<masak> as an author of grammars, I still find it annoying and difficult that
grammars generally fail, that is, return a failing match -- *unless* you use
the `~` combinator, in which case they nqp::die with an exception that can't be
properly caught using a CATCH.
<masak> in effect, that gives grammars three possible outcomes: success,
failure, or death-because-of-failgoal.
<masak> oh, I've kvetched about that before, it seems:
https://rt.perl.org/Ticket/Display.html?id=62086
<jnthn> masak: Why can't it be caught with a CATCH?
<jnthn> masak: Also, you're free to override FAILGOAL...
<masak> I don't rightly know why it can't be. I'm having trouble reproducing
the error in golfed code.
<masak> m: grammar G { regex TOP { a ~ c <foo> }; regex foo { b } }; say
?G.parse("abbc"); CATCH { when X::AdHoc { say "caught" } }
<camelia> rakudo-moar 20aa85: OUTPUT«caught»
<masak> ok, here I *could* catch it. weird.
<jnthn> In all the cases we use it for in the Perl 6 grammar, it'd be useless
if it didn't throw, fwiw.
* masak adds that to the ticket
<jnthn> Of course, I guess we could have a default FAILGOAL that just fails and
override it with one that throws.
<masak> if all outcomes of a grammar return a failed Match object *except* for
FAILGOAL, then I'd like there to be a good theoretical explanation for FAILGOAL
behaving differently.
<masak> and not just "we need to convey an error message here, so throwing an
exception feels right"
<jnthn> panic is another example
<masak> categorically, it's still just a failed match. it feels like with the
current setup, we're committing a category error.
<masak> or rather, what precise thing is it that makes a parse failure so
severe that it promotes from "falsy" to "die"?
<masak> right now, from what I can see, the need to attach an error message.
<jnthn> That, and also wanting to give up at that point because there's no sane
way to proceed and you'd never want something further up to try another path.
<masak> oh, right. control flow.
<masak> still, a use case I mention in earlier parts of that ticket is "I just
want to parse something in order to find out whether it's valid or not". having
to deal with three-valued logic in that case is just... cruft.
<jnthn> Then don't use ~
<masak> I suppose.
