4.29 Finding all Solutions to a Goal

findall(+Template, +Goal, -Bag)

Creates a list of the instantiations Template gets successively on backtracking over Goal and unifies the result with Bag. Succeeds with an empty list if Goal has no solutions. findall/3 is equivalent to bagof/3 with all free variables bound with the existential operator (^), except that bagof/3 fails when goal has no solutions.

bagof(+Template, +Goal, -Bag)

Unify Bag with the alternatives of Template, if Goal has free variables besides the one sharing with Template bagof will backtrack over the alternatives of these free variables, unifying Bag with the corresponding alternatives of Template. The construct +Var^Goal tells bagof not to bind Var in Goal. bagof/3 fails if Goal has no solutions.

The example below illustrates bagof/3 and the ^ operator. The variable bindings are printed together on one line to save paper.

2 ?- listing(foo).

foo(a, b, c).
foo(a, b, d).
foo(b, c, e).
foo(b, c, f).
foo(c, c, g).

Yes
3 ?- bagof(C, foo(A, B, C), Cs).

A = a, B = b, C = G308, Cs = [c, d] ;
A = b, B = c, C = G308, Cs = [e, f] ;
A = c, B = c, C = G308, Cs = [g] ;

No
4 ?- bagof(C, A^foo(A, B, C), Cs).

A = G324, B = b, C = G326, Cs = [c, d] ;
A = G324, B = c, C = G326, Cs = [e, f, g] ;

No
5 ?-

setof(+Template, +Goal, -Set)

Equivalent to bagof/3, but sorts the result using sort/2 to get a sorted list of alternatives without duplicates.