Useful Tip 5

For some tasks you can save space by using failure driven loops. For example, suppose we want to write a predicate, read_file/1, which will read each clause, C, from a given file and assert them into the database as a series of facts of the form cl(C). The first part of this predicate takes care of opening the file; calling the subgoal, read_all_clauses, which does the reading and asserting; and closing the file at the end.

read_file(File):-
    open(File, read, Stream),
    set_input(Stream),
    read_all_clauses,
    close(Stream).

read_all_clauses:-
    read(Clause),
    ((Clause = end_of_file, !) ;
     (assert(cl(Clause)), read_all_clauses)).

read_all_clauses:-
    repeat,
    read(Clause),
    ((Clause = end_of_file, !) ;
     (assert(cl(Clause)), fail)).