[Philadelphia-pm] syms in grammars

[Brian Duggan <bduggan@matatu.org>]

Hi Folks,

Thanks to those who came yesterday for the clinic :-)

Following up on the conversation from dinner -- here's
an example of using syms to simplify alternation --

Before:
  grammar g {
    rule TOP { 1 <op> 2 }
    token op { <x> | <y> }
    token x { x }
    token y { y | Y }
  }

  class a {
    method op($/) {
      say "op is " ~ ( $<x> // $<y> )  # we want to avoid this
    }
  }

After:
  grammar g {
    rule TOP { 1 <op> 2 }
    proto token op { <...> }
    token op:sym<x> { x }
    token op:sym<y> { y | Y }
  }

  class a {
    method op:sym<x> ($/) { say 'op is x'; }
    method op:sym<y> ($/) { say 'op is y or Y'; }
  }

  my $actions = a.new;
  say g.parse('1 x 2', :$actions);
  say '---';
  say g.parse('1 y 2', :$actions);
  say '---';
  say g.parse('1 Y 2', :$actions);

which produces

  op is x
  ｢1 x 2｣
   op => ｢x｣
  ---
  op is y or Y
  ｢1 y 2｣
   op => ｢y｣
  ---
  op is y or Y
  ｢1 Y 2｣
   op => ｢Y｣
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