retract/1 — The Logtalk Handbook v3.47.0 documentation

# `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