When the clause heads of a predicate have portions of arguments common
to several clauses, indexing on the principal functor of one argument
may not be sufficient. Indexing may be improved in such cases by the
use of unification factoring. Unification Factoring is a program
transformation that ``factors out'' common parts of clause heads,
allowing differing parts to be used for indexing, as illustrated by
the following example:
p(f(a),X) :- q(X).
p(f(b),X) :- r(X).
p(f(X),Y) :- _$p(X,Y).
_$p(a,X) :- q(X).
_$p(b,X) :- r(X).
The transformation thus effectively allows to be indexed
on atoms and . Unification Factoring is transparent
to the user; predicates created by the transformation are internal
to the system and do not appear during tracing.
The following compiler directives control the use of unification
factoring:^{3.4}.
:- ti(F/A).
Specifies that predicate should be
compiled with unification factoring enabled.
:- ti_off(F/A).
Specifies that predicate should be
compiled with unification factoring disabled.
:- ti_all.
Specifies that all predicates defined in the
file should be compiled with unification factoring enabled.
:- ti_off_all.
Specifies that all predicates defined in
the file should be compiled with unification factoring disabled.
By default, higher-order predicates (more precisely, predicates named
apply with arity greater than 1) are compiled with unification
factoring enabled. It can be disabled using the ti_off
directive. For all other predicates, unification factoring must be
enabled explicitly via the ti or ti_all directive. If
both :- ti(F/A). ( :- ti_all.) and :- ti_off(F/A).
( :- ti_off_all.) are specified, :- ti_off(F/A). (
:- ti_off_all.) takes precedence. Note that unification factoring
may have no effect when a predicate is well indexed to begin
with. For example, unification factoring has no effect on the
following program:
p(a,c,X) :- q(X).
p(b,c,X) :- r(X).
even though the two clauses have in common. The user may
examine the results of the transformation by using the ti_dump
compiler option (see Section 3.8.2).