content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE CPP, NoImplicitPrelude #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Foreign.Marshal.Error
  -- Copyright   :  (c) The FFI task force 2001
  -- License     :  BSD-style (see the file libraries/base/LICENSE)
  -- 
  -- Maintainer  :  ffi@haskell.org
  -- Stability   :  provisional
  -- Portability :  portable
  --
  -- Routines for testing return values and raising a 'userError' exception
  -- in case of values indicating an error state.
  --
  -----------------------------------------------------------------------------
  
  module Foreign.Marshal.Error (
    throwIf,
    throwIf_,
    throwIfNeg,
    throwIfNeg_,
    throwIfNull,
  
    -- Discard return value
    --
    void
  ) where
  
  import Foreign.Ptr
  
  #ifdef __HADDOCK__
  import Data.Bool
  import System.IO.Error
  #endif
  import GHC.Base
  import GHC.Num
  import GHC.IO.Exception
  
  -- exported functions
  -- ------------------
  
  -- |Execute an 'IO' action, throwing a 'userError' if the predicate yields
  -- 'True' when applied to the result returned by the 'IO' action.
  -- If no exception is raised, return the result of the computation.
  --
  throwIf :: (a -> Bool)  -- ^ error condition on the result of the 'IO' action
          -> (a -> String) -- ^ computes an error message from erroneous results
                          -- of the 'IO' action
          -> IO a         -- ^ the 'IO' action to be executed
          -> IO a
  throwIf pred msgfct act  = 
    do
      res <- act
      (if pred res then ioError . userError . msgfct else return) res
  
  -- |Like 'throwIf', but discarding the result
  --
  throwIf_                 :: (a -> Bool) -> (a -> String) -> IO a -> IO ()
  throwIf_ pred msgfct act  = void $ throwIf pred msgfct act
  
  -- |Guards against negative result values
  --
  throwIfNeg :: (Ord a, Num a) => (a -> String) -> IO a -> IO a
  throwIfNeg  = throwIf (< 0)
  
  -- |Like 'throwIfNeg', but discarding the result
  --
  throwIfNeg_ :: (Ord a, Num a) => (a -> String) -> IO a -> IO ()
  throwIfNeg_  = throwIf_ (< 0)
  
  -- |Guards against null pointers
  --
  throwIfNull :: String -> IO (Ptr a) -> IO (Ptr a)
  throwIfNull  = throwIf (== nullPtr) . const
  
  -- |Discard the return value of an 'IO' action
  --
  void     :: IO a -> IO ()
  void act  = act >> return ()
  {-# DEPRECATED void "use 'Control.Monad.void' instead" #-} -- deprecated in 7.6
  