[Haskell-cafe] Why am I not allowed to use CStringLen in foreignexport?

[Brian Hulley]

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.
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