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

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

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

       hs_exit();
       return 0;
    }

Now, first compile the Haskell file:

   $ ghc -c -O A.hs

Which creates some *.c and *.h headers, 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 A.o A_stub.o -o test

Then run your C code:

   $ ./test 
   Fibonacci: 267914296

And that's it.