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{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
-- This one sent an earlier version of GHC into a
-- loop in the simplfier, because we allowed a RULE
-- to fire on a loop-breaker
--
-- Discovered by Roman L, Nov 09
module Roman where
data T a = T (T a)
type family F a
type instance F (T a) = Wrap (T a)
class B (F a) => C a where
toF :: a -> F a
go :: C a => a -> Int
{-# INLINE go #-}
go x = gow (toF x)
instance C a => C (T a) where
{-# INLINE toF #-}
toF x = Wrap x
newtype Wrap a = Wrap a
class B a where
dummy :: a
gow :: a -> Int
instance C a => B (Wrap a) where
{-# INLINE gow #-}
gow (Wrap x) = go x
dummy = error "urk"
foo :: C a => T a -> Int
foo t = let t' = T t in go t'
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