Hi Alexis, That helps immensely. It was difficult to come up with a "minimal working example" without dumping a large parser, so thank you for being willing to read between the... productions? Lines? *cough*
I have found the library a a joy to work with, and as I got deeper into working with it, I suspected I was not using it as well as I might have. You are right that I did not realize what a "hammer" the backtracking was. This definitely gets me unstuck, and I will see where it takes me. Many thanks, Matt On Fri, Feb 21, 2020 at 11:54 PM Alexis King <[email protected]> wrote: > Hi Matt, > > I think you probably want to read this section of the docs: > https://docs.racket-lang.org/megaparsack/parsing-branching.html#%28part._.Backtracking_with_caution%29 > > The core idea is that `try/p` is a heavy hammer. It causes any failure > inside its scope to backtrack, so you might end up accidentally ruining > your error messages. Usually, what you really want is to do a small amount > of lookahead to determine which branch is the correct one to take, then > commit to the branch so that future parse failures are reported immediately. > > Without seeing your `base-type/p`, `convert/p`, and `chain/p` parsers, > it’s hard to suggest a concrete solution. But consider using `try/p` to do > only whatever parsing you need to do in order to disambiguate the parsers, > then exit the scope of `try/p`. Something like this: > > (or/p (do (try/p base-type-initial/p) base-type-remainder/p) > (do (try/p convert-initial/p) convert-remainder/p) > ...) > > There are other approaches as well, and if you provide more information I > might be able to suggest something better. > > This complication is unfortunately fundamental to the Parsec parsing > model. The syntax/parse model of tracking “progress” and reporting the > error associated with the parse that made it the farthest is much nicer, > but syntax/parse has the luxury of parsing a well-known tree structure. A > more tractable improvement might be to add some kind of explicit committing > construct. > > Hope this helps, > Alexis > > > On Feb 21, 2020, at 20:19, Matt Jadud <[email protected]> wrote: > > > > Hi all, > > > > This might be a Lexi question, but perhaps someone else will have some > insight as well. > > > > I'm wrestling with how to get errors to propagate down in megaparsack. > For example: > > > > (define convert/p > > (do (string/p "convert") > > ... > > [assigns ← (many/p #:min 0 > > assignment-statement/p > > )] > > ... > > (pure ...))) > > > > (Assume I have a bunch of other parsing bits around the call to > `assignment-statement/p`.) > > > > Currently, if I have a malformed assignment statement, the error is at > the top level of `convert`. `convert/p` is part of a backtracking > conditional: > > > > (define conversion/p > > (do > > [result ← (many/p (or/p (try/p base-type/p) > > (try/p convert/p) > > (try/p chain/p) > > ) > > #:sep space0+/p > > )] > > eof/p > > (pure result))) > > > > What should I do to get the error to report/fail down the parse tree, as > opposed to the top? I would rather know that there's something wrong down > in my assignment statement, as opposed to getting an error that "c" was > unexpected (because the entire conversion/p failed on account of an error > somewhere down inside). > > > > I need to give the docs a more careful read, but I thought I'd ask, as > it seems both simple and, given the nature of the parsing tools, possibly > subtle. > > > > Many thanks, > > Matt > > > > > > -- > > You received this message because you are subscribed to the Google > Groups "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to [email protected]. > > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAAGM45761tRo%2Bj0Rh078ngriYiMDum%3DyDyRgwQ1LLiuPZDFj6A%40mail.gmail.com > . > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAAGM456wdPp_Jg9O2xubf98SQ4rA-b5QpLU2%2BBqD5cvSdZyPQg%40mail.gmail.com.
