@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2009,
@c   2010, 2011, 2012  Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@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 entering the following:

@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.

@item @nicode{#:max-expr-width} @var{columns}
The maximum width of an expression.  The default is 50.
@end table
@end deffn


@cindex truncated printing
Also exported by the @code{(ice-9 pretty-print)} module is
@code{truncated-print}, a procedure to print Scheme datums, truncating
the output to a certain number of characters. This is useful when you
need to present an arbitrary datum to the user, but you only have one
line in which to do so.

@lisp
(define exp '(a b #(c d e) f . g))
(truncated-print exp #:width 10) (newline)
@print{} (a b . #)
(truncated-print exp #:width 15) (newline)
@print{} (a b # f . g)
(truncated-print exp #:width 18) (newline)
@print{} (a b #(c ...) . #)
(truncated-print exp #:width 20) (newline)
@print{} (a b #(c d e) f . g)
(truncated-print "The quick brown fox" #:width 20) (newline)
@print{} "The quick brown..."
(truncated-print (current-module) #:width 20) (newline)
@print{} #<directory (gui...>
@end lisp

@code{truncated-print} will not output a trailing newline. If an expression does
not fit in the given width, it will be truncated -- possibly
ellipsized@footnote{On Unicode-capable ports, the ellipsis is represented by
character `HORIZONTAL ELLIPSIS' (U+2026), otherwise it is represented by three
dots.}, or in the worst case, displayed as @nicode{#}.

@deffn {Scheme Procedure} truncated-print obj [port] [keyword-options]
Print @var{obj}, truncating the output, if necessary, to make it fit
into @var{width} characters. By default, @var{obj} will be printed using
@code{write}, though that behavior can be overridden via the
@var{display?} keyword argument.

The default behaviour is to print depth-first, meaning that the entire
remaining width will be available to each sub-expression of @var{obj} --
e.g., if @var{obj} is a vector, each member of @var{obj}. One can attempt to
``ration'' the available width, trying to allocate it equally to each
sub-expression, via the @var{breadth-first?} keyword argument.

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{#:width} @var{columns}
Print within the given @var{columns}.  The default is 79.

@item @nicode{#:breadth-first?} @var{flag}
If @var{flag} is true, then allocate the available width breadth-first
among elements of a compound data structure (list, vector, pair,
etc.). The default is @code{#f} which means that any element is
allowed to consume all of the available width.
@end table
@end deffn


@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 arg @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}), 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), in a locale-independent way.

@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).  However, when
your intent is to write numbers in a way that follows typographical
conventions, using @nicode{~h} is recommended.

@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 "~6,,,'xf" 12345) @print{} 12345.
(format #t "~5,,,'xf" 12345) @print{} xxxxx
@end example

@item @nicode{~h}
Localized number@footnote{The @nicode{~h} format specifier first
appeared in Guile version 2.0.6.}.  Parameters: @var{width},
@var{decimals}, @var{padchar}.

Like @nicode{~f}, output an exact or floating point number, but do so
according to the current locale, or according to the given locale object
when the @code{:} modifier is used (@pxref{Number Input and Output,
@code{number->locale-string}}).

@example
(format #t "~h" 12345.5678)  ; with "C" as the current locale
@print{} 12345.5678

(format #t "~14,,'*:h" 12345.5678
        (make-locale LC_ALL "en_US"))
@print{} ***12,345.5678

(format #t "~,2:h" 12345.5678
        (make-locale LC_NUMERIC "fr_FR"))
@print{} 12 345,56
@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, i.e. 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}
Structured printing.  Parameters: @var{width}.

@nicode{~y} outputs an argument using @code{pretty-print}
(@pxref{Pretty Printing}). The result will be formatted to fit within
@var{width} columns (79 by default), consuming multiple lines if
necessary.

@nicode{~@@y} outputs an argument using @code{truncated-print}
(@pxref{Pretty Printing}). The resulting code will be formatted to fit
within @var{width} columns (79 by default), on a single line. The
output will be truncated if necessary.

@nicode{~:@@y} is like @nicode{~@@y}, except the @var{width} parameter
is interpreted to be the maximum column to which to output. That is to
say, if you are at column 10, and @nicode{~60:@@y} is seen, the datum
will be truncated to 50 columns.

@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 position 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.

@c  @print{} on a new line here to avoid overflowing page width in DVI
@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.

@c  @print{} on a new line here to avoid overflowing page width in DVI
@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

@cindex file system traversal
@cindex directory traversal

The functions in this section traverse a tree of files and
directories.  They come in two flavors: the first one is a high-level
functional interface, and the second one is 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

@deffn {Scheme Procedure} file-system-tree file-name [enter? [stat]]
Return a tree of the form @code{(@var{file-name} @var{stat}
@var{children} ...)} where @var{stat} is the result of @code{(@var{stat}
@var{file-name})} and @var{children} are similar structures for each
file contained in @var{file-name} when it designates a directory.

