[Haskell-beginners] Parsing Revisited

Jeffrey Drake jeffd at techsociety.ca
Tue Nov 11 01:20:15 EST 2008

I have decided to go in another direction, so the parser here is going
to be different, but it works without error correction. I am using a
wiki style right here, where I am testing headers level 1 to 5. 

I believe I need to essentially throw an error here, normally I would
think <?> might be appropriate, but it doesn't seem to apply in this

The problem is that a1, a2 must satisfy two conditions:

	a1 == a2
	1 <= a1 <= 5

If it doesn't, then an error must be output.

It appears that <?> is just an operator for 'label'. Which goes to
labels, which does eventually go to construct Error, but not sure in the
correct context.

Again, any help is appreciated, I am getting a feeling for this I think.
- Jeff.

Code is below:

module Main where

import Control.Monad

import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Char
import Text.ParserCombinators.Parsec.Combinator

data Wiki 
    =   Heading Int String
    |   Other String
    deriving Show

    a1/a2 heading level
    2       5
    3       4
    4       3
    5       2
    6       1
    :. 7 - a1       
heading :: Parser Wiki
heading = do    (a1, s, a2) <- within
                        (length `liftM` many1 (char '='))
                        (length `liftM` many1 (char '='))
                        (many1 (alphaNum <|> space))
                return $ Heading (7 - a1) s
          where within open close p = do
                                        a1 <- open
                                        x <- p
                                        a2 <- close
                                        return (a1, x, a2)

On Mon, 2008-11-10 at 06:10 +0100, Tillmann Rendel wrote:
> Jeffrey Drake wrote:
> > This helps a lot, and I can go over this in the morning. The only final
> > question I have is what you would use to apply all this to an arbitrary
> > string.
> You can use parseTest for testing, e.g. in ghci.
>    parseTest texList "hello\world {example}"
> That will either print the resulting {TeX], or a parser error message.
> For normal processing, use parse.
>    case parse texList "<no source>" "hello\world {example}" of
>      Left problem -> error (show problem)
>      Right texList -> convertToHTML texList
>    Tillmann

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