13.9.6 Use of Term Expansion

This example illustrates the use of user:term_expansion/[2,4] to augment the built-in predicate expand_term/2, which works as a filter on the input to compile and consult. The code below will allow the declaration `:- wait f/3' as an alias for `:- block f(-,?,?)'. Wait declarations were used in previous versions of SICStus Prolog.

Note the multifile declaration, which prevents this user:term_expansion/[2,4] clause from erasing any other clauses for the same predicate that might have been loaded.

     :- op(1150, fx, [wait]).
     :- multifile user:term_expansion/2.
     user:term_expansion((:- wait F/N), (:- block Head)) :-
             functor(Head, F, N),
             wb_args(N, Head).
     wb_args(0, _Head).
     wb_args(1, Head) :- arg(1, Head, -).
     wb_args(N, Head) :-
             arg(N, Head, ?),
             N1 is N-1,
             wb_args(N1, Head).