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39 lines
928 B
Haskell
39 lines
928 B
Haskell
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-- |
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-- Module : ErrM
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-- Maintainer : AR
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-- Stability : (stable)
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-- Portability : (portable)
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--
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-- > CVS $Date: 2005/04/21 16:22:00 $
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-- > CVS $Author: bringert $
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-- > CVS $Revision: 1.5 $
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--
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-- hack for BNFC generated files. AR 21/9/2003
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-----------------------------------------------------------------------------
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module GF.Data.ErrM (Err(..)) where
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import Control.Monad (MonadPlus(..))
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-- | like @Maybe@ type with error msgs
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data Err a = Ok a | Bad String
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deriving (Read, Show, Eq)
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instance Monad Err where
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return = Ok
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fail = Bad
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Ok a >>= f = f a
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Bad s >>= f = Bad s
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-- | added 2\/10\/2003 by PEB
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instance Functor Err where
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fmap f (Ok a) = Ok (f a)
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fmap f (Bad s) = Bad s
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-- | added by KJ
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instance MonadPlus Err where
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mzero = Bad "error (no reason given)"
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mplus (Ok a) _ = Ok a
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mplus (Bad s) b = b
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