ReadP and choice

25 Feb 2012

      Hi all,

The code below defines a ReadP parser which can parse either a keyword
"wrapper" or an arbitrary identifier. I was hoping that it would give
the output
    (CWrapper1,"")
    (CWrapper2,"")
    (CFunction "wrapper","")
but it actually gives
    (CWrapper2,"")
    (CFunction "wrapper","")
    (CWrapper1,"")
i.e. completed parses are returned before the parse that needs to "look"
at the remaining input.

I can see the logic behind the current behaviour, but in this case at
least it's quite inconvenient.

What do you think?
Did I miss a better way to solve the problem?
Should the behaviour be changed?
If so, is changing it easy?

import Data.Char
import Control.Applicative ((<$>))
import Text.ParserCombinators.ReadP as ReadP

main :: IO ()
main = mapM_ print test

test :: [(CImportSpec, String)]
test = readP_to_S parse "wrapper"
    where parse = do r <- choice [
                              token  "wrapper" >> return CWrapper1,
                              string "wrapper" >> return CWrapper2,
                              CFunction <$> many1 (satisfy isAlpha)
                          ]
                     eof
                     return r

          token str = do _ <- string str
                         toks <- look
                         case toks of
                             c : _
                              | isAlpha c -> pfail
                             _            -> return ()

data CImportSpec = CFunction String
                 | CWrapper1
                 | CWrapper2
    deriving Show

Thanks
Ian

ReadP and choice

Ian Lynagh