Copyright 2010-2015 Mario Blazevic

    This file is part of the Streaming Component Combinators (SCC) project.

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-- | This module defines parsing combinators for incremental parsers with left-biased local choice.
-- The exported 'Parser' type can provide partial parsing results from partial input, as long as the output is a
-- 'Monoid'. Construct a parser using the primitives and combinators, supply it with input using functions 'feed' and
-- 'feedEof', and extract the parsed output using 'results'.
-- Implementation is based on Brzozowski derivatives.

{-# LANGUAGE EmptyDataDecls, FlexibleInstances #-}

module Text.ParserCombinators.Incremental.LeftBiasedLocal (
   module Text.ParserCombinators.Incremental,
   Parser, LeftBiasedLocal, leftmost

import Control.Applicative (Alternative (empty, (<|>), many, some))
import Control.Monad (MonadPlus (mzero, mplus))
import Data.Monoid (Monoid)

import Text.ParserCombinators.Incremental hiding (Parser)
import qualified Text.ParserCombinators.Incremental as Incremental (Parser)

-- | An empty type to specialize 'Parser' for the left-biased 'Alternative' instance.
data LeftBiasedLocal

type Parser s r = Incremental.Parser LeftBiasedLocal s r

-- | Left-biased choice. The right parser is used only if the left one utterly fails.
instance Monoid s => Alternative (Incremental.Parser LeftBiasedLocal s) where
   empty = failure
   p1 <|> p2 = p1 <<|> p2
   many = defaultMany
   some = defaultSome

-- | The 'MonadPlus' instances are the same as the 'Alternative' instances.
instance Monoid s => MonadPlus (Incremental.Parser LeftBiasedLocal s) where
   mzero = failure
   mplus = (<|>)

leftmost :: Parser s r -> Incremental.Parser a s r
leftmost p = mapType leftmost p