forked from GitHub/gf-core
35 lines
736 B
Haskell
35 lines
736 B
Haskell
{-# LANGUAGE CPP #-}
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{-# LANGUAGE DeriveFunctor #-}
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-- 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 GFCC.ErrM where
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-- Control.Monad.Fail import will become redundant in GHC 8.8+
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import qualified Control.Monad.Fail as Fail
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import Control.Monad (ap)
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-- the Error monad: 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, Functor)
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instance Applicative Err where
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pure = Ok
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(<*>) = ap
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instance Monad Err where
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return = Ok
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Ok a >>= f = f a
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Bad s >>= f = Bad s
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#if !(MIN_VERSION_base(4,13,0))
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fail = Bad
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#endif
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instance Fail.MonadFail Err where
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fail = Bad
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