( Moritz: Thanks. Thursday, I grabbed a copy of the book and moved to the latest rakudo release. I didn't find the .perl method yet; so I'll just salivate for a while more. I will look again at the book--that was helpful. )

This is going slow as I have little time (and lots of ignorance) but I will keep scratching away.

#!/home/rir/rakudo/parrot_install/bin/perl6

{
    grammar Calc {
        token TOP { <expression> }
        rule expression { <lhs> <op> <rhs> }
        token lhs { <numeric> }
        token rhs { <numeric> }
        token numeric { \d+[\.\d*]? }
        token op { '-' | '+' | '*' | '/' | 'x' }
    }

    class Calc::Actions {
        method TOP($/) { make $<expression>.ast }
        method expression($/) { make eval "$/<lhs> $/<op>.ast() $/<rhs
+>" }
        method lhs($/) { make $/ }
        method rhs($/) { make $/ }
        method numeric($/) { make $/ }
        method op($/) { if ( $/ eq 'x') { make '*'; } else {make $/; }
+ }
    }

    my $m = Calc.parse( "8.8 x 5.0 - 2",  :actions( Calc::Actions));
    die "dying no match"  unless  $m;
    say "$m<expression> =  $m.ast()";
}
[download]

Ok, I have retrenched. I grabbed Rakudo's/Pug's spectest and found some grammars that compile out of the box. The above is built up from t/spec/S05-grammarr/action-stubs.t . (Running spectest was less onerous than I expected.)

The above is meant to model a cheap calculator.

Questions:

• Is the string eval reasonable here?
• Can the expression rule be handled without the separate naming of the lhs and rhs? Is there a way to access each numeric in something like:

     rule expression { <numeric> <op> <numeric> }
  [download]

• The above can easily be extended by changing the rhs token line to

  token rhs { <numeric> | <expression> }
  [download]

  But this destroys the calculator's left associativity and equal precedence. I suppose this could be addressed by changing the rule expression line to:

     rule expression { <lhs> ( <op> <rhs> )+ }
  [download]

  but I'm getting lost here.

Be well,
rir

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