first-class-families: First class type families

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First class type families, eval-style defunctionalization

See Fcf.


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Versions 0.1.0.0, 0.2.0.0, 0.3.0.0, 0.3.0.1, 0.4.0.0, 0.5.0.0, 0.6.0.0, 0.7.0.0, 0.8.0.0, 0.8.0.0, 0.8.0.1
Change log CHANGELOG.md
Dependencies base (>=4.9 && <4.14) [details]
License MIT
Copyright 2018 Li-yao Xia
Author Li-yao Xia
Maintainer lysxia@gmail.com
Category Other
Home page https://github.com/Lysxia/first-class-families#readme
Source repo head: git clone https://github.com/Lysxia/first-class-families
Uploaded by lyxia at 2020-03-01T22:32:20Z

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Readme for first-class-families-0.8.0.0

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First-class type families Hackage Build Status

For example, consider this simple type family:

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 (fcfs), it translates to a data declaration and instances for a single Eval family:

import Fcf

data FromMaybe :: k -> Maybe k -> Exp k
type instance Eval (FromMaybe a 'Nothing)  = a
type instance Eval (FromMaybe a ('Just b)) = b

That way, the FromMaybe constructor can be partially applied, and passed to higher-order fcfs such as Map:

Eval (Map (FromMaybe 0) '[ 'Just 1, 'Nothing ])  =  '[ 1, 0 ] :: [Nat]

Essential language extensions:

{-# LANGUAGE
    DataKinds,
    PolyKinds,
    TypeFamilies,
    TypeInType,
    TypeOperators,
    UndecidableInstances #-}

See also

Haskell with only one type family (blogpost)


Contributions are welcome. Feel free to open an issue or make a PR on Github!