Havepredicate

Havepredicate is a term used in discussions of reflective programming in logic and related languages. It denotes a function or predicate that queries whether another predicate is defined in the current program or environment. The concept is language-dependent, but it generally answers a simple yes or no about the existence of a given predicate indicator, such as a name and arity, optionally qualified by a module.

In practice, havepredicate typically accepts a predicate indicator in the form Name/Arity or Module:Name/Arity and returns true if a corresponding predicate is declared or defined. Some implementations distinguish between static and dynamic predicates, or between public and private visibility across modules. Different languages may expose havepredicate under slightly different names or with additional parameters, but the core idea remains runtime or compile-time introspection of predicate definitions.

Usage scenarios include conditional imports, generating wrappers for optional features, and meta-programming tasks that adapt behavior based on which predicates are available. For example, a program might check havepredicate(print/2) before attempting to call a printing predicate, or check havepredicate/arity variations to determine supported interfaces.

Relation to related concepts includes predicate_property/2, current_predicate/1, and module scoping rules. predicate_property can reveal attributes of a defined predicate (such as dynamic, discontiguous, or interpreted), while current_predicate or havepredicate focus on existence. Limitations of havepredicate include sensitivity to module boundaries, loading order, and runtime changes that can make a previously true predicate unavailable.

See also: current_predicate, predicate_property, dynamic predicate, module system.