Calling Haskell from C

As part of improving the random number generation story for Haskell, I want to be able to use the testu01 library with the minimal amount of Haskell wrapping. testu01 assumes that there is a C function which returns the random number. The ghc manual gives an example but does not give all the specifics. These are my notes on how to get the example working under OS X (El Capitain 10.11.5 to be precise).

The Haskell:

{-# OPTIONS_GHC -Wall                 #-}

{-# LANGUAGE ForeignFunctionInterface #-}

module Foo where

foreign export ccall foo :: Int -> IO Int

foo :: Int -> IO Int
foo n = return (length (f n))

f :: Int -> [Int]
f 0 = []
f n = n:(f (n-1))

The .cabal:

name:                test-via-c
version:             0.1.0.0
homepage:            TBD
license:             MIT
author:              Dominic Steinitz
maintainer:          idontgetoutmuch@gmail.com
category:            System
build-type:          Simple
cabal-version:       >=1.10

executable Foo.dylib
  main-is: Foo.hs
  other-extensions:    ForeignFunctionInterface
  build-depends:       base >=4.7 && =0.6 && <0.7
  hs-source-dirs:      src
  default-language:    Haskell2010
  include-dirs:        src
  ghc-options:         -O2 -shared -fPIC -dynamic
  extra-libraries:     HSrts-ghc8.0.1

On my computer running

cabal install

places the library in

~/Library/Haskell/ghc-8.0.1/lib/test-via-c-0.1.0.0/bin

The C:

#include 
#include "HsFFI.h"

#include "../dist/build/Foo.dylib/Foo.dylib-tmp/Foo_stub.h"

int main(int argc, char *argv[])
{
  int i;

  hs_init(&argc, &argv);

  for (i = 0; i < 5; i++) {
    printf("%d\n", foo(2500));
  }

  hs_exit();
  return 0;
}

On my computer this can be compiled with

gcc-6 Bar.c
~/Library/Haskell/ghc-8.0.1/lib/test-via-c-0.1.0.0/bin/Foo.dylib
-I/Library/Frameworks/GHC.framework/Versions/8.0.1-x86_64/usr/lib/ghc-8.0.1/include
-L/Library/Frameworks/GHC.framework/Versions/8.0.1-x86_64/usr/lib/ghc-8.0.1/rts
-lHSrts-ghc8.0.1

and can be run with

DYLD_LIBRARY_PATH=
~/Library/Haskell/ghc-8.0.1/lib/test-via-c-0.1.0.0/bin:
/Library/Frameworks/GHC.framework/Versions/8.0.1-x86_64/usr/lib/ghc-8.0.1/rts

N.B. setting DYLD_LIBRARY_PATH like this is not recommended as it is a good way of breaking things. I have tried setting DYLD_FALLBACK_LIBRARY_PATH but only to get an error message. Hopefully, at some point I will be able to post a robust way of getting the executable to pick up the required dynamic libraries.