This predicate assembles a list of solutions for each different instantiation of the free variables in a goal. The elements in the list are in order of finding the solution.
The goal bagof(Term, Goal, List) will produce a 'List' of all the expressions 'term' such that the 'Goal' is satisfied. After each solution the variables in the expression 'term' are made empty so that a new solution can be found, other variables retain their values.
It is possible to specify that a variable must be made empty after a solution without specifying it in the goal 'Goal', the ^ operator must be used for that, for example:
bagof(X, Var1 ^ Var2 ^ Goal, List).
The ^ operator is a predefined infix operator. If the 'Goal' expression contains any ^ operators then it must be rightmost goal of the expression. For example: the expression 'X ^ goal(Q)' is correct and 'goal(Q) ^ X' is not correct.
Assume the database contains the user-defined predicates:
legs(A, 6) :- insect(A). legs(A, 4) :- animal(A). legs(A, 8) :- spider(A). insect(bee) :- true. animal(horse) :- true. animal(dog) :- true. spider(tarantula) :- true. age(peter, 7). age(ann, 5). age(pat, 8). age(tom, 5).
see also: ^/2 findall/3 setof/3 setof_reverse/3
| Examples | |
| bagof(Child, age(Child, 5), List). | List = [ann, tom] |
| bagof(Child, age(Child,Age), List). | Age = 7, List = [peter] Age = 5, List = [ann, tom] Age = 8, List = [pat] |
| bagof(Child/Age, age(Child, Age), List). | List = [peter/7, ann/5, pat/8, tom/5] |
| bagof(Child, Age ^ age(Child, Age), List). | List = [peter, ann, pat, tom] |
| bagof(A, legs(A, N), List). | List = [bee] List = [horse, dog] List = [tarantula] |
| Exceptions | |
| third argument is not a list or a variable. | a type_error(list, A) exception is thrown, A is replaced by the incorrect third argument |
| the second argument must not be an empty variable | an instantiation_error is thrown. |
| the second argument is not a correct goal | a type_error(callable, A) exception is thrown, A is replaced by the incorrect goal |