Sealing up your Haskell library inside a DLL is straightforward; compile up the object files that make up the library, and then build the DLL by issuing a command of the form:

ghc –shared -o foo.dll bar.o baz.o wibble.a -lfooble

By feeding the ghc compiler driver the option –shared, it will build a DLL rather than produce an executable. The DLL will consist of all the object files and archives given on the command line.

A couple of things to notice:

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. This is a special case of Section 8.2.1.2, “Making a Haskell library that can be called from foreign code”; we'll deal with the DLL-specific issues that arise below. Here's an example:

The body of a DllMain() function is an extremely dangerous place! This is because the order in which DLLs are unloaded when a process is terminating is unspecified. This means that the DllMain() for your DLL may be called when other DLLs containing functions that you call when de-initializing your DLL have already been unloaded. In other words, you can't put shutdown code inside DllMain(), unless your shutdown code only requires use of certain functions which are guaranteed to be available (see the Platform SDK docs for more info).

In particular, if you are writing a DLL that's statically linked with Haskell, it is not safe to call hs_exit() from DllMain(), since hs_exit() may make use of other DLLs (see also Section 8.2.4.4, “On the use of hs_exit()”). What's more, if you wait until program shutdown to execute your deinitialisation code, Windows will have terminated all the threads in your program except the one calling DllMain(), which can cause even more problems.

A solution is to always export Begin() and End() functions from your DLL, and call these from the application that uses the DLL, so that you can be sure that all DLLs needed by any shutdown code in your End() function are available when it is called.

The following example is untested but illustrates the idea (please let us know if you find problems with this example or have a better one). Suppose we have a DLL called Lewis which makes use of 2 Haskell modules Bar and Zap, where Bar imports Zap and is therefore the root module in the sense of Section 8.2.1.1, “Using your own main()”. Then the main C++ unit for the DLL would look something like:

 // Lewis.cpp -- compiled using GCC
 #include <Windows.h>
 #include "HsFFI.h"

 #define __LEWIS_DLL_EXPORT
 #include "Lewis.h"

 #include "Bar_stub.h"  // generated by GHC
 #include "Zap_stub.h"

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

 extern "C"{

 LEWIS_API HsBool lewis_Begin(){
   int argc = ...
   char *argv[] = ...

   // Initialize Haskell runtime
   hs_init(&argc, &argv);

   // Tell Haskell about all root modules
   hs_add_root(__stginit_Bar);

   // do any other initialization here and
   // return false if there was a problem
   return HS_BOOL_TRUE;
 }

 LEWIS_API void lewis_End(){
   hs_exit();
 }

 LEWIS_API HsInt lewis_Test(HsInt x){
   // use Haskell functions exported by
   // modules Bar and/or Zap

   return ...
 }

 } // extern "C"

and some application which used the functions in the DLL would have a main() function like:

 // MyApp.cpp
 #include "stdafx.h"
 #include "Lewis.h"

 int main(int argc, char *argv[]){
   if (lewis_Begin()){
      // can now safely call other functions
      // exported by Lewis DLL

   }
   lewis_End();
   return 0;
 }

Lewis.h would have to have some appropriate #ifndef to ensure that the Haskell FFI types were defined for external users of the DLL (who wouldn't necessarily have GHC installed and therefore wouldn't have the include files like HsFFI.h etc).