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-0.4.0.0@sha256:3c4ada413b20b1957e11d712c48f71bfc010acfe022397c4a4ddad7fed5f2cdb,1171

Depends on 1 package(full list with versions):

Used by 1 package in nightly-2019-01-23(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, 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 passed to higher-order fcfs.

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

Essential language extensions:

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

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

Initial version