The non-outer variant works, because then there aren’t higher rank types at all, i.e. `state` of `Handler` is free to flow outwards.

There is two ways to fix issue: Either use `newtype` or use `ImpredicativeTypes`

—

{-# LANGUAGE RankNTypes #-}

module TestTypes where

data State a        = State a

data Dummy          = Dummy

newtype Handler result = Handler { runHandler :: forall state . State state -> IO result }

type Resolver       = String -> Handler String

eventRouter :: Resolver -> String -> IO ()

eventRouter resolver event =

    runHandler (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

-}

eventConsumer :: Resolver -> String -> IO ()

eventConsumer = undefined

{-

rank2.hs:34:17: error:

    • Cannot instantiate unification variable ‘a0’

      with a type involving foralls: Resolver -> String -> IO ()

        GHC doesn't yet support impredicative polymorphism

    • In the expression: undefined

      In an equation for ‘eventConsumer’: eventConsumer = undefined

-}

-- does not type check when the rank 2 type isn't the "outermost" one?

createResolver :: (Resolver, Dummy)

createResolver = (\event -> Handler $ \state -> return "result", Dummy)

processor :: IO ()

processor =

    getLine >>= eventConsumer resolver >> processor

  where

    resolver :: Resolver

    resolver = fst (createResolver :: (Resolver, Dummy))

{-

    • 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)

-}

---

On 29 May 2016, at 21:02, Gabor Greif <ggreif@gmail.com> wrote:

The same bug has bitten git-annex too. IIRC.

Cheers,

    Gabor

Em domingo, 29 de maio de 2016, Michael Karg <mgoremeier@gmail.com> escreveu:

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)
-}

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