
(format #t "~? items" "~d~^ ~d" '(1 2)) @print{} 1 2 items
@end example

The parameters to @nicode{~^} (which are numbers) change the condition
used to terminate.  For a single parameter, termination is when that
value is zero (notice this makes plain @nicode{~^} equivalent to
@nicode{~#^}).  For two parameters, termination is when those two are
equal.  For three parameters, termination is when @math{@var{val1}
@le{} @var{val2}} and @math{@var{val2} @le{} @var{val3}}.

@c  FIXME: Good examples of these?

@item @nicode{~q}
Inquiry message.  Insert a copyright message into the output.

@nicode{~:q} inserts the format implementation version.
@end table

@sp 1
It's an error if there are not enough arguments for the escapes in the
format string, but any excess arguments are ignored.

Iterations @nicode{~@{} @nicode{~@}} and conditionals @nicode{~[}
@nicode{~;} @nicode{~]} can be nested, but must be properly nested,
meaning the inner form must be entirely within the outer form.  So
it's not possible, for instance, to try to conditionalize the endpoint
of an iteration.

@example
(format #t "~@{ ~[ ... ~] ~@}" ...)       ;; good
(format #t "~@{ ~[ ... ~@} ... ~]" ...)   ;; bad
@end example

The same applies to case conversions @nicode{~(} @nicode{~)}, they
must properly nest with respect to iterations and conditionals (though
currently a case conversion cannot nest within another case
conversion).

When a sub-format (@nicode{~?}) is used, that sub-format string must
be self-contained.  It cannot for instance give a @nicode{~@{} to
begin an iteration form and have the @nicode{~@}} up in the
originating format, or similar.
@end deffn

@sp 1
Guile contains a @code{format} procedure even when the module
@code{(ice-9 format)} is not loaded.  The default @code{format} is
@code{simple-format} (@pxref{Writing}), it doesn't support all escape
sequences documented in this section, and will signal an error if you
try to use one of them.  The reason for two versions is that the full
@code{format} is fairly large and requires some time to load.
@code{simple-format} is often adequate too.


@node File Tree Walk
@section File Tree Walk
@cindex file tree walk

The functions in this section traverse a tree of files and
directories, in a fashion similar to the C @code{ftw} and @code{nftw}
routines (@pxref{Working with Directory Trees,,, libc, GNU C Library
Reference Manual}).

@example
(use-modules (ice-9 ftw))
@end example
@sp 1

@defun ftw startname proc ['hash-size n]
Walk the filesystem tree descending from @var{startname}, calling
@var{proc} for each file and directory.

Hard links and symbolic links are followed.  A file or directory is
reported to @var{proc} only once, and skipped if seen again in another
place.  One consequence of this is that @code{ftw} is safe against
circularly linked directory structures.

Each @var{proc} call is @code{(@var{proc} filename statinfo flag)} and
it should return @code{#t} to continue, or any other value to stop.

@var{filename} is the item visited, being @var{startname} plus a
further path and the name of the item.  @var{statinfo} is the return
from @code{stat} (@pxref{File System}) on @var{filename}.  @var{flag}
is one of the following symbols,

@table @code
@item regular
@var{filename} is a file, this includes special files like devices,
named pipes, etc.

@item directory
@var{filename} is a directory.

@item invalid-stat
An error occurred when calling @code{stat}, so nothing is known.
@var{statinfo} is @code{#f} in this case.

@item directory-not-readable
@var{filename} is a directory, but one which cannot be read and hence
won't be recursed into.

@item symlink
@var{filename} is a dangling symbolic link.  Symbolic links are
normally followed and their target reported, the link itself is
reported if the target does not exist.
@end table

The return value from @code{ftw} is @code{#t} if it ran to completion,
or otherwise the non-@code{#t} value from @var{proc} which caused the
stop.

