Oh, it's simpler than that, i.e. use Double instead of CDouble: foreign import ccall "math.h sin" c_sin :: Double -> Double main = do let a = c_sin 0.9 print a $ jhc fore.hs -o bin/fore $ ./bin/fore 0.7833269096274834 On 23 September 2017 at 11:11, Nawal Husnoo wrote:

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)