parametrityyppiin

Parametrityyppiin is a Finnish term used in discussions of type systems to refer to the concept of parametric polymorphism and its associated idea of parametric types—types that are themselves parameterized by other types, i.e., generics. The word blends parametri (parameter) and tyyppi (type), with the suffix -iin indicating an abstract or directional notion. In Finnish academic and technical writing, parametrityyppiin is used to talk about how type parameters enable code to be written without depending on concrete types.

The concept rests on the idea that polymorphic code should behave uniformly across all possible type instantiations. This idea is central to Reynolds’ parametricity, which formalizes the restriction that polymorphic functions cannot rely on the specifics of the type parameter. As a result, such functions must operate in a type-agnostic way, leading to “free theorems” that describe guaranteed behaviors based on type signatures alone.

In practical terms, parametrityyppiin underpins generics and type constructors in modern programming languages. Examples include generic containers like List<T> or Option<T>, where code can be written independently of the element type. Parametricityyppiin highlights the benefits of this abstraction for code reuse, safety, and interoperability, while also clarifying the limits imposed by ad-hoc polymorphism such as overloading or type classes.

See also: parametric polymorphism, Reynolds’ parametricity, generics, type inference, type systems.