Optional argument symbol @code{hash-size} and an integer can be given
to set the size of the hash table used to track items already visited.
(@pxref{Hash Table Reference})

@c  Actually, it's probably safe to escape from ftw, just need to
@c  check it.
@c
In the current implementation, returning non-@code{#t} from @var{proc}
is the only valid way to terminate @code{ftw}.  @var{proc} must not
use @code{throw} or similar to escape.
@end defun


@defun nftw startname proc ['chdir] ['depth] ['hash-size n] ['mount] ['physical]
Walk the filesystem tree starting at @var{startname}, calling
@var{proc} for each file and directory.  @code{nftw} has extra
features over the basic @code{ftw} described above.

Hard links and symbolic links are followed, but a file or directory is
reported to @var{proc} only once, and skipped if seen again in another
place.  One consequence of this is that @code{nftw} is safe against
circular linked directory structures.

Each @var{proc} call is @code{(@var{proc} filename statinfo flag
basename level)} and it should return @code{#t} to continue, or any
other value to stop.

@var{filename} is the item visited, being @var{startname} plus a
further path and the name of the item.  @var{statinfo} is the return
from @code{stat} on @var{filename} (@pxref{File System}).
@var{basename} it the item name without any path.  @var{level} is an
integer giving the directory nesting level, starting from 0 for the
contents of @var{startname} (or that item itself if it's a file).
@var{flag} is one of the following symbols,

@table @code
@item regular
@var{filename} is a file, this includes special files like devices,
named pipes, etc.

@item directory
@var{filename} is a directory.

@item directory-processed
@var{filename} is a directory, and its contents have all been visited.
This flag is given instead of @code{directory} when the @code{depth}
option below is used.

@item invalid-stat
An error occurred when applying @code{stat} to @var{filename}, so
nothing is known about it.  @var{statinfo} is @code{#f} in this case.

@item directory-not-readable
@var{filename} is a directory, but one which cannot be read and hence
won't be recursed into.

@item symlink
@var{filename} is a dangling symbolic link.  Symbolic links are
normally followed and their target reported, the link itself is
reported if the target does not exist.

Under the @code{physical} option described below, @code{symlink} is
instead given for symbolic links whose target does exist.

@item stale-symlink
Under the @code{physical} option described below, this indicates
@var{filename} is a dangling symbolic link, meaning its target does
not exist.  Without the @code{physical} option plain @code{symlink}
indicates this.
@end table

The following optional arguments can be given to modify the way
@code{nftw} works.  Each is passed as a symbol (and @code{hash-size}
takes a following integer value).

@table @asis
@item @code{chdir}
Change to the directory containing the item before calling @var{proc}.
When @code{nftw} returns the original current directory is restored.

Under this option, generally the @var{basename} parameter should be
used to access the item in each @var{proc} call.  The @var{filename}
parameter still has a path as normal and this will only be valid if
the @var{startname} directory was absolute.

@item @code{depth}
Visit files ``depth first'', meaning @var{proc} is called for the
contents of each directory before it's called for the directory
itself.  Normally a directory is reported first, then its contents.

Under this option, the @var{flag} to @var{proc} for a directory is
@code{directory-processed} instead of @code{directory}.

@item @code{hash-size @var{n}}
Set the size of the hash table used to track items already visited.
(@pxref{Hash Table Reference})

@item @code{mount}
Don't cross a mount point, meaning only visit items on the same
filesystem as @var{startname}.  (Ie.@: the same @code{stat:dev}.)

@item @code{physical}
Don't follow symbolic links, instead report them to @var{proc} as
@code{symlink}, and report dangling links as @code{stale-symlink}.
@end table

The return value from @code{nftw} is @code{#t} if it ran to
completion, or otherwise the non-@code{#t} value from @var{proc} which
caused the stop.

@c  For reference, one reason not to esacpe is that the current
@c  directory is not saved and restored with dynamic-wind.  Maybe
@c  changing that would be enough to allow escaping.
@c
In the current implementation, returning non-@code{#t} from @var{proc}
is the only valid way to terminate @code{ftw}.  @var{proc} must not
use @code{throw} or similar to escape.
@end defun


@node Queues
@section Queues
@cindex queues
@tindex Queues

@noindent
The functions in this section are provided by

@example
(use-modules (ice-9 q))
@end example

This module implements queues holding arbitrary scheme objects and
designed for efficient first-in / first-out operations.

