Pretty Printing
***************

The module (ice-9 pretty-print) provides the procedure 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.

     (use-modules (ice-9 pretty-print))

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

     (pretty-print '(define (foo) (lambda (x)
     (cond ((zero? x) #t) ((negative? x) -x) (else (if (= x 1) 2 (* x x x)))))))
     -|
     (define (foo)
       (lambda (x)
         (cond ((zero? x) #t)
               ((negative? x) -x)
               (else (if (= x 1) 2 (* x x x))))))

 -- Scheme Procedure: pretty-print obj [port]
     Print the textual representation of the Scheme object OBJ to PORT.  PORT defaults to the current output port, if not given.

Beware: Since pretty-print uses it's own write procedure, it's output will not be the same as for example the output of write. Consider the following example.

     (write (lambda (x) x))
     -|
     #<procedure #f (x)>

     (pretty-print (lambda (x) x))
     -|
     #[procedure]

The reason is that pretty-print does not know as much about Guile's object types as the builtin procedures.  This is particularly important for smobs, for which a write procedure can be defined and be used by write, but not by pretty-print.

Formatted Output
****************

Outputting messages or other texts which are composed of literal strings, variable contents, newlines and other formatting can be cumbersome, when only the standard procedures like display, write and newline are available.  Additionally, one often wants to collect the output in strings.  With the standard routines, the user is required to set up a string port, add this port as a parameter to the output procedure calls and then retrieve the resulting string from the string port.

The format procedure, to be found in module (ice-9 format), can do all this, and even more.  If you are a C programmer, you can think of this procedure as Guile's fprintf.

 -- Scheme Procedure: format destination format-string args ...
     The first parameter is the DESTINATION, it determines where the output of format will go.

     #t
          Send the formatted output to the current output port and return #t.

     #f
          Return the formatted output as a string.

     Any number value
          Send the formatted output to the current error port and return #t.

     A valid output port
          Send the formatted output to the port DESTINATION and return #t.

     The second parameter is the format string.  It has a similar function to the format string in calls to printf or fprintf in C. It is output to the specified destination, but all escape sequences are replaced by the results of formatting the corresponding sequence.

     Note that escape sequences are marked with the character ~ (tilde), and not with a % (percent sign), as in C.

     The escape sequences in the following table are supported.  When there appears ``corresponding ARG', that means any of the additional arguments, after dropping all arguments which have been used up by escape sequences which have been processed earlier.  Some of the format characters (the characters following the tilde) can be prefixed by :, @, or :@, to modify the behaviour of the format character.  How the modified behaviour differs from the default behaviour is described for every character in the table where appropriate.

     ~~
          Output a single ~ (tilde) character.

     ~%
          Output a newline character, thus advancing to the next output line.

     ~&
          Start a new line, that is, output a newline character if not already at the start of a line.

     ~_
          Output a single space character.

     ~/
          Output a single tabulator character.

     ~|
          Output a page separator (formfeed) character.

     ~t
          Advance to the next tabulator position.

     ~y
          Pretty-print the correspinding ARG.

     ~a
          Output the corresponding ARG like display.

     ~s
          Output the corresponding ARG like write.

     ~d
          Output the corresponding ARG as a decimal number.

     ~x
          Output the corresponding ARG as a hexadecimal number.

     ~o
          Output the corresponding ARG as an octal number.

     ~b
          Output the corresponding ARG as a binary number.

     ~r
          Output the corresponding ARG as a number word, e.g. 10 prints as ten.  If prefixed with :, tenth is printed, if prefixed with :@, roman numbers are printed.

     ~f
          Output the corresponding ARG as a fixed format floating point number, such as 1.34.

     ~e
          Output the corresponding ARG in exponential notation, such as 1.34E+0.

     ~g
          This works either like ~f or like ~e, whichever produces less characters to be written.

     ~$
          Like ~f, but only with two digits after the decimal point.

     ~i
          Output the corresponding ARG as a complex number.

     ~c
          Output the corresponding ARG as a character.  If prefixed with @, it is printed like with write.  If prefixed with :, control characters are treated specially, for example #\newline will be printed as ^J.

     ~p
          ``Plural''.  If the corresponding ARG is 1, nothing is printed (or y if prefixed with @ or :@), otherwise s is printed (or ies if prefixed with @ or :@).

     ~?, ~k
          Take the corresponding argument as a format string, and the following argument as a list of values.  Then format the values with respect to the format string.

     ~!
          Flush the output to the output port.

     ~#\newline (tilde-newline)
          Continuation lines.

     ~*
          Argument jumping. Navigate in the argument list as specified by the corresponding argument.  If prefixed with :, jump backwards in the argument list, if prefixed by :@, jump to the parameter with the absolute index, otherwise jump forward in the argument list.

