2018-07-11 07:31:05 +03:00
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# 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)
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2018-07-11 07:27:07 +03:00
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2018-12-17 20:01:42 +03:00
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For example, consider this simple type family:
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```haskell
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type family FromMaybe (a :: k) (m :: Maybe k) :: k
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type instance FromMaybe a 'Nothing = a
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type instance FromMaybe a ('Just b) = b
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```
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2020-01-06 13:10:45 +03:00
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With first-class-families (fcfs), it translates to a `data` declaration
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2018-12-17 20:01:42 +03:00
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and instances for a single `Eval` family:
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```haskell
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import Fcf
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data FromMaybe :: k -> Maybe k -> Exp k
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type instance Eval (FromMaybe a 'Nothing) = a
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type instance Eval (FromMaybe a ('Just b)) = b
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```
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2018-09-12 22:53:12 +03:00
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2020-01-06 13:10:45 +03:00
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That way, the `FromMaybe` constructor can be partially applied,
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and passed to higher-order fcfs such as `Map`:
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2018-12-17 20:08:29 +03:00
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```haskell
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Eval (Map (FromMaybe 0) '[ 'Just 1, 'Nothing ]) = '[ 1, 0 ] :: [Nat]
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```
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2018-12-17 20:01:42 +03:00
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Essential language extensions:
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```haskell
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{-# LANGUAGE
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DataKinds,
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PolyKinds,
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TypeFamilies,
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TypeInType,
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TypeOperators,
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UndecidableInstances #-}
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```
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## See also
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[Haskell with only one type family](http://blog.poisson.chat/posts/2018-08-06-one-type-family.html) (blogpost)
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2018-07-11 07:27:07 +03:00
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---
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Contributions are welcome. Feel free to open an issue or make a PR on
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[Github](https://github.com/Lysxia/first-class-families)!
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