From nawal at husnoo.com  Sat Sep 23 10:11:48 2017
From: nawal at husnoo.com (Nawal Husnoo)
Date: Sat, 23 Sep 2017 11:11:48 +0100
Subject: [jhc] ffi: context reduction,
	no instance for: Jhc.Class.Real.Fractional Jhc.Type.C.CDouble
Message-ID: <CAMC00mkCopzaMO2xZtw9za2496Rk_=rSdqdGN=GOVHH9g0DpgA@mail.gmail.com>

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)
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From nawal at husnoo.com  Sat Sep 23 14:21:56 2017
From: nawal at husnoo.com (Nawal Husnoo)
Date: Sat, 23 Sep 2017 15:21:56 +0100
Subject: [jhc] ffi: context reduction,
 no instance for: Jhc.Class.Real.Fractional Jhc.Type.C.CDouble
In-Reply-To: <CAMC00mkCopzaMO2xZtw9za2496Rk_=rSdqdGN=GOVHH9g0DpgA@mail.gmail.com>
References: <CAMC00mkCopzaMO2xZtw9za2496Rk_=rSdqdGN=GOVHH9g0DpgA@mail.gmail.com>
Message-ID: <CAMC00mkhE29=ut9-5OHfWOahNd9GsveGVxXWxxGYkCUPyd45LA@mail.gmail.com>

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 <nawal at husnoo.com> 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)
>
>
>
>
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