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

Subject: Re: [proto] Visitor Design Pattern
From: Joel de Guzman (joel_at_[hidden])
Date: 2010-10-22 05:29:07

On 10/22/10 4:17 PM, Thomas Heller wrote:
> 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 :)

It's just the metaphor! And what I saying is that you will get into
confusion land if you mix metaphors from different domains. Proto uses
the parsing domain and it makes sense (*). It may (and I say may) be
possible to extend that metaphor and in the end it may be possible
to incorporate that into proto instead of phoenix (if it is indeed
conceptually understandable and reusable) --an opportunity that may
be missed if you shut the door and dismiss the idea prematurely.

It is OK to switch metaphors and have a clean cut. But again,
my point is: use only one metaphor. Don't mix and match ad-hoc.

(* regardless if it doesn't do any parsing at all!)

>>>>> 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 ...

Again I should warn against mixing metaphors. IMO, that is the basic
problem why it is so deceptively unclear. There's no clear model
that conceptualizes all this, and thus no way to reason out on
an abstract level. Not good.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net