calculus/Control/Monad/Classes/Instance.hs

   1 {-# LANGUAGE ConstraintKinds #-}
   2 {-# LANGUAGE DataKinds #-}
   3 {-# LANGUAGE ExistentialQuantification #-}
   4 {-# LANGUAGE TypeFamilies #-}
   5 module Control.Monad.Classes.Instance where
   6 import Data.Bool (Bool(..))
   7 import GHC.Prim (Constraint)
   8
   9 -- | A data type to existentially wrap a value of type @ty@,
  10 -- when it is an instance of the type class @cl@.
  11 data Instance (cl:: * -> Constraint)
  12  = forall ty. cl ty => Instance ty
  13
  14 -- | An open type family to know (at the type level) whether a type @ty@
  15 -- has an instance of the type class @cl@.
  16 --
  17 -- NOTE: currently, users have to manually define type family instances of 'Class'
  18 -- to indicate to the compiler which type support which type class.
  19 -- It is definitively redundant to have to define
  20 -- both a type class instance and a type family instance,
  21 -- but I cannot find a way to automatically synchronize
  22 -- the compiler's knowledge between these two levels.
  23 type family Class (cl:: * -> Constraint) (ty:: *) :: Bool