A simple example of FFI in Haskell using the standard library, a source file, and a library.

Main.hs

{-# LANGUAGE ForeignFunctionInterface #-}

module Main (main) where

import Foreign.C.Types (CDouble (..))

foreign import ccall safe "math.h sin" c_sin :: CDouble -> CDouble
foreign import ccall safe "public.h square" c_square :: CDouble -> CDouble
foreign import ccall safe "private.h half" c_half :: CDouble -> CDouble

main :: IO ()
main = do
    print $ sin 1.2
    print $ c_sin 1.2
    print $ 1.2 * 1.2
    print $ c_square 1.2
    print $ 1.2 / 2
    print $ c_half 1.2

Makefile

.PHONY: all build clean

all: build

build:
	clang -c private.c
	ghc --make Main.hs public.c private.o

clean:
	rm -f Main Main.hi Main.o public.o private.o

private.c

double half ( double x ) { return x / 2 ; }

private.h

double half ( double x ) ;

public.c

double square ( double x ) { return x * x ; }

public.h

double square ( double x ) ;

# clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.2.0
Thread model: posix

# ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.6.3