foreign export stdcall on user-defined type and function

[Simon Marlow]

On 08 November 2004 02:49, David Lo wrote:

> 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 ()

It would help if you posted the entire code, rather than just a snippet.
We can't tell for sure where the errors are without seeing the
definitions for some of the types you've used, for example.

> 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 () 

The error indicates that you have declared a foreign exported function
with a type that does not have a direct translation into a C type.  The
legal types for foreign exported functions are described in the FFI
specification, which can be found online here:

  http://www.cse.unsw.edu.au/~chak/haskell/ffi/ffi.pdf

> 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 ?

Perhaps you could give more details about what you want to do.  What
Haskell functions do you want to export to C#, and what types do they
have (both in Haskell and C#)?  If you want to export Haskell data
structures to C#, then you have to "marshal" the data into C#, by
converting the Haskell representation into a representation that C# can
understand.  Using strings is one possibility, but it's unlikely to be
the best.

Cheers,
	Simon