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{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
-- Shouldn't work because we don't accept multiple occurrences of a binding variable.
foo :: forall a. Monoid a => Maybe a -> a
foo (Nothing @a) = (mempty :: a)
foo (Just @a x) = (x :: a)
main = do
print (foo @String Nothing)
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