Making Haskell libraries into DLLs doesn't work on Windows at the moment; however, all the machinery is still there. If you're interested, contact the GHC team. Note that building an entire Haskell application as a single DLL is still supported: it's just multi-DLL Haskell programs that don't work. The Windows distribution of GHC contains static libraries only.
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 ––mk-dll -o foo.dll bar.o baz.o wibble.a -lfooble
By feeding the ghc compiler driver the option ––mk-dll, 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:
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.
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>
extern void__stginit_Adder(void);
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.