The optional @var{enter?} predicate is invoked as @code{(@var{enter?}
@var{name} @var{stat})} and should return true to allow recursion into
directory @var{name}; the default value is a procedure that always
returns @code{#t}.  When a directory does not match @var{enter?}, it
nonetheless appears in the resulting tree, only with zero children.

The @var{stat} argument is optional and defaults to @code{lstat}, as for
@code{file-system-fold} (see below.)

The example below shows how to obtain a hierarchical listing of the
files under the @file{module/language} directory in the Guile source
tree, discarding their @code{stat} info:

@example
(use-modules (ice-9 match))

(define remove-stat
  ;; Remove the `stat' object the `file-system-tree' provides
  ;; for each file in the tree.
  (match-lambda
    ((name stat)              ; flat file
     name)
    ((name stat children ...) ; directory
     (list name (map remove-stat children)))))

(let ((dir (string-append (assq-ref %guile-build-info 'top_srcdir)
                          "/module/language")))
  (remove-stat (file-system-tree dir)))

@result{}
("language"
 (("value" ("spec.go" "spec.scm"))
  ("scheme"
   ("spec.go"
    "spec.scm"
    "compile-tree-il.scm"
    "decompile-tree-il.scm"
    "decompile-tree-il.go"
    "compile-tree-il.go"))
  ("tree-il"
   ("spec.go"
    "fix-letrec.go"
    "inline.go"
    "fix-letrec.scm"
    "compile-glil.go"
    "spec.scm"
    "optimize.scm"
    "primitives.scm"
    @dots{}))
  @dots{}))
@end example
@end deffn

@cindex file system combinator

It is often desirable to process directories entries directly, rather
than building up a tree of entries in memory, like
@code{file-system-tree} does.  The following procedure, a
@dfn{combinator}, is designed to allow directory entries to be processed
directly as a directory tree is traversed; in fact,
@code{file-system-tree} is implemented in terms of it.

@deffn {Scheme Procedure} file-system-fold enter? leaf down up skip error init file-name [stat]
Traverse the directory at @var{file-name}, recursively, and return the
result of the successive applications of the @var{leaf}, @var{down},
@var{up}, and @var{skip} procedures as described below.

Enter sub-directories only when @code{(@var{enter?} @var{path}
@var{stat} @var{result})} returns true.  When a sub-directory is
entered, call @code{(@var{down} @var{path} @var{stat} @var{result})},
where @var{path} is the path of the sub-directory and @var{stat} the
result of @code{(false-if-exception (@var{stat} @var{path}))}; when it is
left, call @code{(@var{up} @var{path} @var{stat} @var{result})}.

For each file in a directory, call @code{(@var{leaf} @var{path}
@var{stat} @var{result})}.

When @var{enter?} returns @code{#f}, or when an unreadable directory is
encountered, call @code{(@var{skip} @var{path} @var{stat}
@var{result})}.

When @var{file-name} names a flat file, @code{(@var{leaf} @var{path}
@var{stat} @var{init})} is returned.

When an @code{opendir} or @var{stat} call fails, call @code{(@var{error}
@var{path} @var{stat} @var{errno} @var{result})}, with @var{errno} being
the operating system error number that was raised---e.g.,
@code{EACCES}---and @var{stat} either @code{#f} or the result of the
@var{stat} call for that entry, when available.

The special @file{.} and @file{..} entries are not passed to these
procedures.  The @var{path} argument to the procedures is a full file
name---e.g., @code{"../foo/bar/gnu"}; if @var{file-name} is an absolute
file name, then @var{path} is also an absolute file name.  Files and
directories, as identified by their device/inode number pair, are
traversed only once.

The optional @var{stat} argument defaults to @code{lstat}, which means
that symbolic links are not followed; the @code{stat} procedure can be
used instead when symbolic links are to be followed (@pxref{File System,
stat}).

The example below illustrates the use of @code{file-system-fold}:

@example
(define (total-file-size file-name)
  "Return the size in bytes of the files under FILE-NAME (similar
to `du --apparent-size' with GNU Coreutils.)"

  (define (enter? name stat result)
    ;; Skip version control directories.
    (not (member (basename name) '(".git" ".svn" "CVS"))))
  (define (leaf name stat result)
    ;; Return RESULT plus the size of the file at NAME.
    (+ result (stat:size stat)))

  ;; Count zero bytes for directories.
  (define (down name stat result) result)
  (define (up name stat result) result)

  ;; Likewise for skipped directories.
  (define (skip name stat result) result)

  ;; Ignore unreadable files/directories but warn the user.
  (define (error name stat errno result)
    (format (current-error-port) "warning: ~a: ~a~%"
            name (strerror errno))
    result)

  (file-system-fold enter? leaf down up skip error
                           0  ; initial counter is zero bytes
                           file-name))

(total-file-size ".")
@result{} 8217554

(total-file-size "/dev/null")
@result{} 0
@end example
@end deffn

The alternative C-like functions are described below.

