Re: [Haskell-cafe] Unboxing VT_VARIANT in hscom

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 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 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 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

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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