Monday, July 31, 2006

July 31st, 2006 -- Parsec emitter and underlay problems

Main goal of this week was to complete the Parsec emitter. However, due to some natural limitation, it's not possible to really "complete" it, I just try my best to complete as many components as I can and hope those be enough for writing complete Perl 6 grammar. The accepting grammar constructions implemented from last post are:
  • :sigspace option
  • complete \X syntax (but not \Xxxx)
  • numbered captures
  • subrule with parameters
  • non-capture group

And I mailed pmichaud++ about the different semantics of "non-backtrack" in Perl 6 rules and Parsec parsing strategy. The :ratchet option, which is turned on by rule and token, is to make backtrack over atom fail. But different branches are still tried even if first several atoms in one branch are matched. In Parsec, the whole parsing fails if it goes into some branch, consumes some tokens and is unable to go on further. However, this can be changed by wrapping the branch with "try," which, as the name tells, tries to match the branch but will try other ones if failed. Such action is like adding a "::" after each atom instead of adding ":", which is done by rule and token in Perl 6 grammar.

One way to solve it is to add "try" everywhere. But that means giving up the high-efficiency parsing provided by Parsec. When the grammar is not LL, it's unavoidable. Parsec performs best on LL grammar (from official page), so in this stage, I'll feed only LL grammar to it and no additional "try" is added.

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