Differences in library building with using Cabal and GHC

Question

I'd like to build library from Haskell code, and further use this library (shared library: dll or so) in my C++ project.

I found simple tutorial: http://blogging.makesmeanerd.com/?p=367 And successfull repeat this example.

Further, I simplify this example, and get next code:

{-# LANGUAGE ForeignFunctionInterface #-}

module Grep where

import Foreign
import Foreign.C.String
import Data.Char

printCString :: CString -> IO ()
printCString s = do
    ss <- peekCString s
    putStrLn ss

getCStringFromKey :: IO CString
getCStringFromKey = do
    guess <- getLine
    newCString guess

foreign export ccall printCString :: CString -> IO ()
foreign export ccall getCStringFromKey :: IO CString

It's very simple program. I typed next commands:

>ghc -c -O grep.hs
>ghc -shared -o grep.dll grep.o
Creating library file: grep.dll.a

After, I have a few files: grep.dll, grep.dll.a and grep_stub.h (header file for my C++ project). I successfull use this library in C++ project. C++ code is very simple (I used MS Visual Studio):

#include <iostream>
#include <string>
#include "grep_stub.h"

int main(int argc, char* argv[])
{
    std::string testStr;
    hs_init(&argc, &argv);
    HsPtr str1 = getCStringFromKey();
    std::cout << "We've get from Haskell: " << (char*)str1 << std::endl;

    HsPtr ss = "Hello from C++!";
    printCString(ss);

    std::cout << "Test application" << std::endl;
    std::cin.get();
    hs_exit();
    return 0;
}

After compilation, this code works very well.

If I build same Haskell code (grep.hs) with using Cabal build system:

name:                grep
version: 1.0
synopsis:            example shared library for C use
build-type:          Simple
cabal-version:       >=1.10

library
  default-language:    Haskell2010
  exposed-modules:     Grep
  extra-libraries:     HSrts-ghc7.6.3
  extensions: ForeignFunctionInterface 
  build-depends:       base >= 4

And run Cabal build system:

>cabal configure --enable-shared
>cabal build
...
Creating library file: dist\build\libHSgrep-1.0-ghc7.6.3.dll.a

I got another dll (with small size), but I can't use this dll in MS VS, because I get a lot of linker errors (ever if I get dll.a files from Haskell Platform).

Main questions:

• What is difference between build library with Cabal and ghc?
• How can I build the same dll with Cabal, as I get with GHC?

Answer 1

You can set additional options in the cabal file by adding ghc-options to your library settings. Not sure what is needed in your case, but I have run into the same issue (small lib, linker errors) and for me the following setting solved it:

ghc-options: -staticlib

But I use that in an iOS project in Xcode.