Re: [Haskell-cafe] Parsing of bytestrings with non-String errors?

On 22/02/10 18:44, Bryan O'Sullivan wrote:

On Sun, Feb 21, 2010 at 4:36 AM, Magnus Therning mailto:magnus@therning.org> wrote:

I've looked at polyparse and attoparsec and they seem to have in common that the error always is a String. My current ideas for a project would be a lot easier if I could just return some other type, something that I can pattern match on.

It would be easy enough to add this, but you'd end up with a slightly convoluted API. Because of the presence of fail in all monadic APIs, you'd have to support only-a-string as a failure result in some form, so your failure type would have to be something like Either String a.

My thoughts went more like a parser type like data Parser e a = ... Possibly with the addition that 'e' implements a class that goes something like class ParserError e where baseError :: e addError :: e -> e -> e (At first I thought that maybe Monoid would do, but both a identity and associativity feels awkward in this case. :-) With 'String' implemented something like instance ParserError String where baseError = "Parser error, expected:\n" addError = (++) Then I believe 'Parser String' would be equivalent to the existing attoparsec parser (as found in the 0.7 series). I still haven't convinced myself that this will work though. Also, I had a look at attoparsec on bitbucket, and there are some major changes between 0.7 and 0.8. I realised I'll have to spend a lot more time understanding the code than I initially hoped. Right now that is unlikely to happen any time soon :(

There's no support for this in attoparsec simply because I haven't needed it. I suspect the same is true of other libraries, nothing deeper.

Yeah, that's what I thought. In a current project I just have a need to differentiate between errors in different parts of the parser. And handling those errors would just be simple if I could use pattern matching rather than inspect strings. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe