选择正确的ReadP解析结果

问题描述：

我试图解析RFC5322电子邮件地址。我的解析器的工作原理是，在结果中，其中一个是正确的。但是，我该如何选择“正确”的结果呢？选择正确的ReadP解析结果

鉴于字符串Foo Bar <[email protected]>，我的解析器应该产生一个值Address (Just "Foo Bar") "[email protected]"。

另外，给定字符串[email protected]，我的分析器应该产生一个值Address Nothing "[email protected]"。

包含名称的值是首选。

我的解析器是这样的：

import   Control.Applicative 
import   Data.Char 
import qualified Data.Text      as T 
import   Text.ParserCombinators.ReadP 

onlyEmail :: ReadP Address 
onlyEmail = do 
    skipSpaces 
    email <- many1 $ satisfy isAscii 
    skipSpaces 
    return $ Address Nothing (T.pack email) 

withName :: ReadP Address 
withName = do 
    skipSpaces 
    name <- many1 (satisfy isAscii) 
    skipSpaces 
    email <- between (char '<') (char '>') (many1 $ satisfy isAscii) 
    skipSpaces 
    return $ Address (Just $ T.pack name) (T.pack email) 

rfc5322 :: ReadP Address 
rfc5322 = withName <|> onlyEmail

当我运行readP_to_S rfc5322 "Foo Bar <[email protected]>"分析器，它产生以下结果：

[ (Address {addressName = Nothing, addressEmail = "F"},"oo Bar <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Fo"},"o Bar <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo"},"Bar <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo "},"Bar <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo B"},"ar <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Ba"},"r <[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar"},"<[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar "},"<[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <"},"[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <f"},"[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <fo"},"[email protected]>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo"},"@bar.com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"bar.com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"ar.com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"r.com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},".com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"com>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"om>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},"m>") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]"},">") 
, (Address {addressName = Just "Foo Bar", addressEmail = "[email protected]"},"") 
, (Address {addressName = Just "Foo Bar ", addressEmail = "[email protected]"},"") 
, (Address {addressName = Nothing, addressEmail = "Foo Bar <[email protected]>"},"") 
]

在这种情况下，结果其实我是想出现三阶最后在列表中。我如何表达这种偏好？

答

你不应该做偏好。你的问题是你的部分解析器正在接受比真正需要更大的字符串集。

例如，我的解决方案：

import   Control.Bool 
import   Control.Applicative 
import   Data.Char 
import qualified Data.Text      as T 
import   Data.Text (Text) 
import   Text.ParserCombinators.ReadP 

email :: ReadP Text 
email = do 
    l <- part 
    a <- char '@' 
    d <- part 
    return . T.pack $ l ++ a:d 
    where 
    part = munch1 (isAscii <&&> (/='@') <&&> (/='<') <&&> (/='>')) 

name :: ReadP Text 
name = T.pack <$> chainr1 part sep 
    where 
    part = munch1 (isAlpha <||> isDigit <||> (=='\'')) 
    sep = (\xs ys -> xs ++ ' ':ys) <$ munch1 (==' ') 

onlyEmail :: ReadP Address 
onlyEmail = Address Nothing <$> email 

withName :: ReadP Address 
withName = do 
    n <- name 
    skipSpaces 
    e <- between (char '<') (char '>') email 
    return $ Address (Just n) e 

address :: ReadP Address 
address = skipSpaces *> (withName <|> onlyEmail) 

main = print $ readP_to_S address "Foo Bar <[email protected]>"

将被打印：

[(Address (Just "Foo Bar") "[email protected]","")]

选择正确的ReadP解析结果

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