Calling Haskell from C

It is not uncommon to want to call a Haskell function from C code. Here's how to do that.

We define the fibonacci function in Haskell:

{-# LANGUAGE ForeignFunctionInterface #-}

module Safe where

import Foreign.C.Types

fibonacci :: Int -> Int
fibonacci n = fibs !! n
    where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral

foreign export ccall fibonacci_hs :: CInt -> CInt

Note the foreign export. When GHC sees this, it will generate stubs for C, to help it work out the Haskell types.

And call it from C:

#include <HsFFI.h>
#ifdef __GLASGOW_HASKELL__
#include "Safe_stub.h"
extern void __stginit_Safe(void);
#endif
#include <stdio.h>

int main(int argc, char *argv[])
{
    int i;
    hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
    hs_add_root(__stginit_Safe);
#endif

    i = fibonacci_hs(42);
    printf("Fibonacci: %d\n", i);

    hs_exit();
    return 0;
}

Now, first compile the Haskell file:

$ ghc -c -O Safe.hs

Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, which you import into your C program. Now compile your C code with ghc (!), passing the Haskell objects on the command line:

$ ghc -optc-O test.c Safe.o Safe_stub.o -o test

Note, this will not work for ghc >= 7.2 due to ghc not generating Safe_stub.o file (thanks to Daniel Fischer for pointing this out), use the following command instead:

$ ghc --make -no-hs-main -optc-O test.c Safe -o test

Then run your C code:

$ ./test 
Fibonacci: 267914296

And that's it.