On Fri, Oct 29, 2010 at 5:42 PM, Simon Peyton-Jones
<simo...@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
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