     ~(
          Case conversion begin.  If prefixed by :, the following output string will be capitalized, if prefixed by @, the first character will be capitalized, if prefixed by :@ it will be upcased and otherwise it will be downcased.  Conversion stops when the ``Case conversion end'' ~)sequence is encountered.

     ~)
          Case conversion end.  Stop any case conversion currently in effect.

     ~[
          Conditional begin.

     ~;
          Conditional separator.

     ~]
          Conditional end.

     ~{
          Iteration begin.

     ~}
          Iteration end.

     ~^
          Up and out.

     ~'
          Character parameter.

     ~0 ... ~9, ~-, ~+
          Numeric parameter.

     ~v
          Variable parameter from next argument.

     ~#
          Parameter is number of remaining args.  The number of the remaining arguments is prepended to the list of unprocessed arguments.

     ~,
          Parameter separators.

     ~q
          Inquiry message.  Insert a copyright message into the output.

     If any type conversions should fail (for example when using an escape sequence for number output, but the argument is a string), an error will be signalled.

You may have noticed that Guile contains a format procedure even when the module (ice-9 format) is not loaded.  The default format procedure does not support all escape sequences documented in this chapter, and will signal an error if you try to use one of them.  The reason for providing two versions of format is that the full-featured module is fairly large and requires some time to get loaded.  So the Guile maintainers decided not to load the large version of format by default, so that the start-up time of the interpreter is not unnecessarily increased.

The Rx Regular Expression Library
*********************************

[FIXME: this is taken from Gary and Mark's quick summaries and should be reviewed and expanded.  Rx is pretty stable, so could already be done!]

The guile-lang-allover package provides an interface to Tom Lord's Rx library (currently only to POSIX regular expressions).  Use of the library requires a two step process: compile a regular expression into an efficient structure, then use the structure in any number of string comparisons.

For example, given the regular expression abc. (which matches any string containing abc followed by any single character):

     guile> (define r (regcomp "abc."))
     guile> r
     #<rgx abc.>
     guile> (regexec r "abc")
     #f
     guile> (regexec r "abcd")
     #((0 . 4))
     guile>

The definitions of regcomp and regexec are as follows:

 -- Scheme Procedure: regcomp pattern [flags]
     Compile the regular expression pattern using POSIX rules.  Flags is optional and should be specified using symbolic names:
      -- Variable: REG_EXTENDED
          use extended POSIX syntax

      -- Variable: REG_ICASE
          use case-insensitive matching

      -- Variable: REG_NEWLINE
          allow anchors to match after newline characters in the string and prevents . or [^...] from matching newlines.

     The logior procedure can be used to combine multiple flags. The default is to use POSIX basic syntax, which makes + and ?  literals and \+ and \? operators.  Backslashes in PATTERN must be escaped if specified in a literal string e.g., "\\(a\\)\\?".

 -- Scheme Procedure: regexec regex string [match-pick] [flags]
     Match STRING against the compiled POSIX regular expression REGEX. MATCH-PICK and FLAGS are optional.  Possible flags (which can be combined using the logior procedure) are:

      -- Variable: REG_NOTBOL
          The beginning of line operator won't match the beginning of STRING (presumably because it's not the beginning of a line)

      -- Variable: REG_NOTEOL
          Similar to REG_NOTBOL, but prevents the end of line operator from matching the end of STRING.

     If no match is possible, regexec returns #f.  Otherwise MATCH-PICK determines the return value:

     #t or unspecified: a newly-allocated vector is returned, containing pairs with the indices of the matched part of STRING and any substrings.

     "": a list is returned: the first element contains a nested list with the matched part of STRING surrounded by the the unmatched parts. Remaining elements are matched substrings (if any).  All returned substrings share memory with STRING.

     #f: regexec returns #t if a match is made, otherwise #f.

     vector: the supplied vector is returned, with the first element replaced by a pair containing the indices of the matched portion of STRING and further elements replaced by pairs containing the indices of matched substrings (if any).

     list: a list will be returned, with each member of the list specified by a code in the corresponding position of the supplied list:

     a number: the numbered matching substring (0 for the entire match).

     #\<: the beginning of STRING to the beginning of the part matched by regex.

     #\>: the end of the matched part of STRING to the end of STRING.

     #\c: the "final tag", which seems to be associated with the "cut operator", which doesn't seem to be available through the posix interface.

     e.g., (list #\< 0 1 #\>).  The returned substrings share memory with STRING.

Here are some other procedures that might be used when using regular expressions:

 -- Scheme Procedure: compiled-regexp? obj
     Test whether obj is a compiled regular expression.

 -- Scheme Procedure: regexp->dfa regex [flags]

 -- Scheme Procedure: dfa-fork dfa

 -- Scheme Procedure: reset-dfa! dfa

 -- Scheme Procedure: dfa-final-tag dfa

 -- Scheme Procedure: dfa-continuable? dfa

 -- Scheme Procedure: advance-dfa! dfa string