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{-# LANGUAGE ImpredicativeTypes #-}
-- Sept 16: now scraping through with -XImpredicateTypes
module Main where
data Foo x y = Foo {foo1 :: x, foo2 :: y}
instance Functor (Foo x) where
fmap f (Foo x y) = Foo x (f y)
bar :: a -> Foo (forall s. s) a
bar a = Foo undefined a
main = print (foo2 (fmap (*2) (bar 2)))
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