Communication between Java and Haskell

Question

I googled and got some answers that communication between Java and Haskell can be done by GCJNI(Now the site is down) and LambdaVM.. To use the LambdaVM/GCJNI, whether I nee

Answer 1

Calling Haskell from C appears quite easy, and thus can also be easily called from Java with JavaCPP. For example, to call the fibonacci_hs() function from the sample code Safe.hs:

{-# LANGUAGE ForeignFunctionInterface #-}

module Safe where

import Foreign.C.Types

fibonacci :: Int -> Int
fibonacci n = fibs !! n
    where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral

foreign export ccall fibonacci_hs :: CInt -> CInt

we can do it this way from Java:

import org.bytedeco.javacpp.*;
import org.bytedeco.javacpp.annotation.*;

@Platform(include={"","Safe_stub.h"})
public class Safe {
    static { Loader.load(); }
    public static native void hs_init(int[] argc, @Cast("char***") @ByPtrPtr PointerPointer argv);
    public static native int fibonacci_hs(int i);
    public static void main(String[] args) {
        hs_init(null, null);
        int i = fibonacci_hs(42);
        System.out.println("Fibonacci: " + i);
    }
}

Under Linux, the compilation procedure looks like this:

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

And the program runs normally with the usual java command:

$ java -cp javacpp.jar:. Safe
Fibonacci: 267914296

---

Edit: I have taken the liberty to do some microbenchmarking of the calling overhead. With the following C header file Safe.h:

inline int fibonacci_c(int n) {
    return n < 2 ? n : fibonacci_c(n - 1) + fibonacci_c(n - 2);
}

the following Java class:

import org.bytedeco.javacpp.*;
import org.bytedeco.javacpp.annotation.*;

@Platform(include={"","Safe_stub.h", "Safe.h"})
public class Safe {
    static { Loader.load(); }
    public static native void hs_init(int[] argc, @Cast("char***") @ByPtrPtr PointerPointer argv);
    public static native int fibonacci_hs(int i);
    public static native int fibonacci_c(int n);
    public static int fibonacci(int n) {
        return n < 2 ? n : fibonacci(n - 1) + fibonacci(n - 2);
    }
    public static void main(String[] args) {
        hs_init(null, null);

        for (int i = 0; i < 1000000; i++) {
            fibonacci_hs(0);
            fibonacci_c(0);
            fibonacci(0);
        }
        long t1 = System.nanoTime();
        for (int i = 0; i < 1000000; i++) {
            fibonacci_hs(0);
        }
        long t2 = System.nanoTime();
        for (int i = 0; i < 1000000; i++) {
            fibonacci_c(0);
        }
        long t3 = System.nanoTime();
        for (int i = 0; i < 1000000; i++) {
            fibonacci(0);
        }
        long t4 = System.nanoTime();
        System.out.println("fibonacci_hs(0): " + (t2 - t1)/1000000 + " ns");
        System.out.println("fibonacci_c(0): "  + (t3 - t2)/1000000 + " ns");
        System.out.println("fibonacci(0): "    + (t4 - t3)/1000000 + " ns");
    }
}

outputs this with an Intel Core i7-3632QM CPU @ 2.20GHz, Fedora 20 x86_64, GCC 4.8.3, GHC 7.6.3, and OpenJDK 8:

fibonacci_hs(0): 200 ns
fibonacci_c(0): 9 ns
fibonacci(0): 2 ns

In all fairness, I should mention that it is also pretty expensive to call into the JVM as well...

---

Update: With recent changes to JavaCPP, users can now access callback function (pointers) by name from C/C++, making it possible to call into the JVM from Haskell easily. For example, based on information found on a wiki page regarding Haskell's FFI, with the following code placed in Main.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Main where

import Foreign.C -- get the C types
import Foreign.Ptr (Ptr,nullPtr)

-- impure function
foreign import ccall "JavaCPP_init" c_javacpp_init :: CInt -> Ptr (Ptr CString) -> IO ()
javacpp_init :: IO ()
javacpp_init = c_javacpp_init 0 nullPtr

-- pure function
foreign import ccall "fibonacci" c_fibonacci :: CInt -> CInt
fibonacci :: Int -> Int
fibonacci i = fromIntegral (c_fibonacci (fromIntegral i))

main = do
  javacpp_init
  print $ fibonacci 42

and a fibonacci function defined in Java this way:

import org.bytedeco.javacpp.*;
import org.bytedeco.javacpp.annotation.*;

@Platform
public class Main {
    public static class Fibonacci extends FunctionPointer {
        public @Name("fibonacci") int call(int n) {
            return n < 2 ? n : call(n - 1) + call(n - 2);
        }
    }
}

we may build under Linux x86_64 with something like:

$ javac -cp javacpp.jar Main.java
$ java -jar javacpp.jar Main -header
$ ghc --make Main.hs linux-x86_64/libjniMain.so

and the program executes correctly giving this output:

$ ./Main
267914296