Metacode mechanics

[Georg Wrede]

Don Clugston wrote:
> Georg Wrede wrote:
> 
>> Back to the issue at hand:
>>
>> What I'm still saying, is that (even though Walter and Don have made 
>> completely kick-ass progress here), there's a glass ceiling waiting 
>> for us at this rate. By the time we have evolved the D meta business 
>> into something that has cleared all major obstacles, it will be a hard 
>> to use, cumbersome, and unobvious metalanguage. (( but still a hell of 
>> a lot nicer and of course much more powerful than C++ templates ever ))
> 
> 
> I'm not sure about this. What do you see as the major obstacles?
> I had a long list of them once, it's very short now.
> Note that almost all of the improvements which have been made, have made
> metaprogramming *more* similar to ordinary D, not less.
> 
> The main reason D metaprogramming is so much simpler than C++, is that 
> we have "static if" instead of using partial template specialisation.
> 
> I'll just summarise the changes I proposed that Walter has implemented:
> 
> * strings and reals as template value arguments.
>  ---> makes metafunctions able to have a greater variety of parameters,
>       and therefore more like normal D.
> * Improved compile-time constant folding, most notably [] and [..]
>  ---> makes compile-time arrays more similar to runtime arrays.
> * template quirks fixed. Most notably scope of static if.
>  ---> less weird workarounds
> * .mangleof property.
>  OK, this one's a bit weird. But, it does establish a closer link 
> between the two languages: it means that any type can be expressed in 
> the run-time language.
> 
> The remaining changes that are accepted for D 1.0
> * implicit function template instantiation
> --> makes invoking a template function look the same as invoking a 
> normal function.
> 
> and beyond D 1.0
> * array literals
>  ---> makes compile-time arrays more similar to runtime arrays.
> 
> The only other metaprogramming functionality I'd really like, but don't 
> have, is full compile-time reflection, and possibly the ability to 
> convert strings into variables.
> 
> I've spent a few months pushing the boundaries of what's achievable with 
> D, and frankly, I'm running out of ideas. Practically everything I've 
> wished I could do in C++ seems to already be possible in D, with the 
> exceptions listed above.
> 
> In C++, metaprogramming feels like programming in an extremely 
> impoverished functional programming language. But, in D, it feels like 
> programming in a subset of D -- (you can only assign to variables at 
> construction, you must use recursion for looping) -- a metafunction can 
> be trivially converted to a runtime function (change 'static if' to 
> 'if', remove all of the "const"s from the variable declarations, remove 
> all the "!", and change  "template abc(char [] str)" to abc(char [] 
> str)"). Normally you don't use recursion much in a D runtime program, 
> but it will work.
> 
> So, I don't see any signs of D metaprogramming becoming more cumbersome 
> than it is now. I really don't see grounds for pessimism.
> BUT...
> it's probable we could do better. I feel, though, that we should be 
> aiming for a metalanguage that is more similar to ordinary D, rather 
> than try to completely separate them.
> 
>> Thus, it would behoove us to start examining the features and 
>> properties of a from-ground-up designed meta level language for D. 
>> (Definitely not ever even intended to anything earlier than D2.0!)
>>
>> A year ago this was a pipe dream. But now, with the experience we gain 
>> from the existing meta language, and (foremost) since I'm not alone 
>> anymore in thinking that such an undertaking is actually within reach, 
>> we really could start outlining it now!
>>
>> ---
>>
>> So I suggest that we:
>>
>>  - continue developing the current D metalanguage
>>  - continue to have both Don's and Phobos' regexps
>>  - in this NG start the development of requirements for KBDM
>>
>> KBDM being Kick-Butt D Metalanguage. :-)
> 
> 
> I'll be interested to hear any further ideas you have.

Not least due to a few recent "experiences", I'll refrain from answering 
your _valid_ questions, for now.

Before that, and mostly because you seemed pretty happy with the 
existing meta-stuff, I think I'll "do my homework" (i.e. "better know 
what we have now, before touting for changes").

---

While I'm at it, probably I'll use this NG for "meta newbie" questions. 
My method of learning (which has thoroughly frustrated, ehhh, a number 
of people) is to linger at the "dumb questions level" until I really 
feel that I've got a Crystal Clear picture.

georg

PS, I haven't skipped the "glass ceiling idea" yet.....