Udo Stenzel wrote:
Chris Kuklewicz wrote:
Again, Parsec requires you to put "try" where you need it
I'm pretty sure it does, although this
Udo Stenzel wrote:
countBetween 0 n p = p <:> countBetween 0 (n-1) p <|> return []
is a place where it's not needed in general.
No, see my example below. It is not needed in the specific case where p can only consume a single token.
You should know what you're doing, though. And I like ReadP better in general, exactly because 'try' is a bit trippy.
And I have not used ReadP enough to be good with it.
Udo.
As long as you test with 'p' that consumes a single token, there is no difference using try (<:>) = liftM2 (:) countBetween 0 0 _ = return [] countBetween 0 n p = p <:> countBetween 0 (n-1) p <|> return [] countBetween m n p = p <:> countBetween (m-1) (n-1) p Expand countBetween 0 n p = (do value_p <- p value_rest <- countBetween 0 (n-1) p return (value : value_rest) <|> (return []) If p fails without consuming tokens, then the <|> runs (return []). If p fails after consuming tokens, then the <|> will fail instead of running (return []). By replacing p with (try p) this will work.
test = countBetween 3 5 (string "ab") go = runParser test () "" "abababac"
Without try this fails to parse with an error message: Left (line 1, column 7): unexpected "c" expecting "ab" With the try this succeeds: Right ["ab","ab","ab"] I assume you think the former is problem. If you do not then we are trying to define different operations. The problem is that string "ab" consumes the 'a' character/token before it fails. The 'try' wraps this failure, pretending p did not consume any input, so the <|> will then run (return []) as we want. It is not needed in the line countBetween m n p = p <:> countBetween (m-1) (n-1) p because if p fails then the whole operation should fail. There is a benefit to this difficult behavior! The <|> choice point for backtracking is expensive. By committing to the first branch when it consumes a token, the Parsec monad can completely forget the other branch of <|> which allows it to be garbage collected. This default is handy for preventing memory leaks, but it has to be overridden by 'try'. One can package <|> and 'try' in useful combinators such as countBetween so the use of them becomes slightly easier. -- Chris