The D language basically does that. You can write D programs that evaluate D code at compile time to generate strings of new D code which you can then basically compile-time eval into your code as needed. Combined with the extremely powerful compile-time reflection capabilities of D it's the closest thing I've seen to Lisp metaprogramming outside of that family of languages and it's easier to read than Rust macros or C++ template metaprogramming.
Every time I hear about D it sounds awesome. I actually used it to prototype an image collage-composing algorithm which I then rewrote in Scala[1], and the D version might have been nicer to write.
The only reason I didn't write more stuff in D was that the stack traces from my programs were pretty much useless. Maybe I was supposed to set a --better-stack-traces flag when I compiled it or something idk.
> Combined with the extremely powerful compile-time reflection capabilities of D it's the closest thing I've seen to Lisp metaprogramming outside of that family of languages ...
Scala gets pretty close to LISP-level of metaprogramming support between its intrinsic support for macros[0] (not to be confused with the C/C++ preprocessor of the same name), the Scalameta project[1], and libraries such as Shapeless[2].
Not comparing Scala to D, just identifying a language with similar functionality.
D is really, really good. I hope it gets more love soon. D's focus on just getting shit done, lightning builds, QOL improvements all over the place, actually good modules, templates and metaprogramming that work, simpler more regular syntax, any memory management paradigm you want, being fully batteries included, being super easy to cross compile, being able to span all the way from Python/C# slop all the way down to tight-as-you-like C code... It's an amazing language and is getting better all the time. A real C++ successor. It has become my secret weapon! Maybe I actually don't want it to blow up soon, since it gives me a huge edge on anyone stuck with C++, which gets worse every release (how slow do builds have to get before people lose it completely?).
Many of the functions in Phobos are not `@nogc` compatible, yes.
That being said, I can't think of many scenarios in which an application where user-code is all `@nogc` would be hindered by occasional GC'ed stdlib methods.
One standout example of viability is the "dplug" library for realtime audio processing plugins and the commercial AuburnSounds VST's written by the author.
In D, you can write Python-style slop code and/or C#-style slop code. Two very different styles of slop code, both of which can be written in D. Or even mixed and matched.
It would be cool, except for the entire language that is available at compile-time being C++, and thus entirely unsuitable for manipulating C++ programs.
Not at all, originally template metaprogramming was discovered by accident.
Cannot recall any longer if the original article on the matter appeared on The C/C++ Users Journal or Dr. Dobbs.
Eventually it started to get abused and the Turing completeness has been discovered.
Since C++11, the approach to a more sane way to do metaprogramming with templates has been improving.
Instead of tag dispatch, ADL and SFINAE, we can make use of concepts, if constexpr/eval/init, type traits, and eventually reflection, instead of the old clunky ways.
Templates are semi-accidentally Turing-complete. They were intended for writing compile-time-generic, run-time-concrete functions and types - but it turned out you could use them, along with the overload resolution mechanisms, to compute things. The Turing-completeness involves recursive use.
Computing things using templates is not intuitive. Many of us have gotten used to it - but that's because that's all we had for many years. It's a different sub-language within C++. As constexpr capabilities widen, we can avoid "tortuted" templates and can just write our compile-time checks and figurings in plain C++ - more or less.
Sometimes, enhanced language features in C++ allow us to actually throw away and forget about other existing features, or at least - complex and brittle idioms using existing features. Like SFINAE :-)
C++23 doesn't have full reflection yet. That's coming in C++26.
I've seen the vast majority of build time in a very large C++23 project be taken up by reflection in fmtlib and magic_enum because both have to use templates (I think).
