guile/doc/sources/format.texi

@menu
* Format Interface::            
* Format Specification::        
@end menu

@node Format Interface, Format Specification, Format, Format
@subsection Format Interface

@defun format destination format-string . arguments
An almost complete implementation of Common LISP format description
according to the CL reference book @cite{Common LISP} from Guy L.
Steele, Digital Press.  Backward compatible to most of the available
Scheme format implementations.

Returns @code{#t}, @code{#f} or a string; has side effect of printing
according to @var{format-string}.  If @var{destination} is @code{#t},
the output is to the current output port and @code{#t} is returned.  If
@var{destination} is @code{#f}, a formatted string is returned as the
result of the call.  NEW: If @var{destination} is a string,
@var{destination} is regarded as the format string; @var{format-string} is
then the first argument and the output is returned as a string. If
@var{destination} is a number, the output is to the current error port
if available by the implementation. Otherwise @var{destination} must be
an output port and @code{#t} is returned.@refill

@var{format-string} must be a string.  In case of a formatting error
format returns @code{#f} and prints a message on the current output or
error port.  Characters are output as if the string were output by the
@code{display} function with the exception of those prefixed by a tilde
(~).  For a detailed description of the @var{format-string} syntax
please consult a Common LISP format reference manual.  For a test suite
to verify this format implementation load @file{formatst.scm}. Please
send bug reports to @code{lutzeb@@cs.tu-berlin.de}.

Note: @code{format} is not reentrant, i.e. only one @code{format}-call
may be executed at a time.

@end defun

@node Format Specification,  , Format Interface, Format
@subsection Format Specification (Format version 3.0)

Please consult a Common LISP format reference manual for a detailed
description of the format string syntax.  For a demonstration of the
implemented directives see @file{formatst.scm}.@refill

This implementation supports directive parameters and modifiers
(@code{:} and @code{@@} characters). Multiple parameters must be
separated by a comma (@code{,}).  Parameters can be numerical parameters
(positive or negative), character parameters (prefixed by a quote
character (@code{'}), variable parameters (@code{v}), number of rest
arguments parameter (@code{#}), empty and default parameters.  Directive
characters are case independent. The general form of a directive
is:@refill

@noindent
@var{directive} ::= ~@{@var{directive-parameter},@}[:][@@]@var{directive-character}

@noindent
@var{directive-parameter} ::= [ [-|+]@{0-9@}+ | '@var{character} | v | # ]


@subsubsection Implemented CL Format Control Directives

Documentation syntax: Uppercase characters represent the corresponding
control directive characters. Lowercase characters represent control
directive parameter descriptions.

