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{-# LANGUAGE ScopedTypeVariables, InstanceSigs #-}
module Foo where
data T a = T a
class C a where
foo :: b -> a -> (a, [b])
instance C a => C (T a) where
foo :: forall b. b -> T a -> (T a, [b])
foo x (T y) = (T y, xs)
where
xs :: [b]
xs = [x,x,x]
instance Functor T where
fmap :: (a -> b) -> T a -> T b
fmap f (T x) = T (f x)
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