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{-# LANGUAGE ConstraintKinds, DataKinds, GADTs, TypeFamilies, TypeOperators #-}
module T9747 where
import Data.List (intercalate)
import Data.Proxy
import GHC.Prim (Constraint)
data HList :: [*] -> * where
Nil :: HList '[]
Cons :: a -> HList as -> HList (a ': as)
type family HListAll (c :: * -> Constraint) (ts :: [*]) :: Constraint where
HListAll c '[] = ()
HListAll c (t ': ts) = (c t, HListAll c ts)
showHList :: HListAll Show ts => HList ts -> String
showHList = ("[" ++ ) . (++"]") . intercalate ", " . go
where
go :: HListAll Show ts => HList ts -> [String]
go Nil = []
go (Cons x xs) = show x : go xs
-- Things work okay up to this point
test :: String
test = showHList (Cons (2::Int)
(Cons (3.1 :: Float)
(Cons 'c' Nil)))
type family ConFun (t :: *) :: * -> Constraint
data Tag
type instance ConFun Tag = Group
class (Show a, Eq a, Ord a) => Group a
-- This is notionally similar to showHList
bar :: HListAll (ConFun l) ts => Proxy l -> HList ts -> ()
bar _ _ = ()
baz :: (ConFun l a, ConFun l b) => Proxy l -> HList [a,b] -> ()
baz = bar
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