2018-07-11 07:31:05 +03:00
|
|
|
# First-class type families [![Hackage](https://img.shields.io/hackage/v/first-class-families.svg)](https://hackage.haskell.org/package/first-class-families) [![Build Status](https://travis-ci.org/Lysxia/first-class-families.svg)](https://travis-ci.org/Lysxia/first-class-families)
|
2018-07-11 07:27:07 +03:00
|
|
|
|
2018-12-17 20:01:42 +03:00
|
|
|
For example, consider this simple type family:
|
|
|
|
|
|
|
|
```haskell
|
|
|
|
type family FromMaybe (a :: k) (m :: Maybe k) :: k
|
|
|
|
type instance FromMaybe a 'Nothing = a
|
|
|
|
type instance FromMaybe a ('Just b) = b
|
|
|
|
```
|
|
|
|
|
|
|
|
With first-class-families, it translates to a `data` declaration
|
|
|
|
and instances for a single `Eval` family:
|
|
|
|
|
|
|
|
```haskell
|
|
|
|
import Fcf
|
|
|
|
|
|
|
|
data FromMaybe :: k -> Maybe k -> Exp k
|
|
|
|
type instance Eval (FromMaybe a 'Nothing) = a
|
|
|
|
type instance Eval (FromMaybe a ('Just b)) = b
|
|
|
|
```
|
2018-09-12 22:53:12 +03:00
|
|
|
|
2018-12-17 20:01:42 +03:00
|
|
|
Essential language extensions:
|
|
|
|
|
|
|
|
```haskell
|
|
|
|
{-# LANGUAGE
|
|
|
|
DataKinds,
|
|
|
|
PolyKinds,
|
|
|
|
TypeFamilies,
|
|
|
|
TypeInType,
|
|
|
|
TypeOperators,
|
|
|
|
UndecidableInstances #-}
|
|
|
|
```
|
|
|
|
|
|
|
|
## See also
|
|
|
|
|
|
|
|
[Haskell with only one type family](http://blog.poisson.chat/posts/2018-08-06-one-type-family.html) (blogpost)
|
2018-07-11 07:27:07 +03:00
|
|
|
|
|
|
|
---
|
|
|
|
|
|
|
|
Contributions are welcome. Feel free to open an issue or make a PR on
|
|
|
|
[Github](https://github.com/Lysxia/first-class-families)!
|