Language/Symantic/Typing/Family.hs

   1 {-# LANGUAGE DataKinds #-}
   2 {-# LANGUAGE PolyKinds #-}
   3 {-# LANGUAGE ConstraintKinds #-}
   4 {-# LANGUAGE FlexibleContexts #-}
   5 {-# LANGUAGE FlexibleInstances #-}
   6 {-# LANGUAGE GADTs #-}
   7 {-# LANGUAGE MultiParamTypeClasses #-}
   8 {-# LANGUAGE ScopedTypeVariables #-}
   9 {-# LANGUAGE TypeFamilies #-}
  10 {-# LANGUAGE TypeOperators #-}
  11 {-# LANGUAGE UndecidableInstances #-}
  12 {-# OPTIONS_GHC -fconstraint-solver-iterations=7 #-}
  13 module Language.Symantic.Typing.Family where
  14
  15 import Data.Maybe (fromMaybe)
  16 import Data.Proxy
  17
  18 import Language.Symantic.Typing.Type
  19
  20 type family Fam fam h0 :: *
  21
  22 -- * Type 'Proj_Fam'
  23 -- | Convenient type synonym wrapping 'Proj_FamR'
  24 -- to initiate its recursion.
  25 type Proj_Fam cs = Proj_FamR cs cs
  26
  27 -- | Project the 'Constraint' indexed by the given 'Type'
  28 -- onto its proof, captured by 'Con' when it holds.
  29 proj_fam
  30  :: forall fam cs h. Proj_Fam cs fam
  31  => Proxy fam
  32  -> Type cs h
  33  -> Maybe (Type cs (Fam fam h))
  34 proj_fam = proj_famR (Proxy::Proxy cs)
  35
  36 -- ** Class 'Proj_FamR'
  37 -- | Intermediate type class to construct an instance of 'Proj_Fam'
  38 -- from many instances of 'Proj_FamC', one for each item of @cs@.
  39 --
  40 -- * @cs@: starting  list of /type constants/.
  41 -- * @rs@: remaining list of /type constants/.
  42 class Proj_FamR cs rs fam where
  43         proj_famR :: Proxy rs -> Proxy fam -> Type cs h -> Maybe (Type cs (Fam fam h))
  44         proj_famR _rs _fam _ty = Nothing
  45
  46 -- | Test whether @c@ handles the work of 'Proj_Fam' or not,
  47 -- or recurse on @rs@, preserving the starting list of /type constants/.
  48 instance
  49  ( Proj_FamC cs    fam c
  50  , Proj_FamR cs rs fam
  51  ) => Proj_FamR cs (c ': rs) fam where
  52         proj_famR _rs fam ty =
  53                 proj_famR (Proxy::Proxy rs) fam ty `fromMaybe`
  54                 proj_famC (Proxy::Proxy c)  fam ty
  55 -- | End the recursion.
  56 instance Proj_FamR cs '[] fam
  57
  58 -- ** Class 'Proj_FamC'
  59 -- | Handle the work of 'Proj_Fam' for a given /type family/ @fam@ and /type constant/ @c@,
  60 -- that is: maybe it handles the given /type family/,
  61 -- and if so, maybe the /type family/ has an instance for the given 'Type'.
  62 class Proj_FamC cs fam c where
  63         proj_famC :: Proxy c -> Proxy fam -> Type cs h -> Maybe (Maybe (Type cs (Fam fam h)))
  64         proj_famC _c _fam _ty = Nothing