@table @asis
@item @code{~A}
Any (print as @code{display} does).
@table @asis
@item @code{~@@A}
left pad.
@item @code{~@var{mincol},@var{colinc},@var{minpad},@var{padchar}A}
full padding.
@end table
@item @code{~S}
S-expression (print as @code{write} does).
@table @asis
@item @code{~@@S}
left pad.
@item @code{~@var{mincol},@var{colinc},@var{minpad},@var{padchar}S}
full padding.
@end table
@item @code{~D}
Decimal.
@table @asis
@item @code{~@@D}
print number sign always.
@item @code{~:D}
print comma separated.
@item @code{~@var{mincol},@var{padchar},@var{commachar}D}
padding.
@end table
@item @code{~X}
Hexadecimal.
@table @asis
@item @code{~@@X}
print number sign always.
@item @code{~:X}
print comma separated.
@item @code{~@var{mincol},@var{padchar},@var{commachar}X}
padding.
@end table
@item @code{~O}
Octal.
@table @asis
@item @code{~@@O}
print number sign always.
@item @code{~:O}
print comma separated.
@item @code{~@var{mincol},@var{padchar},@var{commachar}O}
padding.
@end table
@item @code{~B}
Binary.
@table @asis
@item @code{~@@B}
print number sign always.
@item @code{~:B}
print comma separated.
@item @code{~@var{mincol},@var{padchar},@var{commachar}B}
padding.
@end table
@item @code{~@var{n}R}
Radix @var{n}.
@table @asis
@item @code{~@var{n},@var{mincol},@var{padchar},@var{commachar}R}
padding.
@end table
@item @code{~@@R}
print a number as a Roman numeral.
@item @code{~:@@R}
print a number as an ``old fashioned'' Roman numeral.
@item @code{~:R}
print a number as an ordinal English number.
@item @code{~:@@R}
print a number as a cardinal English number.
@item @code{~P}
Plural.
@table @asis
@item @code{~@@P}
prints @code{y} and @code{ies}.
@item @code{~:P}
as @code{~P but jumps 1 argument backward.}
@item @code{~:@@P}
as @code{~@@P but jumps 1 argument backward.}
@end table
@item @code{~C}
Character.
@table @asis
@item @code{~@@C}
prints a character as the reader can understand it (i.e. @code{#\} prefixing).
@item @code{~:C}
prints a character as emacs does (eg. @code{^C} for ASCII 03).
@end table
@item @code{~F}
Fixed-format floating-point (prints a flonum like @var{mmm.nnn}).
@table @asis
@item @code{~@var{width},@var{digits},@var{scale},@var{overflowchar},@var{padchar}F}
@item @code{~@@F}
If the number is positive a plus sign is printed.
@end table
@item @code{~E}
Exponential floating-point (prints a flonum like @var{mmm.nnn}@code{E}@var{ee}).
@table @asis
@item @code{~@var{width},@var{digits},@var{exponentdigits},@var{scale},@var{overflowchar},@var{padchar},@var{exponentchar}E}
@item @code{~@@E}
If the number is positive a plus sign is printed.
@end table
@item @code{~G}
General floating-point (prints a flonum either fixed or exponential).
@table @asis
@item @code{~@var{width},@var{digits},@var{exponentdigits},@var{scale},@var{overflowchar},@var{padchar},@var{exponentchar}G}
@item @code{~@@G}
If the number is positive a plus sign is printed.
@end table
@item @code{~$}
Dollars floating-point (prints a flonum in fixed with signs separated).
@table @asis
@item @code{~@var{digits},@var{scale},@var{width},@var{padchar}$}
@item @code{~@@$}
If the number is positive a plus sign is printed.
@item @code{~:@@$}
A sign is always printed and appears before the padding.
@item @code{~:$}
The sign appears before the padding.
@end table
@item @code{~%}
Newline.
@table @asis
@item @code{~@var{n}%}
print @var{n} newlines.
@end table
@item @code{~&}
print newline if not at the beginning of the output line.
@table @asis
@item @code{~@var{n}&}
prints @code{~&} and then @var{n-1} newlines.
@end table
@item @code{~|}
Page Separator.
@table @asis
@item @code{~@var{n}|}
print @var{n} page separators.
@end table
@item @code{~~}
Tilde.
@table @asis
@item @code{~@var{n}~}
print @var{n} tildes.
@end table
@item @code{~}<newline>
Continuation Line.
@table @asis
@item @code{~:}<newline>
newline is ignored, white space left.
@item @code{~@@}<newline>
newline is left, white space ignored.
@end table
@item @code{~T}
Tabulation.
@table @asis
@item @code{~@@T}
relative tabulation.
@item @code{~@var{colnum,colinc}T}
full tabulation.
@end table
@item @code{~?}
Indirection (expects indirect arguments as a list).
@table @asis
@item @code{~@@?}
extracts indirect arguments from format arguments.
@end table
@item @code{~(@var{str}~)}
Case conversion (converts by @code{string-downcase}).
@table @asis
@item @code{~:(@var{str}~)}
converts by @code{string-capitalize}.
@item @code{~@@(@var{str}~)}
converts by @code{string-capitalize-first}.
@item @code{~:@@(@var{str}~)}
converts by @code{string-upcase}.
@end table
@item @code{~*}
Argument Jumping (jumps 1 argument forward).
@table @asis
@item @code{~@var{n}*}
jumps @var{n} arguments forward.
@item @code{~:*}
jumps 1 argument backward.
@item @code{~@var{n}:*}
jumps @var{n} arguments backward.
@item @code{~@@*}
jumps to the 0th argument.
@item @code{~@var{n}@@*}
jumps to the @var{n}th argument (beginning from 0)
@end table
@item @code{~[@var{str0}~;@var{str1}~;...~;@var{strn}~]}
Conditional Expression (numerical clause conditional).
@table @asis
@item @code{~@var{n}[}
take argument from @var{n}.
@item @code{~@@[}
true test conditional.
@item @code{~:[}
if-else-then conditional.
@item @code{~;}
clause separator.
@item @code{~:;}
default clause follows.
@end table
@item @code{~@{@var{str}~@}}
Iteration (args come from the next argument (a list)).
@table @asis
@item @code{~@var{n}@{}
at most @var{n} iterations.
@item @code{~:@{}
args from next arg (a list of lists).
@item @code{~@@@{}
args from the rest of arguments.
@item @code{~:@@@{}
args from the rest args (lists).
@end table
@item @code{~^}
Up and out.
@table @asis
@item @code{~@var{n}^}
aborts if @var{n} = 0
@item @code{~@var{n},@var{m}^}
aborts if @var{n} = @var{m}
@item @code{~@var{n},@var{m},@var{k}^}
aborts if @var{n} <= @var{m} <= @var{k}
@end table
@end table


