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{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
data Foo a where
MkFoo :: a -> (a -> String) -> Foo a
-- Shouldn't work because we don't accept multiple occurrences of a binding variable.
foo :: Foo (String, String) -> String
foo (MkFoo @(a,a) (x,y) f) = f (x :: a, y :: a)
main = do
print (foo (MkFoo ("hello", "goodbye") (\(x,y) -> reverse y ++ reverse x)))
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