# Difference between revisions of "GHC/Kinds"

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== Which data types are promotable? == |
== Which data types are promotable? == |
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− | A data type is promotable if its type constructor and data constructors are. A type constructor is promotable if it is of the form <hask>* -> .. * -> *</hask> 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 [http:// |
+ | A data type is promotable if its type constructor and data constructors are. A type constructor is promotable if it is of the form <hask>* -> .. * -> *</hask> 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 [http://dreixel.net/research/pdf/ghp.pdf 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: |
A simple way to decide if your data type is promotable is to see if you can write without the where-clause like this: |

## Revision as of 13:18, 13 December 2011

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, he will be 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]
```