Type family

In computer science, a type family associates data types with other data types, using a type-level function defined by an open-ended collection of valid instances of input types and the corresponding output types.

Type families are a feature of some type systems that allow partial functions between types to be defined by pattern matching. This is in contrast to data type constructors, which define injective functions from all types of a particular kind to a new set of types, and type synonyms (a.k.a. typedef), which define functions from all types of a particular kind to another existing set of types using a single case.