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{-# LANGUAGE RankNTypes #-}
-- This one killed GHC 5.05 and earlier
-- The problem was in a newtype with a record selector, with
-- a polymorphic argument type. MkId generated a bogus selector
-- function
module ShouldCompile where
type M3 a = forall r. (forall b. M3' b -> (b -> M3' a) -> r) -> r
newtype M3' a = M3' { mkM3' :: M3 a }
flop :: forall a b. M3' b -> (b -> M3' a) -> Int
flop = \m' k -> mkM3' m' (\bm k1 -> error "urk")
-- Suppose mkM3' has the straightforward type:
-- mkM3' :: forall a. M3' a -> M3 a
-- Then (mkM3' m') :: forall r. (forall b. ...) -> r
-- If we simply do a subsumption check of this against
-- alpha -> Int
-- where alpha is the type inferred for (\bm k1 ...)
-- this won't work.
-- But if we give mkM3' the type
-- forall a r. M3' a -> (forall b. ...) -> r
-- everthing works fine. Very very delicate.
---------------- A more complex case -------------
bind :: M3 a -> (a -> M3 b) -> M3 b
bind m k b = b (M3' m) (\a -> M3' (k a))
observe :: M3 a -> a
observe m
= m (\m' k -> mkM3' m'
(\bm k1 -> observe (bind (mkM3' bm)
(\a -> bind (mkM3' (k1 a)) (\a -> mkM3' (k a)))))
)
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