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-- trac #1406: Constraint doesn't reduce in the presence of quantified
-- type variables
{-# LANGUAGE FlexibleInstances, UndecidableInstances, Rank2Types,
MultiParamTypeClasses, FunctionalDependencies #-}
module Problem where
data Z
data S a
class HPrefix l
instance (NSub (S Z) ndiff, HDrop ndiff l l) => HPrefix l
class NSub n1 n3 | n1 -> n3
instance NSub Z Z
instance NSub n1 n3 => NSub (S n1) n3
class HDrop n l1 l2 | n l1 -> l2
instance HDrop Z l l
t_hPrefix :: HPrefix l => l -> ()
t_hPrefix = undefined
-- In ghc 6.6.1 this works...
thr' :: (forall r. l -> a) -> a
thr' f = f undefined
thP4' = thr' t_hPrefix
-- ... but this doesn't work...?
thr :: (forall r. r -> a) -> a
thr f = f undefined
thP4 = thr t_hPrefix
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