Re: [Haskell-cafe] parsec: how to get end location

* Antoine Latter [2010-06-13 16:47:28-0500]

On Sun, Jun 13, 2010 at 4:17 PM, Roman Cheplyaka wrote:

...

Suppose I have some parser 'p'. I want to parse it as well as get its span in the text. So I could write

\begin{code] pWithLocation = do  loc_start <- getPosition  pval <- p  loc_end <- getPosition  return (pval,loc_start,loc_end) \end{code}

except that loc_end gives me the location _after_ 'p'. In case when 'p' has consumed trailing newline I see no way to recover the end location of 'p' (it is the last column of the previous line).

So, can I do anything with this without patching parsec?

I use parsec3 if it matters.

Can you use a parser which doesn't consume leading/trailing whitespace?

Or somehow layer the parsers so that the whitepsace munching happens outside of parseWithLocation.

Of course most parsers don't consume trailing newlines. But I was writing general function to use in many places in the code which would recover the end location. In most cases it just subtracts 1 from the column number, but what if it just happened so that column number is 1? So I got an impression that this way of obtaining the end location is at least ugly. It would be better if there was a function parseWithLocation :: Parser a -> Parser (a, SourcePos, SourcePos) which, I guess, is easy to implement within Parsec without any such hacks. -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain