Making DLLs to be called from other languages

The Glasgow Haskell Compiler User's Guide, Version 5.04
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# 11.4. Making DLLs to be called from other languages

If you want to package up Haskell code to be called from other languages, such as Visual Basic or C++, there are some extra things it is useful to know. The dirty details are in the Foreign Function Interface definition, but it can be tricky to work out how to combine this with DLL building, so here's an example:

- 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:

      gcc -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.

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