Calling Haskell from c++

Question

I'm trying to call Haskell from c++.

I tried to use this explanation; and already asked one question on SO.

However, I haven't got any answer so I'd like to reformulate to a Minimal, Complete, and Verifiable example.

I'm using Debian and this is what I have (In the same folder):

c++:

// main.cpp
#include <iostream>
#include "Hello_stub.h"

int main(int argc, char** argv) {
  hs_init(&argc, &argv);
  std::cout << "Hello from C++\n";
  helloFromHaskell();
  hs_exit();
  return 0;
}

Haskell:

// hello.hs
module Hello where

foreign export ccall helloFromHaskell :: IO ()

helloFromHaskell :: IO ()
helloFromHaskell = putStrLn "Hello from Haskell"

MakeFile:

CPP_SOURCES = main.cpp
HASKELL_SOURCES = Hello.hs
CFLAGS = -Wall -g -fno-stack-protector
HFLAGS = -fforce-recomp

all: main; ./main

main: $(CPP_SOURCES) HaskellPart; g++ \
    $(CFLAGS) -o main $(CPP_SOURCES) Hello.o \
    -I/usr/lib/ghc/include \
    -liconv \
    -I/usr/lib/ghc/ghc-8.0.1/include \
    -L/usr/lib/ghc/ghc-8.0.1 \
    -L/usr/lib/ghc/base-4.9.0.0 \
    -lHSbase-4.9.0.0 \
    -L/usr/lib/ghc/ghc-prim-0.5.0.0 \
    -lHSghc-prim-0.5.0.0 \
    -L/usr/lib/ghc/integer-gmp-1.0.0.1 \
    -lHSinteger-gmp-1.0.0.1 \
    -lHSghc-prim-0.5.0.0 \
    -L/usr/lib/ghc/rts \
    -lHSrts \

HaskellPart: $(HASKELL_SOURCES); ghc $(HFLAGS) $(HASKELL_SOURCES)

clean: ; rm -rf main && rm -rf *.o && rm -rf *.hi && rm -rf *_stub.h

Here's the output. It seems to be a bunch of errors of the form

/usr/bin/ld: Hello.o: relocation R_X86_64_32S against symbol `stg_bh_upd_frame_info' can not be used when making a shared object; recompile with -fPIC

What's wrong? Thank you for your help !

Answer 1

Not sure whether that's actually in your file or whether it's just in the version you put in your question but "// hello.hs" won't compile. Comments are -- in Haskell not //.

Anyway on to the interesting part...

First you need to import the HsFFI.h header file into your C++ code.

#include <iostream>
#include "Hello_stub.h"
#include <HsFFI.h>

Then use ghc to link the files after compiling them. Open a command prompt / terminal and navigate the directory containing your C++ and Haskell files. Then run the following commands:

ghc -c -XForeignFunctionInterface -O hello.hs
g++ -c -O main.cpp -I "C:\Program Files\Haskell Platform\7.10.3\lib\include"                         
ghc -no-hs-main hello.o main.o -lstdc++

The filepath in the second command is to the directory containing the HsFFI.h file.

Running main then outputs:

Hello from C++
Hello from Haskell