first-class-families

First class type families

MIT licensed by Li-yao Xia

Maintained by [email protected]

This version can be pinned in stack with:first-class-families-0.7.0.0@sha256:4759bada637c8ee53758495e938591a14d259db8343ab7b4b313bbb1dfc770cf,1574

Depends on 1 package(full list with versions):

Used by 3 packages in lts-15.3(full list with versions):

First-class type families

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 #-}

Add Unfoldr, Concat, ConcatMap, Replicate, Take, Drop, TakeWhile, DropWhile, Reverse to Data.List. (gspia)
Change Elem, Lookup, Zip to be data instead of type synonyms.
Fix performance of Filter and Find.

New functions (blmage)
- LiftM, LiftM2, LiftM3
- (<=), (>=), (<), (>)
- Guarded, Guard((:=)), Otherwise

Initial version