How to pass a string from Haskell to C?

Question

All I want to do is pass a plain-text string from Haskell to C. However, it says that [Char] is an unacceptable return type. I can't find anywhere why they think it is, nor what acceptable return types are.

I'm trying to make a very simple OS image that I can boot with Qemu.

Does anyone know how to do this? Thanks.

    {-# LANGUAGE ForeignFunctionInterface #-}

    module Hello where

    import Foreign
    import Foreign.C.String
    import Foreign.C.Types

    hello :: String -> (CString -> IO a) -> IO a
    hello = "Hello, world!"

    foreign export ccall hello :: String -> (CString -> IO a) -> IO a

Answer 1

You want a CString.

Going from CString to String:

peekCString :: CString -> IO String

Going from String to CString:

withCString :: String -> (CString -> IO a) -> IO a

There's also Haddock documentation for module Foreign.C.String.

The general list of types that can be used in foreign declarations is specified as part of the Foreign Function Interface in the Haskell Report.

Edit

Ok, here's a very small example of a thing you can do, somewhat based on your sample code. Create a Haskell file CTest.hs with the following contents:

module CTest where

import Foreign.C

hello :: IO CString
hello = newCString "hello"

foreign export ccall hello :: IO CString

Then create a C file ctest.c with the following contents:

#include <stdio.h>
#include "CTest_stub.h"

int main (int argc, char *argv[]) {
  hs_init(&argc, &argv);
  printf("%s\n", hello());
  hs_exit();
  return 0;
}

Then compile and run as follows:

$ ghc CTest
[1 of 1] Compiling CTest            ( CTest.hs, CTest.o )
$ ghc -o ctest ctest.c CTest.o -no-hs-main
$ ./ctest
hello

Answer 2

I think what you need is System.IO.Unsafe.unsafePerformIO to convert IO CString to CString before sending the CString to C. newCString will convert a Haskell String to IO CString. Thus System.IO.Unsafe.unsafePerformIO $ newCString a can be passed to your C routine which will accept input of type char*. If your C routine returns static char* then System.IO.Unsafe.unsafePerformIO $ peekCString will give you back a Haskell string. You need to import System.IO.Unsafe. unsafePerformIO has an implementation in Foreign.C.String (or Foreign.C.Types ?) which is deprecated, so you have to use the full path. I had a hell of a time before I could find unsafePerformIO - probably because people are allergic to something that is so dangerous as to force declaration of impure to pure. newCString can lead to memory leaks if used repeatedly without cleaning. withCString may be a better option - will learn that later.