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27 lines
592 B
Haskell
27 lines
592 B
Haskell
-- BNF Converter: Error Monad
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-- Copyright (C) 2004 Author: Aarne Ranta
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-- This file comes with NO WARRANTY and may be used FOR ANY PURPOSE.
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module GF.Devel.Compile.ErrM where
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-- the Error monad: like Maybe type with error msgs
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import Control.Monad (MonadPlus(..), liftM)
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data Err a = Ok a | Bad String
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deriving (Read, Show, Eq, Ord)
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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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instance Functor Err where
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fmap = liftM
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instance MonadPlus Err where
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mzero = Bad "Err.mzero"
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mplus (Bad _) y = y
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mplus x _ = x
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