{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies #-}

--
-- (c) 2012 Wei Ke
--      license:          GPL-3
--      license-file:     LICENSE
--
-- |
-- The "ErrMsg" module defines the 'ErrMsg' monad for error handling
-- in the type checking and the runtime system,
-- and some error message operators for convenience. 
--
module ErrMsg where

--
-- monadic ErrMsg
--

data ErrMsg a = Err String
              | OK a
              
instance Monad ErrMsg where
    Err x >>= _ = Err x
    OK x  >>= f = f x
    return x    = OK x
    fail x      = Err x

instance Show a => Show (ErrMsg a) where
    show (Err x) = "Err# " ++ x
    show (OK x) = "OK " ++ show x
    
isOK :: ErrMsg a -> Bool
isOK (Err _) =  False
isOK _       =  True

isErr :: ErrMsg a -> Bool
isErr (Err _) =  True
isErr _       =  False

infixl 2 ++#

(++#) :: ErrMsg a -> String -> ErrMsg a
Err x ++# s = Err (x ++ s)
x ++# _     = x

--
-- converters to ErrMsg
--

infixl 2 |?
    
class PlusMsg a b | a -> b where
    (|?) :: a -> String -> ErrMsg b

instance PlusMsg Bool () where
    False |? s = Err s
    True  |? _ = OK ()
    
instance PlusMsg (Maybe a) a where
    Nothing |? s = Err s
    Just x  |? _ = OK x

--
-- constants
--

errCtx :: String -> String
errCtx s = "\n    in " ++ s

--
-- end of ErrMsg
--
-- --$Id: ErrMsg.hs 1191 2012-11-15 05:13:17Z wke@IPM.EDU.MO $