From Haskell to C and C to Haskell

A classic usage of the FFI is calling C functions from Haskell. So let's start with that. Consider that you wrote a function in C, for instance the recursive nth Fibonacci number like the fib.c file here:

/* file: fib.c */

int fib_c(int num)
{
  if (num <= 2)
  {
    return 1;
  }
  else
  {
    return(fib_c(num - 1) + fib_c(num - 2));
  }
}

Although naive, this implementation is still faster than the Haskell equivalent.

Now, to call this fast naive fib_c function from Haskell, at its simplest we could just add the following line to a Haskell source file and then we would have a fib_c :: Int → Int Haskell function:

-- file: ffi-fib.hs

foreign import ccall
  fib_c :: Int -> Int

main = print $ fib_c 20

The only FFI-specific thing here is foreign import. Note that, with some earlier versions of GHC, it was necessary to explicitly enable the FFI with -XForeignFunctionInterface.

To compile this program, we should give both the Haskell and C...