My first instinct is that no, there's not much you can do with this. You could wrap every combinator to return more detailed error messages, which might be *okay*. Or you could use the heavier parsing libraries and get all the error reporting for free. On Sat, Mar 10, 2018 at 2:25 AM, Hilco Wijbenga wrote:

Hi all,

I recently discovered ReadP (Text.ParserCombinators.ReadP) and I wondered if I could use it to write simple parsers _with decent error reporting_.

Let's say I want to parse "[ab] [12]" (i.e. either 'a', or 'b', followed by whitespace, followed by '1', or '2'). I believe something like this does the trick:

parse = do name <- ReadP.choice [ReadP.char 'a', ReadP.char 'b'] ReadP.skipMany1 (ReadP.satisfy (\ch -> ch == ' ' || ch == '\t')) value <- ReadP.choice [ReadP.char '1', ReadP.char '2'] return (name, value)

How do you add error handling to this? Obviously, I could change the signature to return an Either but I don't see how I "return" anything other than Right.

E.g., I would want to give a warning like "Expected 'a', or 'b'" if anything other than 'a', or 'b' is used as first character. Similarly for '1' and '2'. The obvious(?) way would be to add an error result as the last option. The best I can come up with is adding ReadP.pfail to the list of choices but that doesn't allow me to add a useful error. Is ReadP simply not powerful enough for this? Or am I overlooking something obvious?

I did some googling and there is plenty about creating parsers with parser combinators but I could not find anything about error handling.

Cheers, Hilco _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners