foo.c

#include "header.h"

int q() {
  int e = foo(0);
  return(0);
}

header.h

#include <HsFFI.h>

int foo(int);

Main.hs

{-# LANGUAGE TemplateHaskell #-}
module Main where

import P

main = $([| return () |])

P.hs

module P where

foreign export ccall foo :: Int -> IO Int

foo :: Int -> IO Int
foo n = return (length (f n))

f :: Int -> [Int]
f 0 = []
f n = n:(f (n-1))