[Parseperl-discuss] What should 5.10's switch statement parse like?

[Elliot S. <pe...@ga...>]

Let's discuss the following code:

    given ($foo) {
        when (@blah) { }
        default { }
    }

I would have thought that given would be a compound statement (especially in the face of the ability to use when/default in foreach loops); however, perlsyn does not include it in its list of compound statements, so it sounds like we'd need a new subclass of Statement for it.  (Also, this would help differentiation in Perl::Critic, especially in the determination of whether a specific Policy applies to an Element.)  Or we could ignore perlsyn's omission and treat it as Compound.

Should the parentheses be a Structure::Condition?  Or something analogous to Structure::ForLoop? (Again, from the P::C perspective, having a new class would be better.)

Then, the when/default.  The actual statements, like the given, could be Statement::Compound instances, and the parentheses after when could just be Conditions again.  Even if there's a new class, the when and default could be instances of the same one, since perlsyn says that default is precisely equivalent to "when (1 == 1) { ... }".

So, ignoring whitespace tokens, as I see it, here's the most conservative way of this parsing:

PPI::Document
  PPI::Statement::Compound
    PPI::Token::Word  	'given'
    PPI::Structure::Condition  	( ... )
      PPI::Statement::Expression
        PPI::Token::Symbol  	'$foo'
    PPI::Structure::Block  	{ ... }
      PPI::Statement::Compound
        PPI::Token::Word  	'when'
        PPI::Structure::Condition  	( ... )
          PPI::Statement::Expression
            PPI::Token::Symbol  	'@blah'
        PPI::Structure::Block  	{ ... }
      PPI::Statement::Compound
        PPI::Token::Word  	'default'
        PPI::Structure::Block  	{ ... }

OTOH, here's the most favorable structure from P::C's perspective (modulo the class names changing):

PPI::Document
  PPI::Statement::Switch
    PPI::Token::Keyword  	'given'
    PPI::Structure::Given  	( ... )
      PPI::Statement::Expression
        PPI::Token::Symbol  	'$foo'
    PPI::Structure::Block  	{ ... }
      PPI::Statement::When
        PPI::Token::Keyword  	'when'
        PPI::Structure::Match  	( ... )
          PPI::Statement::Expression
            PPI::Token::Symbol  	'@blah'
        PPI::Structure::Block  	{ ... }
      PPI::Statement::When
        PPI::Token::Keyword  	'default'
        PPI::Structure::Block  	{ ... }


Yes, I'm thinking of trying to implement this.  First, I need to figure out the parser and see whether I can fix some existing parsing problems.