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 source file to...