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Subject: Re: P2320: "The Syntax of Static Reflection" feedback request
From: Herb Sutter (hsutter_at_[hidden])
Date: 2021-02-21 18:27:45

Patterns: Yes, reflection+generation will let us write lots of* patterns as libraries, and even many language features as libraries, and that's huge. (* not "all"... careful not to oversell)

Examples: Yes, please be example-driven. Most of the page count of my 45-page P0707<http://wg21.link/p707> is examples with full code, because I spent a lot of time trying out how to write specific patterns (many of them compiling and running using an initial prototype) before I felt the paper was ready to publish as an R0. In fact those examples take up most of the 45-page paper, including that I wrote code for the following:

  * interface
  * base_class
  * final
  * ordered
  * value (aka regular)
  * struct (plain_struct to avoid the keyword)
  * copyable_pointer
  * enum_class
  * flag_enum
  * bitfield
  * union (safe_union to avoid the keyword)
  * namespace (namespace_class to avoid the keyword)
  * signal / slot
  * property

This was a lot of work, and much of the P0707-syntax code should translate well to another syntax so I hope this can help accelerate others writing examples in their own syntax.

A request please: If you're proposing a syntax, please try to demonstrate at least some of the P0707 examples in your alternative syntax. It would be super useful to see what's easier/different - I'd love to see a Tony table of a P0707 example compared to the equivalent in another proposal!

I would be happy to help and answer questions if you like, just reach out privately please.


> -----Original Message-----

> From: SG7 <sg7-bounces_at_[hidden]> On Behalf Of David Rector via SG7

> Sent: Sunday, February 21, 2021 3:13 PM

> To: Ville Voutilainen <ville.voutilainen_at_[hidden]>

> Cc: David Rector <davrec_at_[hidden]>; Andrew Sutton via SG7

> <sg7_at_[hidden]>

> Subject: Re: [SG7] P2320: "The Syntax of Static Reflection" feedback request




> > On Feb 21, 2021, at 4:14 PM, Ville Voutilainen

> <ville.voutilainen_at_[hidden]<mailto:ville.voutilainen_at_[hidden]>> wrote:

> >

> > On Sun, 21 Feb 2021 at 22:54, David Rector <davrec_at_[hidden]<mailto:davrec_at_[hidden]>> wrote:

> >> Well said. Some leisurely thoughts expanding on this:

> >>

> >> Note too that such a metafunction, properly implemented, would indeed

> make that design pattern /obsolete/ - all the user need do is #include the

> right header and they can type `a + b + ...` to their heart's content, blissfully

> ignorant of the meaning of a "Mediator" and an "Observer" and all the

> expertise and manual labor the previous generation needed to code such

> things.

> >>

> >> Indeed, I would argue is that reflection + injection, properly implemented,

> might well be able to make *every* design pattern obsolete - if it qualifies

> as a "pattern" in any sense, that means it is some call to a metafunction, and

> the only question is whether the programmers themselves must serve as that

> "metafunction", or if they are allowed to just write that metafunction

> explicitly and grab a beer.

> >

> > Well, yeah. Design patterns *are* obsolete in some more powerful

> > languages; instead of being recipes for implementing patterns in

> > various ways with various trade-offs, they're just code where those

> > trade-offs are customizations points of the algorithm. :)

> >

> > As a side note, some of us do this already, to the extent the language

> > limitations allow it. We had a particular policy in multiple places I

> > worked in, which was "never (try to) make your type a Singleton, use

> > ourcompany::Singleton<YourType>.

> > Also, please leave it up to the application programmer to decide how

> > many of YourTypes should exist in their program, and please never make

> > that decision for them." Various other oft-mentioned patterns are

> > feasible to implement with such a common library type/wrapper, but

> > reflection+injection opens quite some doors towards that ability. Like

> > Proxies and Mediators and Template Methods and Bridges etc.

> >

> >> I would thus argue the best source for generating examples is thus any

> design pattern textbook: try to write each one as a metafunction, and thus

> render knowledge of it obsolete altogether.

> >

> > It's certainly one source. The best, I don't know - things like

> > serialization and persistence are oft-mentioned examples of problems

> > where reflection/injection is a big help, but they aren't necessarily

> > well-represented in pattern books.


> Those sound great. Why not try writing one, using the new syntax?


> I think more of us be following Roland's example, trying to actually implement

> these things, and presenting the results to the list. It's a bit strange to be

> considering syntax changes without seeing it in action in large, real-world test

> cases.


> And as Roland found, trying to implement these real world cases often leads

> us to places the proposals have overlooked, thus identifying deeper issues.


> And beyond that, exchanging code instead of words would be a good way to

> lower the temperature in here and keep things in focus.


> But Roland can't be the only one.


> I think I've said my peace and presented a useful test case of my own so I'll

> bow out here and leave space for others, thanks all,


> Dave


> >

> >> FWIW, this belief is why I spent a year awhile back pretty much just

> learning clang and implementing the most general possible alternative to

> Andrew's, without much regard to syntax or cleanliness. The resulting ugly

> "glorified preprocessor"/string mixin approach is a standard by which I think

> Andrew's or any other alternative must be measured, and I will never be okay

> with any alternative that is not as capable as it, no matter how many people I

> admire and respect line up on the other side.

> >>

> >> But I would still very much welcome *improvements* on it that don't

> sacrifice its capabilities - it is definitely not without its problems; e.g. recall

> Richard's line that "a good metaprogram is semantic" - if indeed the

> "semantic" approach could be accomplished without losing the capabilities of

> the "syntactic" metaprogramming approach, I agree it is a big improvement,

> and thus Andrew's endeavor to develop such a semantic approach is certainly

> worthwhile, though incomplete at this point.

> >>

> >> The challenge Andrew et al will face, it seems to me, is that, to implement

> injection properly in things like Sum<T,U> and PreCalcdSum<T,U> and other

> still more complex examples, you really just need to be able to write multiple

> statements in "meta" space, complete with meta variable declarations,

> control flow statements, etc. - *in the midst of writing arbitrary code *in

> "non-meta" space - i.e. in the midst of writing expressions, or initializers, or

> base classes.

> >

> > Well.. I do expect to be able to inject function definitions, but I do

> > expect to do that semantically, meaning that I inject code fragments

> > that have injections in them, as opposed to injecting strings.


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