Since DLLs correspond to packages (see Section 4.8, “ Packages ”) you need to use -package-name dll-name when compiling modules that belong to a DLL if you're going to call them from Haskell. Otherwise, Haskell code that calls entry points in that DLL will do so incorrectly, and crash. For similar reasons, you can only compile a single module tree into a DLL, as startupHaskell needs to be able to call its initialisation function, and only takes one such argument (see Section 11.5.4, “Making DLLs to be called from other languages”). Hence the modules you compile into a DLL must have a common root.

By default, the entry points of all the object files will be exported from the DLL when using ––mk-dll. Should you want to constrain this, you can specify the module definition file to use on the command line as follows:

ghc ––mk-dll -o .... -optdll--def -optdllMyDef.def

See Microsoft documentation for details, but a module definition file simply lists what entry points you want to export. Here's one that's suitable when building a Haskell COM server DLL:

EXPORTS
 DllCanUnloadNow     = DllCanUnloadNow@0
 DllGetClassObject   = DllGetClassObject@12
 DllRegisterServer   = DllRegisterServer@0
 DllUnregisterServer = DllUnregisterServer@0

In addition to creating a DLL, the ––mk-dll option also creates an import library. The import library name is derived from the name of the DLL, as follows:

DLL: HScool.dll  ==> import lib: libHScool_imp.a

The naming scheme may look a bit weird, but it has the purpose of allowing the co-existence of import libraries with ordinary static libraries (e.g., libHSfoo.a and libHSfoo_imp.a. Additionally, when the compiler driver is linking in non-static mode, it will rewrite occurrence of -lHSfoo on the command line to -lHSfoo_imp. By doing this for you, switching from non-static to static linking is simply a question of adding -static to your command line.

Use foreign export declarations to export the Haskell functions you want to call from the outside. For example,

module Adder where

adder :: Int -> Int -> IO Int  -- gratuitous use of IO
adder x y = return (x+y)

foreign export stdcall adder :: Int -> Int -> IO Int

Compile it up:

ghc -c adder.hs -fglasgow-exts

This will produce two files, adder.o and adder_stub.o

compile up a DllMain() that starts up the Haskell RTS-––a possible implementation is:

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

EXTFUN(__stginit_Adder);

static char* args[] = { "ghcDll", NULL };
                       /* N.B. argv arrays must end with NULL */
BOOL
STDCALL
DllMain
   ( HANDLE hModule
   , DWORD reason
   , void* reserved
   )
{
  if (reason == DLL_PROCESS_ATTACH) {
      /* By now, the RTS DLL should have been hoisted in, but we need to start it up. */
      startupHaskell(1, args, __stginit_Adder);
      return TRUE;
  }
  return TRUE;
}

Here, Adder is the name of the root module in the module tree (as mentioned above, there must be a single root module, and hence a single module tree in the DLL). Compile this up:

ghc -c dllMain.c

Construct the DLL:

ghc ––mk-dll -o adder.dll adder.o adder_stub.o dllMain.o

Start using adder from VBA-––here's how I would Declare it:

Private Declare Function adder Lib "adder.dll" Alias "adder@8"
      (ByVal x As Long, ByVal y As Long) As Long

Since this Haskell DLL depends on a couple of the DLLs that come with GHC, make sure that they are in scope/visible.

Building statically linked DLLs is the same as in the previous section: it suffices to add -static to the commands used to compile up the Haskell source and build the DLL.