@deffn {Scheme Procedure} scandir name [select? [entry<?]]
Return the list of the names of files contained in directory @var{name}
that match predicate @var{select?} (by default, all files).  The
returned list of file names is sorted according to @var{entry<?}, which
defaults to @code{string-locale<?} such that file names are sorted in
the locale's alphabetical order (@pxref{Text Collation}).  Return
@code{#f} when @var{name} is unreadable or is not a directory.

This procedure is modeled after the C library function of the same name
(@pxref{Scanning Directory Content,,, libc, GNU C Library Reference
Manual}).
@end deffn

@deffn {Scheme Procedure} ftw startname proc ['hash-size n]
Walk the file system 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 deffn


@deffn {Scheme Procedure} nftw startname proc ['chdir] ['depth] ['hash-size n] ['mount] ['physical]
Walk the file system 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.

Like @code{ftw}, 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{nftw}
is safe against circular linked directory structures.

Each @var{proc} call is @code{(@var{proc} filename statinfo flag
base 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{base}
is an integer offset into @var{filename} which is where the basename
for this item begins.  @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, including 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 stale-symlink
@var{filename} is a dangling symbolic link.  Links are normally
followed and their target reported, the link itself is reported if its
target does not exist.

@item symlink
When the @code{physical} option described below is used, this
indicates @var{filename} is a symbolic link whose target exists (and
is not being followed).
@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{base} parameter to each
@var{proc} call should be used to pick out the base part of the
@var{filename}.  The @var{filename} is still a path but with a changed
directory it won't be valid (unless 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
file system 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}.  Dangling links (those whose target doesn't exist) are
still reported 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 escape 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 deffn


@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 occurrences 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

This section documents Guile's legacy stream module.  For a more
complete and portable stream library, @pxref{SRFI-41}.

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
@deffn {Scheme Procedure} 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 deffn

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

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

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

@deffn {Scheme Procedure} list->stream list
@deffnx {Scheme Procedure} 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 deffn

@deffn {Scheme Procedure} 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 deffn

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

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

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

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

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

@defun stream-fold proc init stream1 stream2 @dots{}
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} elem1 elem2 @dots{} 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 stream1 stream2 @dots{}
Call @var{proc} on the elements from the given @var{stream}s.  The
return value is unspecified.

Each call is @code{(@var{proc} elem1 elem2 @dots{})}, 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 stream1 stream2 @dots{}
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} elem1 elem2 @dots{})}, where each
@var{elem} is from the corresponding @var{stream}.  The new stream
ends when the end of the shortest given @var{stream} is reached.
@end defun


@node Buffered Input
@section Buffered Input
@cindex Buffered input
@cindex Line continuation

The following functions are provided by

@example
(use-modules (ice-9 buffered-input))
@end example

A buffered input port allows a reader function to return chunks of
characters which are to be handed out on reading the port.  A notion
of further input for an application level logical expression is
maintained too, and passed through to the reader.

@deffn {Scheme Procedure} make-buffered-input-port reader
Create an input port which returns characters obtained from the given
@var{reader} function.  @var{reader} is called (@var{reader} cont),
and should return a string or an EOF object.

The new port gives precisely the characters returned by @var{reader},
nothing is added, so if any newline characters or other separators are
desired they must come from the reader function.

The @var{cont} parameter to @var{reader} is @code{#f} for initial
input, or @code{#t} when continuing an expression.  This is an
application level notion, set with
@code{set-buffered-input-continuation?!} below.  If the user has
entered a partial expression then it allows @var{reader} for instance
to give a different prompt to show more is required.
@end deffn

@deffn {Scheme Procedure} make-line-buffered-input-port reader
@cindex Line buffered input
Create an input port which returns characters obtained from the
specified @var{reader} function, similar to
@code{make-buffered-input-port} above, but where @var{reader} is
expected to be a line-oriented.

@var{reader} is called (@var{reader} cont), and should return a string
or an EOF object as above.  Each string is a line of input without a
newline character, the port code inserts a newline after each string.
@end deffn

@deffn {Scheme Procedure} set-buffered-input-continuation?! port cont
Set the input continuation flag for a given buffered input
@var{port}.

An application uses this by calling with a @var{cont} flag of
@code{#f} when beginning to read a new logical expression.  For
example with the Scheme @code{read} function (@pxref{Scheme Read}),

@example
(define my-port (make-buffered-input-port my-reader))

(set-buffered-input-continuation?! my-port #f)
(let ((obj (read my-port)))
  ...
@end example
@end deffn


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