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RyanGlScott/Bug.hs

Created February 13, 2019 16:36
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Pattern synonym existential scoping weirdness
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Bug.hs
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Bug where
import Data.Kind
import Data.Proxy
data T = forall k (a :: k). MkT (Proxy a)
-- Uncomment `k` and it typechecks
pattern P :: forall. () => forall {-k-} (a :: k). Proxy a -> T
pattern P x = MkT (x :: Proxy (a :: k))
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