Functietype

Functietype, or function type, is a type that describes the signature of a function. It specifies the number and types of its input parameters and the type of its return value. In many languages, function types can also express optional or variadic parameters and, in some cases, type parameters for generic functions. Function types enable the compiler to verify that values used as functions conform to a declared interface and to enable higher-order programming, where functions are passed as arguments or returned as results.

A typical functietype encodes a parameter list and a result type. For example, a function type might describe a function that takes a number and a string and returns a boolean. A value assigned to a variable of this type must be a function with a matching signature. In languages with type inference or advanced type systems, function types may be inferred from context or described with explicit type annotations.

Variance is often relevant for function types. In many type systems, parameter types are contravariant while return types are covariant, affecting how function values can be substituted or combined. Function types are usually first-class in modern languages and can be used to describe callbacks, event handlers, higher-order functions, and APIs that accept or return functions.

Different languages express functietypes with different syntax and features. Some use structural typing, where only the signature matters, while others rely on nominal typing or function interfaces. Function types are a core concept in type systems, aiding safety, abstraction, and modular design.