@subsubsection Not Implemented CL Format Control Directives

@table @asis
@item @code{~:A}
print @code{#f} as an empty list (see below).
@item @code{~:S}
print @code{#f} as an empty list (see below).
@item @code{~<~>}
Justification.
@item @code{~:^}
(sorry I don't understand its semantics completely)
@end table


@subsubsection Extended, Replaced and Additional Control Directives

@table @asis
@item @code{~@var{mincol},@var{padchar},@var{commachar},@var{commawidth}D}
@item @code{~@var{mincol},@var{padchar},@var{commachar},@var{commawidth}X}
@item @code{~@var{mincol},@var{padchar},@var{commachar},@var{commawidth}O}
@item @code{~@var{mincol},@var{padchar},@var{commachar},@var{commawidth}B}
@item @code{~@var{n},@var{mincol},@var{padchar},@var{commachar},@var{commawidth}R}
@var{commawidth} is the number of characters between two comma characters.
@end table

@table @asis
@item @code{~I}
print an R5RS complex number as @code{~F~@@Fi} with passed parameters for
@code{~F}.
@item @code{~Y}
Pretty print formatting of an argument for scheme code lists.
@item @code{~K}
Same as @code{~?.}
@item @code{~!}
Flushes the output if format @var{destination} is a port.
@item @code{~_}
Print a @code{#\space} character
@table @asis
@item @code{~@var{n}_}
print @var{n} @code{#\space} characters.
@end table
@item @code{~/}
Print a @code{#\tab} character
@table @asis
@item @code{~@var{n}/}
print @var{n} @code{#\tab} characters.
@end table
@item @code{~@var{n}C}
Takes @var{n} as an integer representation for a character. No arguments
are consumed. @var{n} is converted to a character by
@code{integer->char}.  @var{n} must be a positive decimal number.@refill
@item @code{~:S}
Print out readproof.  Prints out internal objects represented as
@code{#<...>} as strings @code{"#<...>"} so that the format output can always
be processed by @code{read}.
@refill
@item @code{~:A}
Print out readproof.  Prints out internal objects represented as
@code{#<...>} as strings @code{"#<...>"} so that the format output can always
be processed by @code{read}.
@item @code{~Q}
Prints information and a copyright notice on the format implementation.
@table @asis
@item @code{~:Q}
prints format version.
@end table
@refill
@item @code{~F, ~E, ~G, ~$}
may also print number strings, i.e. passing a number as a string and
format it accordingly.
@end table

@subsubsection Configuration Variables

Format has some configuration variables at the beginning of
@file{format.scm} to suit the systems and users needs. There should be
no modification necessary for the configuration that comes with SLIB.
If modification is desired the variable should be set after the format
code is loaded. Format detects automatically if the running scheme
system implements floating point numbers and complex numbers.

@table @asis

@item @var{format:symbol-case-conv}
Symbols are converted by @code{symbol->string} so the case type of the
printed symbols is implementation dependent.
@code{format:symbol-case-conv} is a one arg closure which is either
@code{#f} (no conversion), @code{string-upcase}, @code{string-downcase}
or @code{string-capitalize}. (default @code{#f})

@item @var{format:iobj-case-conv}
As @var{format:symbol-case-conv} but applies for the representation of
implementation internal objects. (default @code{#f})

@item @var{format:expch}
The character prefixing the exponent value in @code{~E} printing. (default
@code{#\E})

@end table

@subsubsection Compatibility With Other Format Implementations

@table @asis
@item SLIB format 2.x:
See @file{format.doc}.

@item SLIB format 1.4:
Downward compatible except for padding support and @code{~A}, @code{~S},
@code{~P}, @code{~X} uppercase printing.  SLIB format 1.4 uses C-style
@code{printf} padding support which is completely replaced by the CL
@code{format} padding style.

@item MIT C-Scheme 7.1:
Downward compatible except for @code{~}, which is not documented
(ignores all characters inside the format string up to a newline
character).  (7.1 implements @code{~a}, @code{~s},
~@var{newline}, @code{~~}, @code{~%}, numerical and variable
parameters and @code{:/@@} modifiers in the CL sense).@refill

@item Elk 1.5/2.0:
Downward compatible except for @code{~A} and @code{~S} which print in
uppercase.  (Elk implements @code{~a}, @code{~s}, @code{~~}, and
@code{~%} (no directive parameters or modifiers)).@refill

@item Scheme->C 01nov91:
Downward compatible except for an optional destination parameter: S2C
accepts a format call without a destination which returns a formatted
string. This is equivalent to a #f destination in S2C. (S2C implements
@code{~a}, @code{~s}, @code{~c}, @code{~%}, and @code{~~} (no directive
parameters or modifiers)).@refill

@end table

This implementation of format is solely useful in the SLIB context
because it requires other components provided by SLIB.@refill