@code{make-q} creates a queue, and objects are entered and removed
with @code{enq!} and @code{deq!}.  @code{q-push!}  and @code{q-pop!}
can be used too, treating the front of the queue like a stack.

@sp 1

@deffn {Scheme Procedure} make-q
Return a new queue.
@end deffn

@deffn {Scheme Procedure} q? obj
Return @code{#t} if @var{obj} is a queue, or @code{#f} if not.

Note that queues are not a distinct class of objects but are
implemented with cons cells.  For that reason certain list structures
can get @code{#t} from @code{q?}.
@end deffn

@deffn {Scheme Procedure} enq! q obj
Add @var{obj} to the rear of @var{q}, and return @var{q}.
@end deffn

@deffn {Scheme Procedure} deq! q
@deffnx {Scheme Procedure} q-pop! q
Remove and return the front element from @var{q}.  If @var{q} is
empty, a @code{q-empty} exception is thrown.

@code{deq!} and @code{q-pop!} are the same operation, the two names
just let an application match @code{enq!} with @code{deq!}, or
@code{q-push!} with @code{q-pop!}.
@end deffn

@deffn {Scheme Procedure} q-push! q obj
Add @var{obj} to the front of @var{q}, and return @var{q}.
@end deffn

@deffn {Scheme Procedure} q-length q
Return the number of elements in @var{q}.
@end deffn

@deffn {Scheme Procedure} q-empty? q
Return true if @var{q} is empty.
@end deffn

@deffn {Scheme Procedure} q-empty-check q
Throw a @code{q-empty} exception if @var{q} is empty.
@end deffn

@deffn {Scheme Procedure} q-front q
Return the first element of @var{q} (without removing it).  If @var{q}
is empty, a @code{q-empty} exception is thrown.
@end deffn

@deffn {Scheme Procedure} q-rear q
Return the last element of @var{q} (without removing it).  If @var{q}
is empty, a @code{q-empty} exception is thrown.
@end deffn

@deffn {Scheme Procedure} q-remove! q obj
Remove all occurences of @var{obj} from @var{q}, and return @var{q}.
@var{obj} is compared to queue elements using @code{eq?}.
@end deffn

@sp 1
@cindex @code{q-empty}
The @code{q-empty} exceptions described above are thrown just as
@code{(throw 'q-empty)}, there's no message etc like an error throw.

A queue is implemented as a cons cell, the @code{car} containing a
list of queued elements, and the @code{cdr} being the last cell in
that list (for ease of enqueuing).

@example
(@var{list} . @var{last-cell})
@end example

@noindent
If the queue is empty, @var{list} is the empty list and
@var{last-cell} is @code{#f}.

An application can directly access the queue list if desired, for
instance to search the elements or to insert at a specific point.

@deffn {Scheme Procedure} sync-q! q
Recompute the @var{last-cell} field in @var{q}.

All the operations above maintain @var{last-cell} as described, so
normally there's no need for @code{sync-q!}.  But if an application
modifies the queue @var{list} then it must either maintain
@var{last-cell} similarly, or call @code{sync-q!} to recompute it.
@end deffn


@node Streams
@section Streams
@cindex streams

A stream represents a sequence of values, each of which is calculated
only when required.  This allows large or even infinite sequences to
be represented and manipulated with familiar operations like ``car'',
``cdr'', ``map'' or ``fold''.  In such manipulations only as much as
needed is actually held in memory at any one time.  The functions in
this section are available from

@example
(use-modules (ice-9 streams))
@end example

Streams are implemented using promises (@pxref{Delayed Evaluation}),
which is how the underlying calculation of values is made only when
needed, and the values then retained so the calculation is not
repeated.

