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Subject: Re: [proto] Thoughts on traversing proto expressions and reusing grammar
From: Eric Niebler (eric_at_[hidden])
Date: 2010-10-09 13:53:27

On 10/8/2010 12:12 AM, Thomas Heller wrote:
> On Thursday 07 October 2010 23:06:24 Eric Niebler wrote:
>> On 10/4/2010 1:55 PM, Eric Niebler wrote:
>>> The idea of being able to specify the transforms separately from the
>>> grammar is conceptually very appealing. The grammar is the control
>>> flow, the transform the action. Passing in the transforms to a grammar
>>> would be like passing a function object to a standard algorithm: a
>>> very reasonable thing to do. I don't think we've yet found the right
>>> formulation for it, though. Visitors and tag dispatching are too
>>> ugly/hard to use.
>>> I have some ideas. Let me think some.
>> Really quickly, what I have been thinking of is something like this:
>> template<class Transforms>
>> struct MyGrammar
>> : proto::or_<
>> proto::when< rule1, typename Transforms::tran1 >
>> , proto::when< rule2, typename Transforms::tran2 >
>> , proto::when< rule3, typename Transforms::tran3 >
>> {};
> I don't think this is far away from what i proposed.
> Consider the following:
> template <typename>
> struct my_grammar
> : proto::or_<
> rule1
> , rule2
> , rule3
> {};
> template <typename> my_transform;
> // corresponding to the tag of expression of rule1

Do you mean expression tag, like proto::tag::plus, or some other more
abstract tag?

> template <> struct my_transform<tag1>
> : // transform
> {};
> // corresponding to the tag of expression of rule2
> template <> struct my_transform<tag2>
> : // transform
> {};
> // corresponding to the tag of expression of rule3
> template <> struct my_transform<tag3>
> : // transform
> {};
> typedef proto::visitor<my_transform, my_grammar>
> algorithm_with_specific_transforms;
> In my approach, both the transform and the grammar can be exchanged at will.

I don't know what this can possibly mean. Grammars and transforms are
not substitutable for each other in *any* context.

> What i am trying to say is, both the transforms and the control flow (aka the
> grammar) intrinsically depend on the tag of the expressions, because the tag
> is what makes different proto expressions distinguishable.

This is where I disagree. There are many cases where the top-level tag
is insufficient to distinguish between two expressions. That's why Proto
has grammars. Proto::switch_ dispatches on tags, but I consider switch_
to be primarily an optimization technique (although it does have that
nice open-extensibility feature that we're using for Phoenix).

> This imminent characteristic of a proto expression is what drove Joel Falcou
> (i am just guessing here) and me (I know that for certain) to this tag based
> dispatching of transforms and grammars.

Understood. OK, the problem you're trying to solve is:

A) Have an openly extensible grammar.
B) Have an equally extensible set of transforms.
C) Be able to substitute out a whole other (extensible) set of transforms.

Is that correct?

Eric Niebler
BoostPro Computing

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