Andreas Marth wrote:
Hi!
I try to export a Haskell function to VBA. (At the moment without COM.) Because VBA seems to expect a String with length information I tried to return a CStringLen as defined in Foreign.C.String. But if I try to compile it I get an "unacceptable argument type in foreign declaration: CStringLen". My reduced test program now is a strange "Hallo world!" program:
module Test where import Foreign.C.String
foreign export stdcall hello :: IO CStringLen hello :: IO CStringLen hello = do newCStringLen ("Hallo world!")
Do I do some thing wrong here? (If I use CString instead of CStringLen and accordingly newCString instead of newCStringLen everything compiles fine but VBA crashes.)
The FFI only supports a very limited range of types. CString is supported because it is just defined by: type CString = Ptr CChar whereas CStringLen is defined as (Ptr CChar, Int) and tuples can't be passed through the FFI. In any case, the BSTR type required by VBA is not so simple as a CStringLen.
From the Microsoft Platfrom SDK docs,
BSTRs are wide, double-byte (Unicode) strings on 32-bit Windows platforms and narrow, single-byte strings on the Apple® PowerMacT. The length is stored as an integer at the memory location preceding the data in the string. I assume that this means that on 32 bit Windows, the format of a BSTR is: Word16 -- low word of length Word16 -- high word of length Word16 -- first char of string ... so you could try creating an array of Word16's in that format and passing them, perhaps something like: import Data.Word import Data.Bits import Foreign.Marshal.Array import Foreign.Ptr type BSTR = Ptr Word16 createBSTR :: [Char] -> IO BSTR createBSTR s = do let len :: Word32 = fromIntegral (length s) low :: Word16 = fromIntegral (len .&. 0xFFFF) high :: Word16 = fromIntegral (shiftR len 16 .&. 0xFFFF) newArray ([low, high] ++ map (fromIntegral . fromEnum) s) foreign export stdcall hello :: IO BSTR hello :: IO BSTR hello = createBSTR "Hello world!" (The above code compiles but is untested) Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com