Play with FFI in Haskell

Lib.hs

{-# LANGUAGE CPP, ForeignFunctionInterface #-}

module Lib where

import Data.Word
import Foreign
import Foreign.C.Types
import Foreign.C.Error
import Foreign.C.String

foreign import ccall "open" cOpenFile :: CString -> CInt -> IO CInt
#define O_RDWR    0x0002
#define O_CREAT   0x0200

foreign import ccall "close" cCloseFile :: CInt -> IO ()

foreign import ccall "write" cWriteFile :: CInt -> Ptr () -> Word -> IO Word

newtype FileDescriptor = FileDescriptor { unFD :: CInt }

getFileDescriptor :: String -> IO FileDescriptor
getFileDescriptor path = do
  pathCStr <- newCString path
  fd <- cOpenFile pathCStr 0x0202
  free pathCStr
  if fd == -1
    then throwErrno "fd == - 1"
    else return $ FileDescriptor fd

deleteFileDescriptor :: FileDescriptor -> IO ()
deleteFileDescriptor descriptor = cCloseFile $ unFD descriptor

writeToFileDescriptor :: FileDescriptor -> String -> IO ()
writeToFileDescriptor (FileDescriptor fd) string = withCStringLen string helper
  where
    helper :: (CString, Int) -> IO ()
    helper (cstr, length) = do
      let ulen = (fromIntegral length) :: Word
      cWriteFile fd (castPtr cstr) ulen
      return ()