clause/2 — The Logtalk Handbook v3.47.0 documentation

# `clause/2`

## Description

    clause(Head, Body)

Enumerates, by backtracking, the clauses of a dynamic predicate.

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

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

## Modes and number of proofs

    clause(+callable, ?body) - zero_or_more

## Errors

`Head` is a variable:
`instantiation_error`
`Head` is a neither a variable nor a callable term:
`type_error(callable, Head)`
`Body` is a neither a variable nor a callable term:
`type_error(callable, Body)`
The predicate indicator of `Head`, `Name/Arity`, is that of a private predicate:
`permission_error(access, private_predicate, Name/Arity)`
The predicate indicator of `Head`, `Name/Arity`, is that of a protected predicate:
`permission_error(access, protected_predicate, Name/Arity)`
The predicate indicator of `Head`, `Name/Arity`, is that of a static predicate:
`permission_error(access, static_predicate, Name/Arity)`
`Head` is not a declared predicate:
`existence_error(predicate_declaration, Name/Arity)`

## Examples

To retrieve a matching clause of a local dynamic predicate or a dynamic predicate in this:
`clause(Head, Body)`
To retrieve a matching clause of a public or protected dynamic predicate in self:
`::clause(Head, Body)`
To retrieve a matching clause of a public dynamic predicate in an explicit object:
`Object::clause(Head, Body)`

See also

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