Scryer Prolog documentation

Module format

:- use_module(library(format)).

This library provides the nonterminal format_//2 to describe formatted strings. format/[2,3] are provided for impure output.

The entire library only works if the Prolog flag double_quotes is set to chars, the default value in Scryer Prolog. This should also stay that way, to encourage a sensible environment.

format_(+FormatString, +Arguments)//


phrase(format_(FormatString, Arguments), Ls)

format_//2 describes a list of characters Ls that are formatted according to FormatString. FormatString is a string (i.e., a list of characters) that specifies the layout of Ls. The characters in FormatString are used literally, except for the following tokens with special meaning:

~w use the next available argument from Arguments here
~q use the next argument here, formatted as by writeq/1
~a use the next argument here, which must be an atom
~s use the next argument here, which must be a string
~d use the next argument here, which must be an integer
~f use the next argument here, a floating point number
~Nf where N is an integer: format the float argument
using N digits after the decimal point
~Nd like ~d, placing the last N digits after a decimal point;
if N is 0 or omitted, no decimal point is used.
~ND like ~Nd, separating digits to the left of the decimal point
in groups of three, using the character "," (comma)
~NU like ~ND, using "_" (underscore) to separate groups of digits
~NL format an integer so that at most N digits appear on a line.
If N is 0 or omitted, it defaults to 72.
~Nr where N is an integer between 2 and 36: format the
next argument, which must be an integer, in radix N.
The characters "a" to "z" are used for radices 10 to 36.
If N is omitted, it defaults to 8 (octal).
~NR like ~Nr, except that "A" to "Z" are used for radices > 9
~| place a tab stop at this position
~N| where N is an integer: place a tab stop at text column N
~N+ where N is an integer: place a tab stop N characters
after the previous tab stop (or start of line)
~t distribute spaces evenly between the two closest tab stops
~`Ct like ~t, use character C instead of spaces to fill the space
~n newline
~Nn N newlines
~i ignore the next argument
~~ the literal ~

Instead of ~N, you can write ~* to use the next argument from Arguments as the numeric argument.


?- phrase(format_("~s~n~`.t~w!~12|", ["hello",there]), Cs).
   Cs = "hello\n......there!".

format(+Fs, +Args)

The predicate format/2 is like format_//2, except that it outputs the text on the terminal instead of describing it declaratively as a list of characters.

If at all possible, format_//2 should be used, to stress pure parts that enable easy testing etc. If necessary, you can emit the described list of characters Ls with maplist(put_char, Ls) or, much faster, with format("~s", [Ls]). Ideally, however, you use phrase_to_file/[2,3] or phrase_to_stream/2 from library(pio) to write the described list directly to a file or stream, respectively: phrase_to_stream(format_(..., [...]), S). The advantage of this is that an ideal implementation writes the characters as they become known, without manifesting the list.

format(Stream, FormatString, Arguments)

Output the described string to the given Stream. If Stream is a binary stream, then the code of each emitted character must be in 0..255.


portray_clause/1 is useful for printing solutions in such a way that they can be read back with read/1.