GHC/Kinds

Kinds (this is not the definitive name) will be a language extension adding a kind level and some kind polymorphic support to Haskell.

What does this extension do?

Haskell defines kinds as κ ::= * | κ -> κ. Nowadays, programmers use type-level computation more often using GADTs and type families. The need of a well-kinded type-level computation has become bigger. This extension provides a simple mechanism called promotion to populate the kind level.

Each time the user defines a promotable data type at the term level they are able to use it at the type level too. For instance, the user can write the following example:

data Nat = Zero | Succ Nat
data Vector :: * -> Nat -> * where
  VNil :: Vector a Zero
  VCons :: a -> Vector a n -> Vector a (Succ n)

data List a = Nil | Cons a (List a)
data HList :: List * -> * where
  HNil :: HList Nil
  HCons :: a -> HList as -> HList (Cons a as)

How do I use it?

Simply build GHC HEAD.

Which data types are promotable?

A data type is promotable if its type constructor and data constructors are. A type constructor is promotable if it is of the form * -> .. * -> * which is a first-order Haskell98 type constructor. All Haskell98 data constructors of a first-order Haskell98 data type are promotable. More details can be found in this paper.

A simple way to decide if your data type is promotable is to see if you can write without the where-clause like this:

data T (a::*) (b::*) (c::*) = A a | B Int b | C (Either a c) [b]