{-# LANGUAGE CPP, ForeignFunctionInterface #-}
{-# OPTIONS_GHC -Wall #-}

module Foreign.C.Error.Errno (withErrno,withErrnoPred)
    where

import Control.Monad.Error
import Foreign.C.Error
import Foreign.C.String
import Foreign.C.Types

#include <errno.h>

getErrorString :: IO String
getErrorString = do
  (Errno i) <- getErrno
  s <- errorString i
  peekCString s

withErrno :: (Integral a, MonadIO m) => m a -> ErrorT String m a
withErrno = withErrnoPred (==(-1))

withErrnoPred :: (Integral a, MonadIO m) => (a -> Bool) -> m a -> ErrorT String m a
withErrnoPred f m = do
  i <- lift m
  if f i 
    then do
      msg <- liftIO getErrorString
      throwError msg
    else return i
      
foreign import ccall "errno.h strerror" errorString :: CInt -> IO CString