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I used the following failure driven loop to list everything without using semicolons.
happiness(fred,5).
happiness(john,3).
happiness(grace,2).
someGoal(X) :-
happiness(X,Y), write(Y), tab(4), fail.
In query mode, I get this as expected
?- someGoal(_).
5 3 2
How can I insert these numbers into a list rather than writing them to screen?
I could not handle this within someGoal since backtracking seems implicit.
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Well, Prolog automatically tries to backtrack when it fails. So, adding a goal which always fails (such as the built in predicate fail/0 which has no effect but to make Prolog fail) at the end of a clause, will force Prolog to backtrack over all the other goals in that clause until it has tried all the possibilities.
As its name suggests, fail/0 is a special symbol that will immediately fail when Prolog encounters it as a goal. That may not sound too useful, but remember: when Prolog fails, it tries to backtrack . Thus fail/0 can be viewed as an instruction to force backtracking.
repeat :- repeat. Thus repeat succeeds when first called, thanks to the first clause. If the Prolog interpreter subsequently backtracks, the second clause ( repeat :- repeat. ) is tried. This initiates a new call to repeat , which succeeds via the first clause, and so on.
You are right, backtracking it's the way Prolog handles alternatives.
Use findall/3, that collect all alternatives using backtracking 'internally':
someGoal(X, Values) :-
findall(Value, happiness(X, Value), Values).
Then ?- someGoal(_, Values). will instance Values = [5, 3, 2]
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