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{-# LANGUAGE ConstraintKinds #-} | |
{-# LANGUAGE FlexibleInstances #-} | |
{-# LANGUAGE Rank2Types #-} | |
{-# LANGUAGE ScopedTypeVariables #-} | |
{-# LANGUAGE TypeFamilies #-} | |
{-# LANGUAGE TypeOperators #-} | |
{-# LANGUAGE UndecidableInstances #-} | |
module DynBounded where | |
import Data.Proxy | |
import Data.Reflection | |
import Data.Constraint | |
import Data.Constraint.Unsafe | |
newtype Lift (p :: * -> Constraint) (a :: *) (s :: *) = | |
Lift { lower :: a } | |
class ReifiableConstraint p | |
where | |
data Def (p :: * -> Constraint) (a :: *) :: * | |
reifiedIns :: Reifies s (Def p a) :- p (Lift p a s) | |
using :: forall p a. ReifiableConstraint p => Def p a -> (p a => a) -> a | |
using d m = | |
reify d $ \(_ :: Proxy s) -> | |
let | |
replaceProof :: Reifies s (Def p a) :- p a | |
replaceProof = trans proof reifiedIns | |
where | |
proof = unsafeCoerceConstraint :: p (Lift p a s) :- p a | |
in | |
m \\ replaceProof | |
instance ReifiableConstraint Bounded | |
where | |
data Def Bounded a = Bounded { minBound_ :: a, maxBound_ :: a } | |
reifiedIns = Sub Dict | |
instance Reifies s (Def Bounded a) => Bounded (Lift Bounded a s) | |
where | |
minBound = Lift $ minBound_ (reflect (Proxy :: Proxy s)) | |
maxBound = Lift $ maxBound_ (reflect (Proxy :: Proxy s)) | |
example1 :: (Int, Int) | |
example1 = | |
(minBound, maxBound) | |
example2 :: (Int, Int) | |
example2 = | |
(using (Bounded 0 42) minBound, using (Bounded 0 42) maxBound) |
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