module Foreign.Marshal.Error
  ( throwIf       -- :: (a -> Bool) -> (a -> String) -> IO a       -> IO a
  , throwIf_      -- :: (a -> Bool) -> (a -> String) -> IO a       -> IO ()
  , throwIfNeg    -- :: (Ord a, Num a) 
                  -- =>                (a -> String) -> IO a       -> IO a
  , throwIfNeg_   -- :: (Ord a, Num a)
                  -- =>                (a -> String) -> IO a       -> IO ()
  , throwIfNull   -- ::                String        -> IO (Ptr a) -> IO (Ptr a)
  , void          -- IO a -> IO ()
  ) where

import Foreign.Ptr

throwIf                 :: (a -> Bool) -> (a -> String) -> IO a -> IO a
throwIf pred msgfct act  = 
  do
    res <- act
    (if pred res then ioError . userError . msgfct else return) res

throwIf_                 :: (a -> Bool) -> (a -> String) -> IO a -> IO ()
throwIf_ pred msgfct act  = void $ throwIf pred msgfct act

throwIfNeg :: (Ord a, Num a) => (a -> String) -> IO a -> IO a
throwIfNeg  = throwIf (< 0)

throwIfNeg_ :: (Ord a, Num a) => (a -> String) -> IO a -> IO ()
throwIfNeg_  = throwIf_ (< 0)

throwIfNull :: String -> IO (Ptr a) -> IO (Ptr a)
throwIfNull  = throwIf (== nullPtr) . const

void     :: IO a -> IO ()
void act  = act >> return ()