Proto for Boost mailing list page: Re: [proto] Visitor Design Pattern

Subject: Re: [proto] Visitor Design Pattern
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2010-10-22 04:17:12

On Friday 22 October 2010 09:58:25 Eric Niebler wrote:
> On 10/22/2010 12:33 AM, Thomas Heller wrote:
> > On Friday 22 October 2010 09:15:47 Eric Niebler wrote:
> >> On 10/21/2010 7:09 PM, Joel de Guzman wrote:
> >>> Check out the doc I sent (Annex A). It's really, to my mind,
> >>> generic languages -- abstraction of rules and templated grammars
> >>> through metanotions and hyper-rules.
> >> 
> >> Parameterized rules. Yes, I can understand that much. My
> >> understanding stops when I try to imagine how to build a parser
> >> that recognizes a grammar with parameterized rules.
> > 
> > And I can't understand how expression templates relate to parsing.
> 
> It doesn't in any practical sense, really. No parsing ever happens in
> Proto. The C++ compiler parses expressions for us and builds the tree.
> Proto grammars are patterns that match trees. (It is in this sense
> they're closer to schemata, not grammars that drive parsers.)
> 
> They're called "grammars" in Proto not because they drive the parsing
> but because they describe the valid syntax for your embedded language.

Ok, this formulation makes it much clearer :)

> >>> I have this strong feeling that that's the intent of Thomas and
> >>> your recent designs. Essentially, making the phoenix language a
> >>> metanotion in itself that can be extended post-hoc through
> >>> generic means.
> >> 
> >> I don't think that's what Thomas and I are doing. vW-grammars
> >> change the descriptive power of grammars. But we don't need more
> >> descriptive grammars. Thomas and I aren't changing the grammar of
> >> Phoenix at all. We're just plugging in different actions. The
> >> grammar is unchanged.
> > 
> > Exactly.
> > Though, I think this is the hard part to wrap the head around. We
> > have a grammar, and this very same grammar is used to describe
> > "visitation".
> 
> It's for the same reason that grammars are useful for validating
> expressions that they are also useful for driving tree traversals:
> pattern matching. There's no law that the /same/ grammar be used for
> validation and evaluation. In fact, that's often not the case.

True.
However it seems convenient to me reusing the grammar you wrote for 
validating your language for the traversal of an expression matching that 
grammar.
This is what we tried with this rule based dispatching to Semantic Actions.
I am currently thinking in another direction, that is separating traversal 
and grammar again, very much like proto contexts, but with this rule 
dispatching and describing it with proto transforms ... the idea is slowly 
materializing in my head ...