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From: Eric Niebler (eric_at_[hidden])
Date: 2006-08-07 00:51:27

Joel de Guzman wrote:
> Andreas Pokorny wrote:
>> For my thesis work I wrote a DSL framework based on fusion-2. It fulfills a
>> similar purpose like boost::xpressive's proto. Compared to proto, the framework
>> has a rule system, allowing the user define the language rules of a domain.

Sounds interesting. Proto would be better with some way of constraining
the legal expressions within a domain.

>> Is there any interest in a library like that?


>> Some additional information:
>> All operators defined by the framework verify the correctness of expressions
>> querying the domain rules the expression parameters belong to, using EnableIf.

This sounds good. I wonder what the impact is on compile time.

>> Foreign types can be integrated into a domain by specifing adaptation rules.
>> The framework separates operators (or DSL functions) from language rules.
>> Consequently rules may
>> introduces this layer of indirection to achieve a consistent
>> definition of language rules.

I'd like an example.

>> Expressions have a varying set of attributes stored in a fusion-2 map, e.g.
>> the expressions domain is identified by an attribute inside that map. The
>> set of attributes is controlled by the domain rules. Apart from returning the
>> validity of expressions, rules also have to provide a result type. Hence the
>> rule code tends to grow complex, with an increasing amount of attributes.

More examples, please!

>> The library lacks a proper documentation, and requires some interface changes
>> to simplify attribute update operations. Before working on that, I wanted to see
>> if there is interest at all.
>> I used the framework to implement a matrix library with a signle frontend and
>> multiple backends, that could be picked and combined on a by expression basis.
> I'm interested. I'd like to see some examples; especially on how
> the "rule system" works. I am also interested to know how a domain
> can be adapted for use by 2 or more libraries: xpressive and spirit
> being main examples. IOTW, in the future, spirit and xpressive
> parts can be shared.

Joel, Hartmut Kaiser and I have spent a lot of time working out the
details of an expression template system that would work for xpressive,
Spirit-2 and Hartmut's Karma library. The discussions were public on
spirit-devel. Search the archives for "Spirit-2 and Subversion",
"Scanner Busines, R.I.P.", "Proto Compiler Visitors", "Fusion-ifying
proto parse trees", and "segmented fusion - a-ha!"

In particular, this is the message where the strategy to unify all the
ET libraries really crystallized:

It would be great if you could share your thoughts on how you would
approach these problems with your new ET library.

> To be honest, right now, for Spirit-2, I am inclined to use Eric's
> proto. It's been there for quite some time now and is quite mature.
> But, like your library, there are no docs yet (as far as I know).
> I'm not sure when Eric will have the time to get it outside xpressive
> as a standalone ET library, for review. At any rate, it would be
> interesting if you can form a collaboration with him. So, instead
> of having 2 separate ET entries, we can just have one that has the
> the best of both. Is that possible at all? Or is your library too
> different from Proto?

I too would be interested in hearing more about your ideas and helping
to build a unified ET framework for boost.

Eric Niebler
Boost Consulting

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