foreign export stdcall on user-defined type and function

[David Lo]

Dear all,

I'm new in haskell. I'm assigned to see that a piece of Haskell code
need to be callable from C#. I find that I can convert Haskell to DLL
using ghc --mk-dll.

I find it is fine for simple function but for the following errors are
reported.

foreign export stdcall doComp :: Pattern -> CString -> IO ()
doComp :: Pattern -> CString -> IO ()

foreign export stdcall evalDisplay :: RealFrac a => CString ->
(Int->VMC (Maybe a)) -> IO ()
evalDisplay :: RealFrac a => CString -> (Int->VMC (Maybe a)) -> IO ()

The errors are as follows:

Compiling Main             ( CPL.hs, interpreted )

CPL.hs:29:
    Unacceptable argument type in foreign declaration:
        forall a b c. (Ind a c, Compare c b) => Pat a b
    When checking declaration:
        foreign export stdcall "doComp" doComp :: Pattern-> CString -> IO ()

CPL.hs:107:
    Unacceptable argument type in foreign declaration: {RealFrac a}
    When checking declaration:
        foreign export stdcall "evalDisplay" evalDisplay :: forall a. (RealFrac
a) => CString-> (Int -> VMC (Maybe a)) -> IO ()

CPL.hs:107:
    Unacceptable argument type in foreign declaration:
        Int -> VMC (Maybe a)
    When checking declaration:
        foreign export stdcall "evalDisplay" evalDisplay :: forall a. (RealFrac
a) =>CString-> (Int -> VMC (Maybe a)) -> IO ()

I would like to inquire on how to use foreign function stdcall on self
defined data structure and function. Can I just simply cast them to
string ?

Thank you in advance for your help and attention.

Best Regards,

David