Choosing the right ReadP parse result - parsing

Question

I'm trying to parse a RFC5322 email address. My parser works in the sense that among the results, one of them is correct. However, how do I go about selecting the “correct” result?
Given the string Foo Bar <foo#bar.com>, my parser should produce a value of Address (Just "Foo Bar") "foo#bar.com".
Alternatively, given the string foo#bar.com, my parser should produce a value of Address Nothing "foo#bar.com".
The value with the name included is preferred.
My parser looks like this:
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
When I run the parser with readP_to_S rfc5322 "Foo Bar <foo#bar.com>", it produces the following results:
[ (Address {addressName = Nothing, addressEmail = "F"},"oo Bar <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Fo"},"o Bar <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo"},"Bar <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo "},"Bar <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo B"},"ar <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Ba"},"r <foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar"},"<foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar "},"<foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <"},"foo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <f"},"oo#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <fo"},"o#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo"},"#bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#"},"bar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#b"},"ar.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#ba"},"r.com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar"},".com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar."},"com>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar.c"},"om>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar.co"},"m>")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar.com"},">")
, (Address {addressName = Just "Foo Bar", addressEmail = "foo#bar.com"},"")
, (Address {addressName = Just "Foo Bar ", addressEmail = "foo#bar.com"},"")
, (Address {addressName = Nothing, addressEmail = "Foo Bar <foo#bar.com>"},"")
]
In this case, the result I actually want appears third-last in the list. How do I express that preference?

Answer 1

You should not to do preference. Your problem is that your partial parsers are accepting the more bigger string set than really need.
For example, my solution:
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 <foo#bar.com>"
Will be printed:
[(Address (Just "Foo Bar") "foo#bar.com","")]