Callables

Callables are values that can be invoked as functions. In programming languages that support first-class functions, a callable value may be a function, a method, or an object implementing a call interface. When invoked, a callable is supplied with arguments, and it returns a result.

Common categories include functions or procedures, lambda expressions, and objects with a special method like __call__ in Python or operator() in C++. In languages with implicit this binding, methods may require a bound receiver or support binding at call time. In JavaScript, functions themselves are objects and can carry properties; in Python, the built-in callable() checks callability.

Callables enable higher-order programming: passing functions as arguments, returning them from other functions, and storing them in data structures. They underpin callbacks, event handlers, and strategies. Some languages differentiate function pointers, functors, and closures; others unify them under a single callable abstraction. The arity and accepted argument types of a callable are determined by the language and the callable’s definition. Variadic or variably-typed callables are common in dynamic languages, while statically typed languages may require explicit type signatures or type inference.

Implementation considerations include binding of context for methods, lifetime and ownership (in languages with manual memory management), and performance overhead for indirection through higher-level callable wrappers. Ensuring consistent behavior across calls, handling exceptions, and documenting expected side effects are typical concerns for API design.