@noindent
Here is a simple example producing a stream of all odd numbers,

@example
(define odds (make-stream (lambda (state)
                            (cons state (+ state 2)))
                          1))
(stream-car odds)              @result{} 1
(stream-car (stream-cdr odds)) @result{} 3
@end example

@noindent
@code{stream-map} could be used to derive a stream of odd squares,

@example
(define (square n) (* n n))
(define oddsquares (stream-map square odds))
@end example

These are infinite sequences, so it's not possible to convert them to
a list, but they could be printed (infinitely) with for example

@example
(stream-for-each (lambda (n sq)
                   (format #t "~a squared is ~a\n" n sq))
                 odds oddsquares)
@print{}
1 squared is 1
3 squared is 9
5 squared is 25
7 squared is 49
@dots{}
@end example

@sp 1
@defun make-stream proc initial-state
Return a new stream, formed by calling @var{proc} successively.

Each call is @code{(@var{proc} @var{state})}, it should return a pair,
the @code{car} being the value for the stream, and the @code{cdr}
being the new @var{state} for the next call.  For the first call
@var{state} is the given @var{initial-state}.  At the end of the
stream, @var{proc} should return some non-pair object.
@end defun

@defun stream-car stream
Return the first element from @var{stream}.  @var{stream} must not be
empty.
@end defun

@defun stream-cdr stream
Return a stream which is the second and subsequent elements of
@var{stream}.  @var{stream} must not be empty.
@end defun

@defun stream-null? stream
Return true if @var{stream} is empty.
@end defun

@defun list->stream list
@defunx vector->stream vector
Return a stream with the contents of @var{list} or @var{vector}.

@var{list} or @var{vector} should not be modified subsequently, since
it's unspecified whether changes there will be reflected in the stream
returned.
@end defun

@defun port->stream port readproc
Return a stream which is the values obtained by reading from
@var{port} using @var{readproc}.  Each read call is
@code{(@var{readproc} @var{port})}, and it should return an EOF object
(@pxref{Reading}) at the end of input.

For example a stream of characters from a file,

@example
(port->stream (open-input-file "/foo/bar.txt") read-char)
@end example
@end defun

@defun stream->list stream
Return a list which is the entire contents of @var{stream}.
@end defun

@defun stream->reversed-list stream
Return a list which is the entire contents of @var{stream}, but in
reverse order.
@end defun

@defun stream->list&length stream
Return two values (@pxref{Multiple Values}) being a list which is the
entire contents of @var{stream}, and the number of elements in that
list.
@end defun

@defun stream->reversed-list&length stream
Return two values (@pxref{Multiple Values}) being a list which is the
entire contents of @var{stream}, but in reverse order, and the number
of elements in that list.
@end defun

@defun stream->vector stream
Return a vector which is the entire contents of @var{stream}.
@end defun

@defun stream-fold proc init stream0 @dots{} streamN
Apply @var{proc} successively over the elements of the given streams,
from first to last until the end of the shortest stream is reached.
Return the result from the last @var{proc} call.

Each call is @code{(@var{proc} elem0 @dots{} elemN prev)}, where each
@var{elem} is from the corresponding @var{stream}.  @var{prev} is the
return from the previous @var{proc} call, or the given @var{init} for
the first call.
@end defun

@defun stream-for-each proc stream0 @dots{} streamN
Call @var{proc} on the elements from the given @var{stream}s.  The
return value is unspecified.

Each call is @code{(@var{proc} elem0 @dots{} elemN)}, where each
@var{elem} is from the corresponding @var{stream}.
@code{stream-for-each} stops when it reaches the end of the shortest
@var{stream}.
@end defun

@defun stream-map proc stream0 @dots{} streamN
Return a new stream which is the results of applying @var{proc} to the
elements of the given @var{stream}s.

Each call is @code{(@var{proc} elem0 @dots{} elemN)}, where each
@var{elem} is from the corresponding @var{stream}.  The new stream
ends when the end of teh shortest given @var{stream} is reached.
@end defun


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