Re: P6: Rule and RPN calc

[ayrnieu]

When I gave up:

#! /usr/bin/env perl
use v6;

my %operators =
  ( '+' => sub { [+] @_ },
    '-' => sub { [-] @_ },
    '*' => sub { [*] @_ },
    '/' => sub { [/] @_ },
  );

grammar rpn_data {
  rule value      { -?\d+ }
  rule operator   { <[+*/\-]> }
  rule expression { }
}

sub rpn_eval (Str $e) returns Num {
  my @stack;
#  die "Invalid RPN: $e" unless $e ~~ /<expression>/;
  for $e.split() -> $tok {
    if $tok ~~ /<value> / { @stack.push($tok); next }
    @stack.push(%operators{$tok}(reverse @stack.pop, @stack.pop));
  }
  @stack[0]
}

say rpn_eval($_) while $_ = =<>

For the most part, writing this was very exciting -- for the first time I delved into the perl6 rules stuff and tried to use it, and from this I now see how I would've solved this problem -- with code blocks after matches to neatly fail if I discovered improper RPN, or even with a modified %operators in the operator rule that evaluated the RPN as it was parsed.

And yet, this was disappointing: none of what I wanted to do was implemented in pugs.

Still, I've now a much better idea of what I've to look forward to. Thanks.

Update: in case anyone asks, there are no checks for overflow/underflow or such in my rpn_eval because I expected to have already verified that the RPN wouldn't have such problems. No point in uglyifying this test code with such checks when I discover that I momentarily can't. With time, I'll have this as the core of a real implementation.