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{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
module Main where
data Foo a where
MkFoo :: Maybe a -> (a -> String) -> Foo a
foo :: Foo String -> String
foo (MkFoo @a (Nothing @b) f) = "nothing"
foo (MkFoo @a (Just @b x) f) = f ((x :: b) :: a)
main = do
print (foo (MkFoo (Just "hello") reverse))
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