Sorry if I'm talking to myself, but I found a solution and thought it could be interesting for other people who need to call GHC created DLLs from Excel.

The solution is based on the information found in : http://haskell.org/haskellwiki/GHC/Using_the_FFI#Debugging_Haskell_DLLs.

As suggested, I added two extra functions in my dllMain.c file (adder_Begin, adder_End) and removed the startupHaskell call from the dllMain function.

adder_Begin contains the startupHaskell call and adder_End the shutdownHaskell. dllMain just returns true. I also adapted the Excel VBA code to call adder_Begin on a WorkBook_Open event and adder_End on a WorkBook_BeforeClose event.

The VBA looks like this:

Public functions in a new "Module1" module (cannot declare Public functions in the ThisWorkbook module):

Public Declare Function adder Lib " adder.dll" Alias "adder@8" (ByVal x As Long, ByVal y As Long) As Long
Public Declare Function adder_Begin Lib "adder.dll" () As Boolean
Public Declare Sub adder_End Lib "adder.dll" ()

Private functions the the "ThisWorkbook" module:

Private Sub Workbook_BeforeClose(Cancel As Boolean)
adder_End
End Sub

Private Sub Workbook_Open()
adder_Begin
End Sub

The GHC dllMain.c looks like this:

#include <windows.h>
#include <Rts.h>

#define __ADDER_DLL_EXPORT
#define ADDER_API _declspec(dllexport)

extern void __stginit_Adder(void);

static char* args[] = { "ghcDll", NULL };
                        /* N.B. argv arrays must end with NULL */

BOOL STDCALL DllMain( HANDLE hModule,
                       DWORD ul_reason_for_call,
                       LPVOID lpReserved
                      ){
return TRUE;
}

ADDER_API BOOL adder_Begin(){
startupHaskell(1, args, __stginit_Adder);
return HS_BOOL_TRUE;
}

ADDER_API void adder_End(){
shutdownHaskell();
}

If somebody wants the code and Excel file, just ask, I'll be happy to provide it.

Thanks,

Olivier.