Example of calling C from Haskell using the FFI

FfiExample.hs

{-# LANGUAGE ForeignFunctionInterface #-}

-- Simple example of calling C from Haskell.
-- 
-- $ ghci
-- > :load FfiExample.hs
-- > c_sin pi
-- 1.2246467991473532e-16
--
-- $ ghc --make FfiExample.hs
-- $ ./FfiExample

module Main where

import Foreign
import Foreign.C.Types
import Foreign.C.String
import Text.Printf

-- http://book.realworldhaskell.org/read/interfacing-with-c-the-ffi.html
foreign import ccall "math.h sin"
    c_sin :: CDouble -> CDouble

foreign import ccall "stdlib.h system"
    c_system :: CString -> IO CInt

sin2 :: Double -> Double
sin2 x =
    realToFrac $ c_sin $ realToFrac x

system :: String -> IO CInt
system cmd =
    withCString cmd c_system

main = do
    putStrLn (printf "sin 3.14 = %.3f" (sin2 3.14))
    putStrLn "ls:"
    system "ls"
    return ()