我想知道解析多个解析器可以成功的输入的最佳方法。我已经概述了我的第一次失败的尝试和一个不优雅的解决方案,我希望它可以变得更加地道。
例如,我想将以下句子中的“the”、“quick”和“fox”应用到它们自己的数据构造函数中:
"the quick brown fox jumps over the lazy dog".
所以给定以下类型构造函数:
data InterestingWord = Quick | The | Fox deriving Show
data Snippet = Word InterestingWord | Rest String deriving Show
我希望解析的输出是:
[Word The,
Rest " ", Word Quick,
Rest " brown ", Word Fox,
Rest " jumped over ", Word The,
Rest " lazy dog"]
以下是两种解决方案:
import Text.Parsec
import Data.Maybe
import Data.Ord
import Data.List
data InterestingWord = Quick | The | Fox deriving Show
data Snippet = Word InterestingWord | Rest String deriving Show
testCase = "the quick brown fox jumped over the lazy dog"
-- Expected output:
-- [Word The,
-- Rest " ", Word Quick,
-- Rest " brown ", Word Fox,
-- Rest " jumped over ", Word The,
-- Rest " lazy dog"]
toString Quick = "quick"
toString The = "the"
toString Fox = "fox"
-- First attempt
-- Return characters upto the intended word along
-- with the word itself
upto word = do
pre <- manyTill anyChar $ lookAhead $ string (toString word)
word' <- try $ string (toString word)
return [Rest pre, Word word]
-- Parsers for the interesting words
parsers = [upto Quick,
upto The,
upto Fox]
-- Try each parser and return its results with the
-- rest of the input.
-- An incorrect result is produced because "choice"
-- picks the first successful parse result.
wordParser = do
snippets <- many $ try $ choice parsers
leftOver <- many anyChar
return $ concat $ snippets ++ [[Rest leftOver]]
-- [Rest "the ",Word Quick,Rest " brown fox jumped over the lazy dog"]
test1 = parseTest wordParser testCase
-- Correct
-- In addition to the characters leading upto the
-- word and the word, the position is also returned
upto' word = do
result <- upto word
pos <- getPosition
return (pos, result)
-- The new parsers
parsers' = [upto' Quick,
upto' The,
upto' Fox]
-- Try each of the given parsers and
-- possibly returning the results and
-- the parser but don't consume
-- input.
tryAll = mapM (\p -> do
r <- optionMaybe $ try (lookAhead p)
case r of
Just result -> return $ Just (p, result)
Nothing -> return $ Nothing
)
-- Pick the parser that has consumed the least.
firstSuccess ps = do
successes <- tryAll ps >>= return . catMaybes
if not (null successes) then
return $ Just (fst $ head (sortBy (comparing (\(_,(pos,_)) -> pos)) successes))
else return $ Nothing
-- Return the parse results for the parser that
-- has consumed the least
wordParser' = do
parser <- firstSuccess parsers'
case parser of
Just p -> do
(_,snippet) <- p
return snippet
Nothing -> parserZero
-- Returns the right result
test2 = parseTest (many wordParser' >>= return . concat) testCase
第一次尝试“test1”不会产生所需的输出,因为“选择”返回第一个成功的解析器,而我真正想要的是第一个成功的解析器,同时消耗最少的字符。这就是我接下来要尝试的方法,方法是保留已解析输入的源位置并使用具有最低源位置的解析器。
这种情况似乎很常见,我觉得我错过了一些明显的组合符咒语。谁能提供更好的建议?
谢谢!
-deech