Using Parsec to parse regular expressions

Question

I\'m trying to learn Parsec by implementing a small regular expression parser. In BNF, my grammar looks something like:

EXP  : EXP *
     | LIT EXP
     | LIT
         


        

Answer 1

Your grammar is left-recursive, which doesn’t play nice with try, as Parsec will repeatedly backtrack. There are a few ways around this. Probably the simplest is just making the * optional in another rule:

lit :: Parser (Char, Maybe Char)
lit = do
  c <- noneOf "*"
  s <- optionMaybe $ char '*'
  return (c, s)

Of course, you’ll probably end up wrapping things in a data type anyway, and there are a lot of ways to go about it. Here’s one, off the top of my head:

import Control.Applicative ((<$>))

data Term = Literal Char
          | Sequence [Term]
          | Star Term

expr :: Parser Term
expr = Sequence <$> many term

term :: Parser Term
term = do
  c <- lit
  s <- optionMaybe $ char '*' -- Easily extended for +, ?, etc.
  return $ if isNothing s
    then Literal c
    else Star $ Literal c

Maybe a more experienced Haskeller will come along with a better solution.