>>>>> "Simon" == Simon Cozens <[EMAIL PROTECTED]> writes:
Simon> [EMAIL PROTECTED] (Damian Conway) writes: Damian> But in Perl 6, the consistency between a method's parameter Damian> list and its argument list *is* checked at run-time, so Damian> passing the wrong number of arguments is (quite literally) Damian> fatal. Simon> But wait! If we can check how many parameters to pass, we know Simon> how many parameters to pass; problem solved. Sure, the parser Simon> has to stay in a superposition of states until the check is Simon> made, but we're close to requiring quantum supercomputers to Simon> run this thing anyway. Simon> I'm afraid I can't tell whether or not I'm being serious any Simon> more. I don't know if this has been discussed before, but there are completely serious parsing algorithms that work this way, "forking" into conceptually separate parsers that parse the input in parallel before either vanishing or collapsing back together. The best known of these is the Generalized LR algorithm (GLR), in which the individual parsers can be like the well-known yacc/bison LALR(1) parsers, except they split whenever there's a conflict in the parse table. When the input has relatively few ambiguities, GLR parsers can be competitive with LALR(1) ones in performance, and if they're implemented right, their worst-case performance can also be tractable. (Cubic in the input length, which is essentially tight for general context free grammars. This holds even when there are exponentially many possible parses, though obviously you only get an implicit representation of them). GLR parsing was originally designed for natural language parsing, but there's been research in the last few years on using it for programming languages[*], and support for it was recently added to Bison. The snag I can see with using GLR parsing for Perl 6 is that it's based on a model where you preprocess the grammar once at compile time into a big fixed table, while Perl 6 would like to have a grammar that can easily be changed on the fly. [*] Disclaimer: some of which I was involved in. A useful link: http://www.cs.berkeley.edu/~smcpeak/elkhound/ -- Stephen
