================ CInterface.hsc =====================
{-# LANGUAGE ForeignFunctionInterface #-}

module CInterface where

import Foreign.C
import Foreign.Storable
import Foreign.Ptr
import Data.Bits

foreign export ccall printInput :: Ptr Input -> IO ()


data Input = Input { alpha       :: Bool
                   , beta        :: Bool } deriving (Eq, Show)

-- from http://www.haskell.org/haskellwiki/FFI_cook_book
#let alignment t = "%lu", (unsigned long)offsetof(struct {char x__; t (y__); }, y__)

#def typedef struct {
  union {
    struct {
      unsigned char alpha : 1;
      unsigned char beta : 1;
      unsigned char reserved : 6;
    } baz;
    unsigned char baz_all;
  };
} input_t;


instance Storable Input where
  alignment _ = #{alignment input_t}
  sizeOf _ = #{size input_t}
  peek ptr = do
    b <- ( #{peek input_t, baz_all} ptr )::IO CUChar
    return Input { alpha       = testBit b 0
                 , beta        = testBit b 1 }
  poke ptr inp = undefined


printInput :: Ptr Input -> IO ()
printInput pInp = do
  inp <- peek pInp
  putStrLn $ show inp

================ ctest.c =====================
#include "HsFFI.h"
#include "CInterface_stub.h"
#include "CInterface_hsc.h"
#include "dll_init.h"

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

  ////////////////////////////////////////
  input_t inp = {0};
  inp.baz.alpha = 1;
  inp.baz.beta = 0;

  printInput( &inp );
  ////////////////////////////////////////

  sp_exit();
  return 0;
}

================ dll_init.h =====================
#pragma once

extern void __stginit_CInterface(void);

extern void sp_init(int*, char***);
extern void sp_exit(void);

================ dll_init.c =====================
#include "HsFFI.h"
#include "dll_init.h"

void sp_init(int* p_argc, char*** p_argv) {
  hs_init(p_argc, p_argv);
  hs_add_root(__stginit_CInterface);
}

void sp_exit(void) {
  hs_exit();
}

=================== test.cabal =====================
name:                test
version:             0.0
cabal-version:	     >1.2
synopsis:            <Project description>
description:         <Project description>
category:            Codec
license:             BSD3
license-file:        LICENSE
author:              # example, `Fred Bloggs
maintainer:          fred@example.net
build-depends:       base >=4 && <5
build-type:          Simple


library
  build-depends:     base >=4 && <5
  exposed-modules:   CInterface
  c-sources:         dll_init.c
  ghc-options:       -threaded

====================== LICENSE ===============

====================== Setup.lhs ==============
#!/usr/bin/env runhaskell
> import Distribution.Simple
> main = defaultMain