The lookahead feature <https://docs.racket-lang.org/megaparsack/reference.html?q=megaparsack#%28def._%28%28lib._megaparsack%2Fmain..rkt%29._lookahead%2Fp%29%29> you've added is precisely what I was missing when I tried megaparsack, although I was able to work around the need for it with some hacks. Excited to give it another try!
On Sun, Jul 4, 2021 at 12:21 PM Alexis King <[email protected]> wrote: > Addendum: the docs are now built, so you can read more at these two links: > > - https://docs.racket-lang.org/megaparsack/state.html > - > > https://docs.racket-lang.org/megaparsack/reference.html#%28part._parser-parameters%29 > > > On Sun, Jul 4, 2021 at 2:21 AM Alexis King <[email protected]> wrote: > >> Hi all, >> >> As some of you may know, I am the maintainer of the Megaparsack >> <https://docs.racket-lang.org/megaparsack/index.html> package, a >> Parsec-style parser combinator library. Though it’s mostly been in >> maintenance mode for some time, the latest release adds a significant new >> feature: *user-defined parser state*. This closes the main expressivity >> gap Megaparsack has historically had relative to Parsack (though Parsack >> continues to be more performant). >> >> As a quick overview to pique your interest, Megaparsack 1.4 provides a >> new abstraction called a *parser parameter*. Parser parameters are >> similar to ordinary Racket parameters, but their state is associated with a >> parsing context rather than with a thread. Their interface should already >> look familiar to Racketeers, so here is an example that uses parser >> parameters to implement an indentation-sensitive syntax: >> >> (define current-indent (make-parser-parameter 0)) >> >> (define skip-current-indent/p >> (do [cols <- (current-indent)] >> (repeat/p cols (char/p #\space)))) >> >> (define markdown-like-list/p >> (do (string/p "* ") >> [blk <- block/p] >> [cols <- (current-indent)] >> [blks <- (parameterize/p ([current-indent (+ cols 2)]) >> (many/p block/p))] >> (pure (cons blk blks)))) >> >> Modifications to parser parameters are automatically reverted when the >> parser backtracks past the point of modification, which allows them to be >> used for accumulating information during parsing even if the current parse >> branch is tentative. To illustrate, note that this example results in 10 >> rather than 15: >> >> (define current-count (make-parser-parameter 0)) >> >> (define (increment/p n) >> (do [count <- (current-count)] >> (current-count (+ count n)))) >> >> > (parse-string >> (do (or/p (try/p (do (string/p "!") (increment/p 5) eof/p)) >> (do (string/p "!!") (increment/p 10) eof/p)) >> (current-count)) >> "!!") >> (success 10) >> >> I generally don’t post about releases to this list, but I happen to know >> Megaparsack has at least a few active users, and this is the first major >> update in a long while, so I figured I’d give a heads-up. If you’ve counted >> the library out in the past for missing this feature, consider giving it >> another look. And if you’re interested in learning more, the gory details >> are described in the documentation, along with some additional examples (at >> the time of this writing, pkg-build hasn’t rebuilt the docs just yet, but >> that should be resolved with tomorrow’s daily refresh). >> >> Happy parsing, >> Alexis >> > -- > 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/CAA8dsafQt%2BCtXPVFKuTYC1ZShC3%3DZgzNBEzfe7_bH67sYCJMCg%40mail.gmail.com > <https://groups.google.com/d/msgid/racket-users/CAA8dsafQt%2BCtXPVFKuTYC1ZShC3%3DZgzNBEzfe7_bH67sYCJMCg%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- 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/CACQBWFnh%2B4DAa%2Bdze%2BADXfKXswtjkMQqkPy0ZV3aFm_2vGrTeQ%40mail.gmail.com.
