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0 pointsplanede3y ago0 comments

That's runtime reflection.

Compile time reflection AFAIK is available in D and Zig, and is planned for C++.

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6 comments · 3 top-level

lmm3y ago· 1 in thread

"Compile time reflection" is an inconsistent and nonstandard concept; originally it seemed to just mean typeclasses for people who hadn't heard of typeclasses.

anonymoushn3y ago

for weirdos who only have ad-hoc constraints instead of knowing what typeclasses are, it means that you can first say "I only have ad-hoc constraints" then say "wait wait I need to make decisions based on the specifics of the type" which may be useful for e.g. generating serializers and deserializers at compile time instead of using code generators like protobuf

GrumpySloth3y ago· 1 in thread

Yup. I consider runtime reflection an antifeature, which has negative performance effects, is unsafe (see e.g. log4j) and leads to fragile code.

I would however welcome static reflection with open arms. In Rust in particular, I’d prefer it if derive was implemented using static reflection, rather than proc macros.

lmm3y ago

Derive or equivalent ought to be something you can implement on top of frunk (so you're ultimately still depending on a proc macro, but the whole ecosystem only needs to depend on that one macro, and tools etc. can build in support for it) - that's how it's usually done in Scala.

elcritch3y ago· 1 in thread

That's right. Nim does as well. It's amazing. Once you get used to having CTTI and being able to use it, it's hard to program without it. Bonus points if you can do basic dependent types too.

In C++ with SFINAE you can effectively do CTTI-style programming in C++. C++ has long had runtime type reflection as well (RTTI), though it needs to be compiled in. Looks like there's a boost library for CTTI.

Conscat3y ago

The C++ reflection improves a lot in C++20, but it's still very limited compared to that aspect of Nim, or even Zig. The std::meta::info and "splices" based on Haskell for C++26 are incredibly exciting to me. I have many use cases in mind. Splices in combination with std::embed will make C++ basically just a bad Racket (but one with inline assembly!).

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