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ErrM.hs

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