See Wikipedia.
Examples
multiple_result_predicate(X,Y,Z) :- Z is X+Y.
multiple_result_predicate(X,Y,Z) :- Z is X*Y.
single_result_predicate(X,Y,Z) :- Z is X+Y.
?- foldl(single_result_predicate, [2,4,7], 0, X).
X = 13
yes
?- foldl(single_result_predicate, [2,4,7], 42, X).
X = 55
yes
?- foldl(single_result_predicate, [2,4,7], 0, 13).
yes
?- foldl(single_result_predicate, [2,4,7], 0, 12).
no
?- foldl(single_result_predicate, [], 7, X).
X = 7
yes
?- foldl(single_result_predicate, [3], 7, X).
X = 10
yes
?- foldl(multiple_result_predicate, [2,4,7], 42, X).
X = 55
yes;
X = 336
yes;
X = 183
yes;
X = 1232
yes;
X = 95
yes;
X = 616
yes;
X = 343
yes;
X = 2352
yes
?- foldl(multiple_result_predicate, [1,2,3], 0, X).
X = 6
yes;
X = 9
yes;
X = 5
yes;
X = 6
yes;
X = 5
yes;
X = 6
yes;
X = 3
yes;
X = 0
yes
?- foldl(multiple_result_predicate, [1,2,3], 0, 6).
yes;
yes;
yes;
no
?- foldl(multiple_result_predicate, [], 7, X).
X = 7
yes
?- foldl(multiple_result_predicate, [3], 7, X).
X = 10
yes;
X = 21
yes four_arg_predicate(1,X,Y,Z) :- Z is X+Y.
four_arg_predicate(2,X,Y,Z) :- Z is X-Y.
four_arg_predicate(3,X,Y,Z) :- Z is X+Y.
four_arg_predicate(3,X,Y,Z) :- Z is X-Y.
four_arg_predicate(a,1,2,3).
four_arg_predicate(a,x,y,3).
four_arg_predicate(a,5,3,12).
four_arg_predicate(a,999,_,99999).
four_arg_predicate(b,_,_,_).
?- foldl(four_arg_predicate(1), [2,4,7], 0, X).
X = 13
yes
?- foldl(four_arg_predicate(2), [2,4,7], 0, X).
X = 5
yes
?- foldl(four_arg_predicate(3), [2,4,7], 0, X).
X = 13
yes;
X = 1
yes;
X = 9
yes;
X = 5
yes;
X = 13
yes;
X = 1
yes;
X = 9
yes;
X = 5
yes
?- foldl(four_arg_predicate(3), [2,4,7], 0, 5).
yes;
yes
?- foldl(four_arg_predicate(a), [B,C], A, X).
A = 2
B = 1
C = 5
X = 12
yes;
A = 2
B = 1
C = 999
X = 99999
yes;
A = y
B = x
C = 5
X = 12
yes;
A = y
B = x
C = 999
X = 99999
yes;
A = 3
B = 5
C = 999
X = 99999
yes;
A = UNINSTANTIATED VARIABLE
B = 999
C = 999
X = 99999
yes
?- foldl(four_arg_predicate(3), [2,4,7], 0, 14).
no
?- foldl(four_arg_predicate(4), [2,4,7], 0, X).
noNote: Unlike SWI Prolog, fails on first evaluation if the second argument is not a concrete list.
?- foldl(single_result_predicate, [2,4,7|T], 0, X).
Expected concrete list but got: .(2, .(4, .(7, T)))
?- foldl(single_result_predicate, L, 0, X).
Expected concrete list but got: L