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{-# LANGUAGE DataKinds, FlexibleContexts, FlexibleInstances, GADTs #-}
{-# LANGUAGE KindSignatures, MultiParamTypeClasses, TypeFamilies,
TypeOperators #-}
module T13784 where
import Data.Kind (Type)
import Data.Monoid ((<>))
data Product :: [Type] -> Type where
(:*) :: a -> Product as -> Product (a : as)
Unit :: Product '[]
infixr 5 :*
instance Show (Product '[]) where
show Unit = "Unit"
instance (Show a, Show (Product as)) => Show (Product (a : as)) where
show (a :* as) = show a <> " :* " <> show as
class Divideable a as where
type Divide a as :: [Type]
divide :: Product as -> (a, Product (Divide a as))
instance Divideable a (a : as) where
-- type Divide a (a : as) = as
-- Conflicting type family instances, seems like OVERLAPS isn't a thing for
-- type families.
divide (a :* as) = (a, as)
instance Divideable b as => Divideable b (a : as) where
type Divide b (a : as) = a : Divide b as
divide (a :* as) = a :* divide as
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