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={"<HsFFI.h>","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={"<HsFFI.h>","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
