Removes the first occurrence of dynamic clause Clause from module M.
callable, must be nonvar
A valid Prolog clause.
retract/1 erases the first clause in the database that matches
Clause. Clause is retracted in module M if
specified. Otherwise, Clause is retracted in the source module.
retract/1 is nondeterminate.
If control backtracks into
the call to
retract/1, then successive clauses matching Clause are
erased. If and when no clauses match, then the call to
If the predicate did not previously exist, then
it is created as a dynamic predicate and
Clause must be of one of the forms:
Head :- Body
where Head is of type callable and the principal functor of Head is the name of a dynamic procedure. If specified, then Module must be an atom.
retract(Head) means retract the unit-clause Head. The exact
same effect can be achieved by
retract((Head :- true)).
Body may be uninstantiated, in which case it will match any body. In
the case of a unit-clause it will be bound to
true. Thus, for
| ?- retract((foo(X) :- Body)), fail.
is guaranteed to retract all the clauses for
foo/1, including any
unit-clauses, providing of course that
foo/1 is dynamic.
Can be used to retract all matching clauses through backtracking.
Head (in Clause) or M is uninstantiated.
Head is not a callable, or M is not an atom, or Body is not a valid clause body.
the procedure corresponding to Head is not dynamic