Re: [Haskell-beginners] Parsing Revisited

11 Nov 2008

      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
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Re: [Haskell-beginners] Parsing Revisited

Daniel Fischer