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clause/[2,3]
ISOclause(+Head, -Body)
clause(+Head, -Body, -Ref)
clause(-Head, -Body, +Ref)
Searches the database for a clause whose head matches Head and whose body matches Body.
callable
A term whose functor names a dynamic procedure.
callable
db_reference
Initially, at least one of Head and Ref must be instantiated.
In the case of unit-clauses, Body is unified with true
.
If a procedure consists entirely of unit-clauses, then there is no point in
calling clause/2
on it. It is simpler and faster to call the procedure.
In clause/3
, either Head or Ref must be
instantiated. If Ref is instantiated, then (Head :-
Body)
is unified with the clause identified by Ref. (If
this clause is a unit-clause, then Body is unified with
true
.)
If the predicate did not previously exist, then
it is created as a dynamic predicate and clause/2
fails. If
Ref is not instantiated, then 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.
instantiation_error
Neither Head nor Ref is instantiated.
type_error
Head is not a callable, or Ref is not a well-formed db_reference
permission_error
Procedure is not dynamic.
existence_error
Ref is a well-formed db_reference but does not correspond to an existing clause or record.
If clause/[2,3]
is called on an undefined procedure, then 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, then 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, then the clauses can be accessed
by first finding the module(s) by means of current_predicate/2
.
For example, if the procedure is f/1
:
| ?- 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.
For example:
| ?- 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.
instance/2
,
assert/[1,2]
,
dynamic/1
,
retract/1
,
ref-mdb-acl.