bagof(X,P,L) produces a list (L) of X for each possible solution of the goal P. If P contains uninstantiated variables, other than X, it is possible that bagof can be successfully evaluated multiple times - for each possible values of the uninstantiated variables. The elements in L will appear in the order they were found and may include duplicates. Fails if P has no solutions.
Examples
z(r).
z(t).
z(y).
x(a,b,c).
x(q,X,e) :- z(X).
x(1,2,3).
x(w,b,c).
x(d,b,c).
x(a,b,c).
?- bagof(X,x(X,Y,Z),L).
L = [a,w,d,a]
X = UNINSTANTIATED VARIABLE
Y = b
Z = c
yes;
L = [q]
X = UNINSTANTIATED VARIABLE
Y = r
Z = e
yes;
L = [q]
X = UNINSTANTIATED VARIABLE
Y = t
Z = e
yes;
L = [q]
X = UNINSTANTIATED VARIABLE
Y = y
Z = e
yes;
L = [1]
X = UNINSTANTIATED VARIABLE
Y = 2
Z = 3
yes
?- bagof(X,x(X,y,z),L).
no
?- bagof(Y, (member(X,[6,3,7,2,5,4,3]), X<4, Y is X*X), L).
L = [9,9]
X = 3
Y = UNINSTANTIATED VARIABLE
yes;
L = [4]
X = 2
Y = UNINSTANTIATED VARIABLE
yes p(a,1).
p(b,2).
p(c,3).
p(d,2).
p(d,2).
?- bagof(X, p(X,Y), List).
List = [a]
X = UNINSTANTIATED VARIABLE
Y = 1
yes;
List = [b,d,d]
X = UNINSTANTIATED VARIABLE
Y = 2
yes;
List = [c]
X = UNINSTANTIATED VARIABLE
Y = 3
yesTODO bagof(X, Y ^ p(X,Y), List)