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September 2017

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ffi: context reduction, no instance for: Jhc.Class.Real.Fractional Jhc.Type.C.CDouble
by Nawal Husnoo 23 Sep '17

23 Sep '17

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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