Haskell Design Patterns

Type-level programming

Type families bring functions to the type-level. Polymorphic kinds bring polymorphism to the kind-level. Type promotion bring datatypes and type-safety to the kind-level.

Two major problems of the Haskell kind system are solved by these extensions:

• The kind system is too restrictive (because it lacks polymorphism)
  • Solution: Provide polymorphism on the kind level (PolyKinds)
• The kind system is too permissive (kinds are too vague)
  • Solution: Promote datatypes to kinds to simulate a type-system on the kind-level (DataKinds)

Haskell98 already carried the seed for type-level programming by including multiparameter type-classes. Since then, the Haskell kind-system has been enriched with functional dependencies, GADTs, type families, ...