My take, based on S05: On Thu, 2005-05-12 at 10:33, Patrick R. Michaud wrote: > I have a couple of questions regarding C< :: > in perl 6 rules. > First, a question of verification -- in > > $rule = rx :w / plane :: (\d+) | train :: (\w+) | auto :: (\S+) / ; > > "travel by plane jet train tgv today" ~~ $rule > > I think the match should fail outright, as opposed to matching "train tgv".
Correct, that's the meaning of :: S05: "Backtracking over a double colon causes the surrounding group of alternations to immediately fail:" Your surrounding group is the entire rule, and thus you fail at that point. > In other words, it acts as though one had written > > $rule = rx :w / plane ::: (\d+) | train ::: (\w+) | auto ::: (\S+) / ; > > and not > > $rule = rx :w /[ plane :: (\d+) | train :: (\w+) | auto :: (\S+) ]/ ; Your two examples fail in the same way because of the fact that the group IS the whole rule. > Next on my list, S05 says "It is illegal to use :: outside of > an alternation", but A05 has > > /[:w::foo bar]/ I can't even figure out what that means. :w turns on word mode (lexically scoped per S05) and "::" is a group-level commit. What are we committing exactly? Looks like a noop to me, which actually might not be so bad. However, you're right: this is an error as there are no alternations. > which leads me to believe that :: isn't illegal here even though there's > no alternation. I'd like to strike that sentence from S05. I don't think it should be removed. You can always use ::: if that's what you wanted. > Also, A05 proposes incorrect alternatives to the above > > /[:w[]foo bar]/ # null pattern illegal, use <null> Correct. > /[:w()foo bar]/ # null capture illegal, and probably undesirable Correct. > /[:w\bfoo bar]/ # not exactly the same as above No, I think that's exactly the same. > So, now then, on to the item that got me here in the first place. > The upshot of all of the above is that > > rx :w /foo bar/ > > is not equivalent to > > rx /:w::foo bar/ If we feel strongly, it could be special-cased, but your <null> solution seems fine to me. -- Aaron Sherman <[EMAIL PROTECTED]> Senior Systems Engineer and Toolsmith "It's the sound of a satellite saying, 'get me down!'" -Shriekback
