3.30.3 Programming Format

format_predicate(+Char, +Head)

If a sequence ~c (tilde, followed by some character) is found, the format derivatives will first check whether the user has defined a predicate to handle the format. If not, the built in formatting rules described above are used. Char is either an ASCII value, or a one character atom, specifying the letter to be (re)defined. Head is a term, whose name and arity are used to determine the predicate to call for the redefined formatting character. The first argument to the predicate is the numeric argument of the format command, or the atom default if no argument is specified. The remaining arguments are filled from the argument list. The example below redefines ~n to produce Arg times return followed by linefeed (so a (Grr.) DOS machine is happy with the output).

:- format_predicate(n, dos_newline(_Arg)). dos_newline(Arg) :- between(1, Ar, _), put(13), put(10), fail ; true.