Calling Haskell function returning String from Java

Question

I want to call a Haskell function that is operating on text and returns text (actually the processing is complicated and the Haskell code is split into few modules, but this probably isn't the case).

I tried the approach described here: Communication between Java and Haskell and modified it to work with Strings.

But I'm getting the error:

 error:   initializing argument 1 of ‘void* myFunction_hs(HsPtr)’ [-fpermissive]
 extern HsPtr myFunction_hs(HsPtr a1);
              ^

The relevant code and compilation is here: in Haskell:

foreign export ccall myFunction_hs :: CString -> IO CString

in Java:

import com.googlecode.javacpp.*;
import com.googlecode.javacpp.annotation.*;

@Platform(include={"<HsFFI.h>","myModule_stub.h"})
public class MyModule {
    static { Loader.load(); }
    public static native void hs_init(int[] argc, @Cast("char***") @ByPtrPtr PointerPointer argv);
    public static native String myFunction_hs(String text);
    public static void main(String[] args) {
        hs_init(null, null);
        String s = myFunction_hs("This is some String.");
        System.out.println("Result: " + s);
    }
}

And the compilation:

$ ghc -fPIC  -c -O myModule.hs
$ javac -cp javacpp.jar MyModule.java 
$ java -jar javacpp.jar -Dcompiler.path=ghc -Dcompiler.output.prefix="-optc-O3 -Wall MyModule.o -dynamic -fPIC -shared -lstdc++ -lHSrts-ghc7.6.3 -o " -Dcompiler.linkpath.prefix2="-optl -Wl,-rpath," MyModule

Do you have any idea what goes wrong?

Answer 1

I managed to solve this issue.

You need to edit the myModule_stub.h file (it is automatically generated). In my case I had to change the line:

extern HsPtr myFunction_hs(HsPtr a1);

to:

extern char* myFunction_hs(const char* a1);

Of course, you should rename your myModule_stub.h into myModule.h after manual editing. Otherwise it will be overwritten by GHC, and your specific types will be lost.