On 10/14/2010 4:46 AM, Eric Niebler wrote:
On 10/13/2010 11:54 AM, Thomas Heller wrote:
I don't get that. There needs to be a phoenex grammar. One, and only
one. I don't see why it's a template, what the tag type is for, or how
one could use a tag to define a "better" phoenix grammar. Because
there's only one. Either an expression is a Phoenix grammar or it isn't,
right?
I disagree here.
End users might want to extend phoenix (the EDSL itself), by introducing new
expressions (right now by introducing a new tag). Now, the user wants to
specifiy what valid expressions look like that can be formed with his new
expression type. Here i currently can imagine two different scenarios:
First, the user just wants to extend our "regular" grammar. This can be
achieved by something like this:
template<>
struct phoenix_grammar<my_new_super_cool_tag>
: // some proto grammar which specifies how valid expressions look like
The second scenario i can imagine, that someone wants to build some kind of
other language reusing phoenix parts, imagine someone who is so obsessed
with functional programming, and doesn't like the current approach and wants
a pure functional programming EDSL. Thus he wants to disallow certain
expressions. The current visitor design makes that perfectly possible!
(I hope this makes things clearer)
No, I'm afraid it doesn't. Extending phoenix by introducing additional
tags doesn't necessitate parameterizing the *whole* grammar on a tag.
The old design handled that scenario just fine by using the openly
extensible proto::switch_. Just one grammar, and users could hook it easily.
And the second scenario also doesn't suggest to me that the grammar
needs parameterization. It suggests to me that someone who wants a
radical customization of Phoenix grammar should make their domain a
sub-domain of Phoenix, reuse the parts they want, and define the grammar
of their new EDSL.
(still trying to catch up with this thread)
At this point, I am having the same difficulty as (probably) with Eric
in that it seems it's too convoluted and complex. I, for one, don't have
any problems with using switch_ and partial specialization and possibly
sub-classing or template-izing (perhaps using CRTP to let the base
template grammars hook up the most derived, ala static-OOP).
(I'll read some more and see if there are new twists in the thread)
Regards,
--
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net
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