[Haskell-cafe] Re: Forall and type synonyms in GHC 7.0

[Mario Blažević]

> I had the exact same problem in my regional-pointers package in the
> withArray function:
>
> withArray ∷ (Storable α, MonadCatchIO pr)
>            ⇒ [α]
>            → (∀ s. RegionalPtr α (RegionT s pr) → RegionT s pr β)
>            → pr β
>
>   I had to replace the original:
>
> withArray vals = withArrayLen vals ∘ const
>
> with:
>
> withArray vals f = withArrayLen vals $ \_ → f
>
> where:
>
> withArrayLen ∷ (Storable α, MonadCatchIO pr)
>              ⇒ [α]
>              → (∀ s. Int → RegionalPtr α (RegionT s pr) → RegionT s pr β)
>              → pr β
>
> So unfortunately you gave to inline the function composition:
>
> pair2 combinator = pair1 $ \b ->  combinator (chooseBinder b)


	This worked for me, thank you! I was worried I'd have to make a 
sweeping change to the module interfaces. I find this solution rather 
surprising, but as long as it's localized I don't mind.


> Note that in the other thread I'm describing a similar problem in my
> usb-safe package. Where in essence the problem is that the following
> won't type check:
>
> foo :: (forall s. ST s a) ->  a
> foo st = ($) runST st
>
> but the following will:
>
> foo :: (forall s. ST s a) ->  a
> foo st = runST st
>
> and surprisingly the following will also type check:
>
> foo :: (forall s. ST s a) ->  a
> foo st = runST $ st


	Yes, I hadn't seen that thread until this morning. The same issue with 
impredicative types appears to cause my problem and both problems you've 
encountered. I wonder what percentage of Hackage libraries will be 
affected by the change.