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-- !!! CPR on newtype with polymorphic argument
{-# OPTIONS -O #-}
module ShouldCompile where
data StateM m s a = STM (s -> m (a,s))
instance Functor m => Functor (StateM m s) where
fmap f (STM xs) = STM (\s -> fmap (\ (x,s') -> (f x, s'))
(xs s)
)
{- With GHC 4.04 (first release) this program gave:
panic! (the `impossible' happened):
mk_cpr_let: not a product
forall a{-ruq-} b{-rur-}.
(a{-ruq-} -> b{-rur-})
-> MonadLibrary.StateM{-r2o,x-} m{-a30Y-} s{-a30Z-} a{-ruq-}
-> MonadLibrary.StateM{-r2o,x-} m{-a30Y-} s{-a30Z-} b{-rur-}
The reason: 'Functor' is a newtype, whose element is a for-all type.
newtype Functor f = Functor (forall a,b. (a->b) -> f a -> f b)
-}
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