# Declarative object-oriented programming

Logtalk is a declarative object-oriented logic programming language. This means that Logtalk shares key concepts with other object-oriented programming languages but abstracts and reinterprets these concepts in the context of declarative logic programming.

The key concepts in declarative object-oriented programming are encapsulation and reuse patterns. Notably, the concept of mutable state, which is an imperative concept, is not a significant concept in declarative object-oriented programming. Declarative object-oriented programming concepts can be materialized in both logic and functional languages. In this section, we focus only in declarative object-oriented logic programming.

The first critical generalization of object-oriented programming concepts is the concept of object itself. What an object encapsulates depends on the base programming paradigm where we apply object-oriented programming concepts. When these concepts are applied to an imperative language, where mutable state and destructive assignment are central, objects naturally encapsulate and abstract mutable state, providing disciplined access and modification. When these concepts are applied to a declarative logic language such as Prolog, objects naturally encapsulate predicates. Therefore, an object can be seen as a theory, expressed by a set of related predicates. Theories are usually static and thus Logtalk objects are static by default. This contrasts with imperative object-oriented languages where usually classes are static and objects are dynamic. This view of an object as a set of predicates also forgo a distinction between data and procedures that is central to imperative object-oriented languages but moot in declarative, homoiconic logic languages.

The second critical generalization concerns the relation between objects and other entities such as protocols (interfaces) and ancestor objects. The idea is that entity relations define reuse patterns and the roles played by the participating entities. A common reuse pattern is inheritance. In this case, an entity inherits, and thus reuses, resources from an ancestor entity. In a reuse pattern, each participating entity plays a specific role. The same entity, however, can play multiple roles depending on its relations with other entities. For example, an object can play the role of a class for its instances, the role of a subclass for its superclasses, and the role of an instance for its metaclass. Another common reuse pattern is protocol implementation. In this case, an object implementing a protocol reuses its predicate declarations by providing an implementation for those predicates and exposing those predicates to its clients. An essential consequence of this generalization is that protocols, objects, and categories are first-class entities while e.g. class, instance, metaclass, subclass, superclass are just roles that an object can play. Moreover, a language can provide multiple reuse patterns instead of selecting a set of patterns and supporting this set as a design choice that excludes other reuse patterns. For example, most imperative object-oriented languages are either class-based or prototype-based. In contrast, Logtalk naturally supports both classes and prototypes by providing the corresponding reuse patterns using objects as first-class entities capable of playing multiple roles.