The same bug has bitten git-annex too. IIRC.
Cheers,
Gabor
Em domingo, 29 de maio de 2016, Michael Karg
Hi devs,
could you please have a look at the following code snippet (modeled after a real-world app of mine)? There's a rank2type involved, and it doesn't type-check anymore when the type is e.g. part of a tuple, whereas everything's fine when it's the "outermost" type.
With GHC7.10 both variants type-check. Could anyone shed some light on what's behind this? Is the way the types are used in the snippet considered dispreferred or wrong under GHC8?
Thanks for having a look and hopefully pointing me to a page/ticket/... providing insight, Michael
--------
{-# LANGUAGE Rank2Types #-}
module TestTypes where
data State a = State a
data Dummy = Dummy
type Handler result = forall state . State state -> IO result
type Resolver = String -> Handler String
eventRouter :: Resolver -> String -> IO () eventRouter resolver event = resolver event state >> return () where state :: State () state = undefined
{- -- does type check createResolver :: Resolver createResolver = \event state -> return "result"
processor :: IO () processor = getLine >>= eventRouter resolver >> processor where resolver = createResolver -}
-- does not type check when the rank 2 type isn't the "outermost" one? createResolver :: (Resolver, Dummy) createResolver = (\event state -> return "result", Dummy)
processor :: IO () processor = getLine >>= eventConsumer resolver >> processor where (resolver, _) = createResolver
{- • Couldn't match type ‘t’ with ‘Resolver’ ‘t’ is a rigid type variable bound by the inferred type of resolver :: t at TestTypes.hs:41:5 Expected type: (t, Dummy) Actual type: (Resolver, Dummy) -}