Type error in GHC-7 but not in GHC-6.12.3

[Bas van Dijk]

On Fri, Oct 29, 2010 at 5:42 PM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> That looks odd.
>
> Can you isolate it for us?  The easiest thing is usually to start with the offending code:
> withDeviceWhich ∷
>  ∀ pr α
>  . MonadCatchIO pr
>  ⇒ USB.Ctx
>  → (USB.DeviceDesc → Bool)
>  → (∀ s. RegionalDeviceHandle (RegionT s pr) → RegionT s pr α)
>  → pr α
> withDeviceWhich ctx p f = do
>  devs ← liftIO $ USB.getDevices ctx
>  useWhich devs withDevice p f
>
> Now add local definitions for each of the functions mentioned, with definition foo = undefined.
>
> useWhich ∷
>  ∀ k desc e (m ∷ * → *) α
>  . (GetDescriptor e desc, MonadIO m)
>  ⇒ [e]
>  → (e → k → m α)
>  → (desc → Bool)
>  → k
>  → m α
> useWhich = undefined.
>
> Now all you need is the types involved, and you can probably define them as
>
> data RegionT s pr a
>
> etc
>
> That should give a standalone test case.
>
> Thanks!
>
> SImon
>

Ok, Here's the more isolated program which still gives the same error
as the full usb-safe (on the latest ghc-HEAD (7.1.20101029)):


USBSafeTest.hs:39:57:
    Couldn't match expected type `forall s.
                                  RegionalDeviceHandle (RegionT s pr)
-> RegionT s pr α'
                with actual type `RegionalDeviceHandle (RegionT s pr)
                                  -> RegionT s pr α'
    In the fourth argument of `useWhich', namely `f'
    In the expression: useWhich devs withDevice p f
    In the expression:
      do { devs <- return [Device];
           useWhich devs withDevice p f }


{-# LANGUAGE UnicodeSyntax
           , KindSignatures
           , RankNTypes
           , MultiParamTypeClasses
           , FunctionalDependencies
  #-}

import Data.List (find)

data Ctx = Ctx
data Device = Device
data DeviceDesc = DeviceDesc
data RegionalDeviceHandle (r ∷ * → *) = RegionalDeviceHandle
data RegionT s (pr ∷ * → *) α = RegionT

instance Monad pr ⇒ Monad (RegionT s pr) where
    return = undefined
    (>>=)  = undefined

runRegionT ∷ (∀ s. RegionT s pr α) → pr α
runRegionT = undefined

openDevice ∷ Device → RegionT s pr (RegionalDeviceHandle (RegionT s pr))
openDevice = undefined

withDevice ∷ Monad pr
           ⇒ Device
           → (∀ s. RegionalDeviceHandle (RegionT s pr) → RegionT s pr α)
           → pr α
withDevice dev f = runRegionT $ openDevice dev >>= f

withDeviceWhich ∷ ∀ pr α
                . Monad pr
                ⇒ Ctx
                → (DeviceDesc → Bool)
                → (∀ s. RegionalDeviceHandle (RegionT s pr) → RegionT s pr α)
                → pr α
withDeviceWhich ctx p f = do devs ← return [Device]
                             useWhich devs withDevice p f

useWhich ∷ ∀ k desc e (m ∷ * → *) α
         . (GetDescriptor e desc)
         ⇒ [e]
         → (e → k → m α)
         → (desc → Bool)
         → k
         → m α
useWhich ds w p f = case find (p . getDesc) ds of
                      Nothing → error "not found"
                      Just d  → w d f

class GetDescriptor α desc | α → desc, desc → α where
    getDesc ∷ α → desc

instance GetDescriptor Device DeviceDesc where
    getDesc = undefined


I could isolate it a bit more if you want.

Thanks,

Bas