built-in method

retract/1

Description

retract(Head)
retract((Head:-Body))

Retracts a clause for an object dynamic predicate. On backtracking, the predicate retracts the next matching clause.

When the predicate indicator for Head is declared in a uses/2 or use_module/2 directive, the clause is retracted in the referenced object or module. When the backend Prolog compiler supports a module system, the predicate argument can also be module qualified.

This method may be used to retract clauses for predicates that are not declared dynamic for dynamic objects provided that the predicates are declared in this.

Modes and number of proofs

retract(+clause) - zero_or_more

Errors

Head is a variable:

instantiation_error

Head is neither a variable nor a callable term:

type_error(callable, Head)

The predicate indicator of Head, Name/Arity, is that of a private predicate:

permission_error(modify, private_predicate, Name/Arity)

The predicate indicator of Head, Name/Arity, is that of a protected predicate:

permission_error(modify, protected_predicate, Name/Arity)

The predicate indicator of Head, Name/Arity, is that of a static predicate:

permission_error(modify, static_predicate, Name/Arity)

The predicate indicator of Head, Name/Arity, is not declared:

existence_error(predicate_declaration, Name/Arity)

Examples

To retract a matching clause of a dynamic predicate in this:

retract(Clause)

To retract a matching clause of a public or protected dynamic predicate in self:

::retract(Clause)

To retract a matching clause of a public dynamic predicate in an explicit object:

Object::retract(Clause)

See also

abolish/1, asserta/1, assertz/1, clause/2, retractall/1, dynamic/0, dynamic/1, uses/2, use_module/2