Re: how to make this fail driven loop succesfull ?

On 9/03/2014, at 10:43 AM, Bob Minors wrote:
> 
> siblings(Father,Mother) :- is_father(X, Father), is_mother(X, Mother), write(X), nl, fail ; true.

By the way, the predicate name here is rather odd.
If I see siblings(X, Y), I assume it means "X and Y
are siblings".

Here, it's not a relation but a command.

It is a really good idea to make commands *obviously*
commands by making their names be verb phrases.  (This
has nothing to do with Prolog.  It's counted as good
practice in every programming language.)

So

print_children_of(Father, Mother) :-
    (   child_father(Child, Father),
        child_mother(Child, Mother),
        write(Child), nl,
        fail
    ;   true
    ).

Now the pattern ( P, Q, fail ; true) is common enough
that it has a name, and SWI supports that.  And the
pattern write(X), nl is common enough that _it_ has
a name, and SWI supports that.

child_father_mother(Child, Father, Mother) :-
    child_father(Child, Father),
    child_mother(Child, Mother).

print_children_of(Father, Mother) :-
    foreach(
        child_father_mother(Child, Father, Mother),
        writeln(Child) ).

and now it's pretty hard to misunderstand.