occurs.pl -- Finding and counting sub-terms

This is a SWI-Prolog implementation of the corresponding Quintus library, based on the generalised arg/3 predicate of SWI-Prolog.

See also

- library(terms) provides similar predicates and is probably more wide-spread than this library.

 contains_term(+Sub, +Term) is semidet

Succeeds if Sub is contained in Term (=, deterministically)

 contains_var(+Sub, +Term) is semidet

Succeeds if Sub is contained in Term (==, deterministically)

 free_of_term(+Sub, +Term) is semidet

Succeeds of Sub does not unify to any subterm of Term

 free_of_var(+Sub, +Term) is semidet

Succeeds of Sub is not equal (==) to any subterm of Term

 occurrences_of_term(@SubTerm, @Term, ?Count) is det

Count the number of SubTerms in Term that unify with SubTerm. As this predicate is implemented using backtracking, SubTerm and Term are not further instantiated. Possible constraints are enforced. For example, we can count the integers in Term using

?- freeze(S, integer(S)), occurrences_of_term(S, f(1,2,a), C).
C = 2,
freeze(S, integer(S)).

See also

- occurrences_of_var/3 for an equality (==/2) based variant.

 occurrences_of_var(@SubTerm, @Term, ?Count) is det

Count the number of SubTerms in Term that are equal to SubTerm. Equality is tested using ==/2. Can be used to count the occurrences of a particular variable in Term.

See also

- occurrences_of_term/3 for a unification (=/2) based variant.

 sub_term(-Sub, +Term)

Generates (on backtracking) all subterms of Term.

 sub_var(-Sub, +Term)

Generates (on backtracking) all subterms (==) of Term.

 sub_term_shared_variables(+Sub, +Term, -Vars) is det

If Sub is a sub term of Term, Vars is bound to the list of variables in Sub that also appear outside Sub in Term. Note that if Sub appears twice in Term, its variables are all considered shared.

An example use-case is refactoring a large clause body by introducing intermediate predicates. This predicate can be used to find the arguments that must be passed to the new predicate.