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?

A C string and a Java string are already quite different beasts, but an IO CString is surely not a string at all. — Ingo
– Ingo, Commented May 13, 2014 at 10:46
I suspect that String in Java and CString in Haskell just do not match. You need C-style string type (perhaps byte[]), I think. — fizruk
– fizruk, Commented May 13, 2014 at 10:51

imz -- Ivan Zakharyaschev · Accepted Answer · 2015-06-10 13:27:44Z

3

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.

edited Jun 10, 2015 at 13:27

imz -- Ivan Zakharyaschev

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answered May 22, 2014 at 7:32

niczka

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2 Comments

imz -- Ivan Zakharyaschev Over a year ago

I have a wish and feature request for GHC for this kind of incoveniences hindering fast integration of Haskell into C: [ more specific types in the generated *_stub.h files ](ghc.haskell.org/trac/ghc/ticket/10505).

imz -- Ivan Zakharyaschev Over a year ago

BTW, it'd be nice if you marked your answer as accepted. As far as I understand, that's the solution so far. (A quite important addition: And 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.)

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