ADDED: library(format), describing strings with format_//2

ADDED: library(format), describing strings with format_//2

This library provides the nonterminal format_//2 to describe
formatted strings. format/2 is provided for impure output.

Usage:
======

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,
        which must be atomic (a current limitation)
  ~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
  ~s    use the next argument here, which must be a string
  ~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 tabstops
  ~`Ct  like ~t, use character C instead of spaces to fill the space
  ~n    newline
  ~~    the literal ~

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

If at all possible, format_//2 should be used, to stress pure parts
that enable easy testing etc. If necessary, you can emit the list Ls
with maplist(write, Ls).

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.

Example:

?- phrase(format_("~s~n~`.t~w!~12|", ["hello",there]), Cs).
%@ Cs = [h,e,l,l,o,'\n','.','.','.','.','.','.',t,h,e,r,e,!] ;
%@ false.