[Haskell-beginners] Parsing Revisited

[Daniel Fischer]

Am Dienstag, 11. November 2008 07:20 schrieb Jeffrey Drake:
> 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
> case.
>
> 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.

<?> gives you the opportunity to construct a helpful error message if the 
parser fails. You provide information what the parser expects and if it fails 
you get the message
"unexpected whatever, expecting x, y, z or w"
>
> 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)

Perhaps

heading :: Parser Wiki
heading = do
	a1 <- length `liftM` many1 (char '=')
	title <- many1 (alphaNum <|> space)
	count a1 (char '=')
	notFollowedBy (char '=')
	return $ Heading (7-a1) title

? Although that doesn't check that there are at most 5 '=', so you might use 
the combinators

upTo 0 p = do
	notFollowedBy p
 	return []
upTo k p = (do
	a <- p
	as <- upTo (k-1) p
	return (a:as)) <|> return []

upTo1 k p = do
	a <- p
	as <- upTo (k-1) p
	return (a:as)

exactly k p = do
	as <- count k p
	notFollowedBy p
	return as

as in

heading = do
	a1 <- length `liftM` upTo1 5 (char '=') <?> ("1 to 5 '='s")
	title <- many1 (alphaNum <|> space)
	exactly a1 (char '=') <?> (show a1 ++ " '='s")
	return $ Heading (7-a1) title

>
> 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
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners