Stephen Tetley wrote:
Malcolm Wallace describes a commit combinator in the paper "Partial parsing: combining choice with commitment" which sounds like what you would want. It is implemented for Polyparse rather than Parsec though.
Yes, I can see how that might help on some kinds of data. Thanks.
If your Parsec parser uses try because it is not especially left-factored, one extra combinator I have found useful for left-factoring on the cheap is optionalSuffix:
optionalSuffix :: (a -> c) -> (a -> b -> c) -> Parser a -> Parser b -> Parser c optionalSuffix f g pbody psuffix = do a <- pbody mb_b <- optionMaybe psuffix return $ maybe (f a) (\b -> g a b) mb_b
Funnily enough, while this function didn't help me directly, it did inspire me to take yet another look at the problem code (I've made at least 4 or 5 attempts to fix it over the last year). Revisiting the problem now, with your code in mind, I came up with a solution in about 5 minutes. Now that I've fixed it, it seems so obvious. That led me to wonder if there was a Parsec best-practices document. Does such a thing exist? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/