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{-# LANGUAGE
FlexibleContexts,
RankNTypes,
TypeFamilies,
MultiParamTypeClasses,
FlexibleInstances #-}
-- See #4178
module T4178 where
data True = T
data False = F
class Decide tf a b where
type If tf a b
nonFunctionalIf :: tf -> a -> b -> If tf a b
instance Decide True a b where
type If True a b = a
nonFunctionalIf T a b = a
instance Decide False a b where
type If False a b = b
nonFunctionalIf F a b = b
useRank2 :: (forall a . a -> b) -> b
useRank2 f = f "foo"
hasTrouble a = nonFunctionalIf F a (2 :: Int)
blurg = useRank2 hasTrouble
hasNoTrouble :: a -> Int
hasNoTrouble = hasTrouble
blurg2 = useRank2 hasNoTrouble
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