Rank2Types example not typechecking w/ GHC8. Bug or feature?
Oleg Grenrus
oleg.grenrus at iki.fi
Sun May 29 18:20:01 UTC 2016
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 at gmail.com> wrote:
>
> The same bug has bitten git-annex too. IIRC.
>
> Cheers,
>
> Gabor
>
> Em domingo, 29 de maio de 2016, Michael Karg <mgoremeier at gmail.com <mailto:mgoremeier at 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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