Haskell FFI examples

cfile.c

#include <stdlib.h> // to use calloc

double cfun (double x){
    return x+1;
}

double foo (double *x){
    return x[1];
}

double* bar (int n, double *arr){
    double* y = calloc(n, sizeof(double));
    for(int i=0; i<n; i++)
      y[i] = arr[i]+1;
    return y;
}

Lib.hs

{-# LANGUAGE ForeignFunctionInterface #-}

module Lib where

import Foreign.C.Types 

import Foreign (peek, newForeignPtr_)
import Foreign.Ptr
import System.IO.Unsafe (unsafePerformIO)
import qualified Data.Vector.Storable as SV

import Foreign.Marshal.Array (peekArray)

foreign import ccall "cfun" c_cfun :: CDouble -> CDouble
cfun :: Double -> Double
cfun d = realToFrac (c_cfun (realToFrac d))

-- https://stackoverflow.com/questions/9854782/can-the-ffi-deal-with-arrays-if-so-how
foreign import ccall "foo" c_foo :: Ptr CDouble -> CDouble
haskellFoo :: SV.Vector CDouble -> CDouble
haskellFoo sv = unsafePerformIO $ SV.unsafeWith sv (return . c_foo)

test_foo :: CDouble
test_foo = haskellFoo (SV.fromList [3,2,1])

foreign import ccall "bar" c_bar :: Ptr CInt -> Ptr CDouble -> Ptr CDouble

haskellBar :: SV.Vector CDouble -> IO [CDouble]
haskellBar sv = do
    let n = SV.length sv
    ptr <- SV.unsafeWith sv (return . (c_bar (fromIntegral n)))
    peekArray n ptr

test_bar :: IO [CDouble]
test_bar = haskellBar (SV.fromList [3,2,1])

haskellBar2 :: SV.Vector CDouble -> IO (SV.Vector CDouble)
haskellBar2 sv = do
    ptr <- SV.unsafeWith sv (return . c_bar)
    fptr <- newForeignPtr_ ptr
    return $ SV.unsafeFromForeignPtr0 fptr (SV.length sv)

test_bar2 :: IO (SV.Vector CDouble)
test_bar2 = haskellBar2 (SV.fromList [3,2,1])
    

testC.cabal

name:                testC
version:             0.1.0.0
-- synopsis:
-- description:
homepage:            https://github.com/githubuser/testC#readme
license:             BSD3
license-file:        LICENSE
author:              Author name here
maintainer:          [email protected]
copyright:           2017 Author name here
category:            Web
build-type:          Simple
extra-source-files:  README.md
cabal-version:       >=1.10
extra-source-files:  C/cfile.h

library
  hs-source-dirs:      src
  exposed-modules:     Lib
  build-depends:       base >= 4.7 && < 5
                     , vector
  default-language:    Haskell2010
  default-extensions:  ForeignFunctionInterface
  include-dirs:        C
  C-sources:           C/cfile.c

source-repository head
  type:     git
  location: https://github.com/githubuser/testC