How can I integrate a Haskell DLL into a C ++ / Qt Windows application?

I recommend using cabal-install

(or stack

) for this. If you plan to distribute this DLL without LPGL compliant licensing, chances are you will want to use the GHC build using (the default) integer-simple

instead integer-gmp

.

---

Haskell side

Check out this GHC tutorial first .

After reading this, you know you need a copy StartEnd.c

. Let's say it's in src\

.

So here's your MyLibrary.cabal

file:

...other cabal stuff...

library
  exposed-modules:     MyModule
  --other-modules:
  other-extensions:    ForeignFunctionInterface
  build-depends:       base
  hs-source-dirs:      src
  c-sources:           src/StartEnd.c  --tells cabal to rebuild if this changes
  default-language:    Haskell2010
  --default-extensions:

  -- Add whatever options you need, in addition to these:
  -- (In my experience, adding `-threaded` can make the DLL 
  --  behave better during debugging; but that only a guess.)
  ghc-options:         -O1 -shared src/StartEnd.c

      

        
        
        
      

    

Now create it with cabal build

(or stack build

if you are using stack

).

Wait for warnings. Cabal is not quite aware of what you are doing.

You will find three important files for you regarding your project:

• HSdll.dll
  
  
• HSdll.dll.a
  
  (Windows usually calls these files .lib
  
  , you can rename it if you like)
• build\...\MyLibrary_stub.h
  
  

Note that this DLL statically binds all Haskell dependencies. Therefore, you don't need to tell Visual Studio about anything other than your DLL.

---

C ++ side

• In C ++ code, you need to add the file MyLibrary_stub.h
  
  generated above.

• You will also need to specify prototypes HsStart
  
  and HsEnd
  
  somewhere:

  extern "C" { void HsStart(); void HsEnd(); }
  
  

• The file _stub.h
  
  contains some headers GHC, so you need to add a folder in the "Advanced Features" in the VS project: <ghc-folder>\ghc-7.10.1\lib\include
  
  .

  • One trick for this is to run where ghc
    
    , drop ghc.exe
    
    and add \..\lib\include
    
    to the output of this.

In a C ++ application, make sure you call HsStart()

and HsEnd()

at the beginning and end of your program. I like to encapsulate them in a RAII framework, so I can't forget to do it right.

---

My experience

I've tried this with versions cabal-install

1.2 * and stack

. I've used GHC 7.8.4 and 7.10.1, each with 32-bit and 64-bit.

Note that there Cabal

will be much better support for this soon .

---

Naming your DLL

Until Cabal gets better support, the only way I've found to give your DLL a name is add -o MyLibrary.dll

in ghc-options

. Even though it works, GHC gives you warnings when you do this, showing that it really doesn't know what's going on. To help you, you can pre-create StartEnd.c

in StartEnd.o

and link this. The command used looks something like this:

cabal exec -- ghc -optc-O -c src/StartEnd.c -o obj/StartEnd.o

      

        
        
        
      

    

Now, in ghc-options

, replace src/StartEnd.c

with obj/StartEnd.o

.

Replacing Cabal

with stack

should also work if you are using stack

.