fobj

FObj, short for functional object, is a conceptual construct used in programming to illustrate objects that behave like functions while maintaining instance state. In this pattern, an object exposes a single callable interface, typically implemented by a method named invoke or by overloading a call operator. The idea blends data encapsulation from object-oriented design with the first-class, higher-order nature of functions.

An FObj is created with internal configuration and state. When invoked with arguments, it performs a computation using both its internal data and the provided inputs, returning a result. This design enables treating behavior as data: an FObj can be passed as a parameter, stored in collections, or composed with other callables in pipelines or strategy patterns. While many implementations favor immutability to preserve referential transparency, mutable variants exist where state changes are needed.

Examples across languages include Python classes that implement call, JavaScript objects with callable interfaces, and C++ functors defined by operator(). Some languages offer dedicated functional abstractions, such as function objects or closures, which share similar semantics but differ in how state is represented.

FObj is often contrasted with plain functions, which typically do not carry identity or internal state. It differs from traditional objects by focusing on a callable interface as the primary means of interaction. Variants of the pattern may emphasize laziness, partial application, or monadic composition to model effects and sequencing.

See also: function object, functor, higher-order function, object-oriented programming, functional programming.