clause(+Head, -Body, -Ref)
clause(-Head, -Body, +Ref)
Searches the database for a clause whose head matches Head and whose body matches Body.
A term whose functor names a dynamic procedure.
Initially, at least one of Head and Ref must be instantiated.
In the case of unit-clauses, Body is unified with
If a procedure consists entirely of unit-clauses then there is no point in
clause/2 on it. It is simpler and faster to call the procedure.
clause/3, either Head or Ref must be
instantiated. If Ref is instantiated,
Body) is unified with the clause identified by Ref. (If
this clause is a unit-clause, Body is unified with
If the predicate did not previously exist, then
it is created as a dynamic predicate and
clause/2 fails. If
Ref is not instantiated,
clause/3 behaves exactly like
clause/2 except that the database reference is returned.
By default, clauses are accessed with respect to the source module.
Can be used to backtrack through all the clauses matching a given Head and Body. It fails when there are no (or no further) matching clauses in the database.
Neither Head nor Ref is instantiated.
Head is not a callable, or Ref is not a well-formed db_reference
Procedure is not dynamic.
Ref is a well-formed db_reference but does not correspond to an existing clause or record.
clause/[2,3] is called on an undefined procedure it fails, but before
failing it makes the procedure dynamic. This can be useful if you wish to
prevent unknown procedure catching from happening on a call to that procedure.
It is not a limitation that Head is required to be instantiated in
clause(Head, Body), because if you want to backtrack
through all clauses for all dynamic procedures this can be achieved by:
| ?- predicate_property(P,dynamic), clause(P,B).
If there are clauses with a given name and arity in several different modules,
or if the module for some clauses is not known, the clauses can be accessed
by first finding the module(s) by means of
For example, if the procedure is
| ?- current_predicate(_,M:f(_)), clause(M:f(X),B).
clause/3 will only access clauses that are defined in, or
imported into, the source module, except that the source module can be
overridden by explicitly naming the appropriate module.
| ?- assert(foo:bar,R). R = '$ref'(771292,1) | ?- clause(H,B,'$ref'(771292,1)). no | ?- clause(foo:H,B,'$ref'(771292,1)). H = bar, B = true
Accessing a clause using
clause/2 uses first argument indexing when
possible, in just the same way that calling a procedure uses first argument
indexing. See Indexing.
clause/2 is part of the ISO Prolog standard;
clause/3 is not.