The issue reported by Praki looks like a GHC bug to me. The smallest test case is at the end. (VarType a) is a GADT and Variant wraps the VarType with a value of type a. The definitions of Variant and VarType require ExistentialQuantification and GADTs extensions and I have declared them. The problem is when someone try to use them. If the client code is compiled without any extensions turned on the error message is:

...

test.hs:35:47: Couldn't match expected type `IDispatch a' against inferred type `a1' `a1' is a rigid type variable bound by the constructor `Variant' at test.hs:34:22 In the second argument of `getMethodID', namely `idisp' In a 'do' expression: dispId <- getMethodID "ExecQuery" idisp In the expression: do dispId <- getMethodID "ExecQuery" idisp return ()

This suggests that the compiler understands that Variant is an existential type but doesn't understand that VarType is a GADT. If the code is compiled with -fglasgow-exts then everything works fine. Why the compiler always understands existential types and not GADT. For me this is unfair against GADT. Shouldn't the compiler always understand GADT as well provided that they are already defined somewhere? Best Regards, Krasimir ------------------------------------------------------------------------------ -- Implementation of Variant ------------------------------------------------------------------------------ {-# LANGUAGE ExistentialQuantification, GADTs #-} module Variant where data VarType a where VT_DISPATCH :: VarType IDispatch data IDispatch = IDispatch data Variant = forall a . Variant (VarType a) a ------------------------------------------------------------------------------ -- Use of Variant ------------------------------------------------------------------------------ import Variant useIDispatch :: IDispatch -> IO () useIDispatch _ = putStrLn "useIDispatch" useVariant (Variant VT_DISPATCH idisp) = useIDispatch idisp On Wed, Aug 20, 2008 at 9:58 AM, Krasimir Angelov <kr.angelov@gmail.com> wrote:

Aha. I got it. You should compile with -fglasgow-exts option. The extra type signatures doesn't matter.

On Wed, Aug 20, 2008 at 8:20 AM, Praki Prakash <praki.prakash@gmail.com> wrote:

...

Krasimir - thanks for your reply. I had tried explicit typing but I still get the same error. I have some code below and the error message.

import Control.Exception import Foreign import Foreign.COM.Client import Foreign.COM.Automation import System.IO

data WmiConnection = WmiConnection { execQuery :: String -> IO () }

main = withCOM $ do conn <- wmiconnect "." "root\\cimv2" execQuery conn "Select * From ..." print "done"

wmiconnect :: String -> String -> IO WmiConnection wmiconnect comp cimroot = do clsid <- progid2clsid "WbemScripting.SWbemLocator" (bracket (createInstance clsid iidIUnknown) release $ \iunkn -> bracket (queryInterface iidIDispatch iunkn) release $ \idisp -> do dispId <- getMethodID "ConnectServer" idisp withBSTR comp $ \pComputer -> withBSTR cimroot $ \pCimRoot -> do (res,args) <- invoke dispId InvokeMethod [Variant VT_BSTR pComputer, Variant VT_BSTR pCimRoot ] idisp print res let conn = WmiConnection{ execQuery = queryFunc res } return conn) where queryFunc :: Variant -> String -> IO () queryFunc (Variant VT_DISPATCH idisp) query = do dispId <- getMethodID "ExecQuery" idisp return () {- queryFunc (Variant vt idisp) query = if vt == VT_DISPATCH then do dispId <- getMethodID "ExecQuery" idisp return "" else fail "error"

return () -}

C:\>ghc --make test.hs [1 of 1] Compiling Main ( test.hs, test.o )

test.hs:35:47: Couldn't match expected type `IDispatch a' against inferred type `a1' `a1' is a rigid type variable bound by the constructor `Variant' at test.hs:34:22 In the second argument of `getMethodID', namely `idisp' In a 'do' expression: dispId <- getMethodID "ExecQuery" idisp In the expression: do dispId <- getMethodID "ExecQuery" idisp return ()

Any further suggestions?

Thanks, Praki

On Tue, Aug 19, 2008 at 1:49 AM, Krasimir Angelov <kr.angelov@gmail.com> wrote:

...

This looks like a GHC bug to me. I am pretty sure that this worked before. Variant is defined like this:

data Variant = forall a . Variant (VarType a) a

data VarType a where .... VT_DISPATCH :: VarType (IDispatch ())

From this it clear that val is of type (IDispatch ()) because the VarType has value VT_DISPATCH. A workaround is to add explicit type singnature for val:

someFunc (Variant VT_DISPATCH val) query = do dispId <- getMethodID "MethodName" (val :: IDispatch ())

I don't know why this doesn't work without the signature.

Regards, Krasimir

On Tue, Aug 19, 2008 at 7:09 AM, Praki Prakash <praki.prakash@gmail.com> wrote:

...

I am a Haskell newbie trying to do COM automation using Haskell. I am using hscom (Krasimir's implementation of COM automation). I have run into a problem and need some help.

I have a Variant returned from a COM method invocation. When I print it, it shows up as below.

Variant VT_DISPATCH <interface 0x00039f00>

I need to invoke methods on the wrapped interface. My attempt to unbox it as below runs into 'rigid type' error.

someFunc (Variant VT_DISPATCH val) query = do dispId <- getMethodID "MethodName" val

The code above generates this error.

Couldn't match expected type `IDispatch a' against inferred type `a1' `a1' is a rigid type variable bound by...

I am probably missing something pretty basic. Any help on this is greatly appreciated!

Thanks

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