Hi,

I can't get the FFI to work as advertised - any ideas?

Thanks,

Nawal.

Tutorial from here:

http://book.realworldhaskell.org/read/interfacing-with-c-the-ffi.html

$ cat fore.hs 

{-# LANGUAGE ForeignFunctionInterface #-}

import Prelude

import Foreign

import Foreign.C.Types

foreign import ccall "math.h sin"

     c_sin :: CDouble -> CDouble

fastsin :: Double -> Double

fastsin x = realToFrac (c_sin (realToFrac x))

main = print (fastsin 0.5)

$ jhc -phaskell2010 fore.hs -o bin/fore

jhc -phaskell2010 fore.hs -o bin/fore

jhc 0.8.2 (mydniquipepo-32)

Finding Dependencies...

Using Ho Cache: '/home/nawal/.jhc/cache'

Main                    [fore.hs]

Typechecking...

[1 of 1] Main             (......................jhc: user error (

What:    failure

Why:     context reduction, no instance for: Jhc.Class.Real.Fractional Jhc.Type.C.CDouble

Where:   on line 12 in fore.hs

in the explicitly typed Main.fastsin Main.1_x

  = Jhc.Num.realToFrac (Main.c_sin (Jhc.Num.realToFrac Main.1_x))               {- on line 12 -}

Compilation of module: Main)