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{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE StandaloneKindSignatures #-}
module T13333 where
import Data.Data
import Data.Kind
data T (phantom :: k) = T
type D :: (k -> Constraint) -> j -> Type
data D p x where
D :: forall k (p :: k -> Constraint) (x :: k). p x => D p x
class Possibly p x where
possibly :: proxy1 p -> proxy2 x -> Maybe (D p x)
dataCast1T :: forall k c t (phantom :: k).
(Typeable k, Typeable t, Typeable phantom, Possibly Data phantom)
=> (forall d. Data d => c (t d))
-> Maybe (c (T phantom))
dataCast1T f = case possibly (Proxy :: Proxy Data) (Proxy :: Proxy phantom) of
Nothing -> Nothing
Just D -> gcast1 f
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