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class NewFunctor f where
inj :: a -> f a
surj :: f a -> a
data N a = Z a
ninj x = (Z x)
nsurj (Z x) = x
instance NewFunctor N where
inj = ninj
surj = nsurj
twice :: NewFunctor f => a -> f (f a)
twice x = inj(inj x)
undo :: NewFunctor f => f (f a) -> a
undo x = surj(surj x)
main = putStr (undo (Z (Z "hello world\n")))
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