Things Zig comptime won't do

Yes!

To me, the uniqueness of Zig's comptime is a combination of two things:

1. comtpime replaces many other features that would be specialised in other languages with or without rich compile-time (or runtime) metaprogramming, and

2. comptime is referentially transparent [1], that makes it strictly "weaker" than AST macros, but simpler to understand; what's surprising is just how capable you can be with a comptime mechanism with access to introspection yet without the referentially opaque power of macros.

These two give Zig a unique combination of simplicity and power. We're used to seeing things like that in Scheme and other Lisps, but the approach in Zig is very different. The outcome isn't as general as in Lisp, but it's powerful enough while keeping code easier to understand.

You can like it or not, but it is very interesting and very novel (the novelty isn't in the feature itself, but in the place it has in the language). Languages with a novel design and approach that you can learn in a couple of days are quite rare.

[1]: In short, this means that you get no access to names or expressions, only the values they yield.

I’ve never managed to understand your year-long[1] manic praise over this feature. Given that you’re a language implementer.

It’s very cool to be able to just say “Y is just X”. You know in a museum. Or at a distance. Not necessarily as something you have to work with daily. Because I would rather take something ranging from Java’s interface to Haskell’s typeclasses since once implemented, they’ll just work. With comptime types, according to what I’ve read, you’ll have to bring your T to the comptime and find out right then and there if it will work. Without enough foresight it might not.

That’s not something I want. I just want generics or parametric polymorphism or whatever it is to work once it compiles. If there’s a <T> I want to slot in T without any surprises. And whether Y is just X is a very distant priority at that point. Another distant priority is if generics and whatever else is all just X undernea... I mean just let me use the language declaratively.

I felt like I was on the idealistic end of the spectrum when I saw you criticizing other languages that are not installed on 3 billion devices as too academic.[2] Now I’m not so sure?

[1] https://news.ycombinator.com/item?id=24292760

[2] But does Scala technically count since it’s on the JVM though?

My "manic praise" extends to the novelty of the feature as Zig's design is revolutionary. It is exciting because it's very rare to see completely novel designs in programming languages, especially in a language that is both easy to learn and intended for low-level programming.

I wait 10-15 years before judging if a feature is "good"; determining that a feature is bad is usually quicker.

> With comptime types, according to what I’ve read, you’ll have to bring your T to the comptime and find out right then and there if it will work. Without enough foresight it might not.

But the point is that all that is done at compile time, which is also the time when all more specialised features are checked.

> That’s not something I want. I just want generics or parametric polymorphism or whatever it is to work once it compiles.

Again, everything is checked at compile-time. Once it compiles it will work just like generics.

> I mean just let me use the language declaratively.

That's fine and expected. I believe that most language preferences are aesthetic, and there have been few objective reasons to prefer some designs over others, and usually it's a matter of personal preference or "extra-linguistic" concerns, such as availability of developers and libraries, maturity, etc..

> Now I’m not so sure?

Personally, I wouldn't dream of using Zig or Rust for important software because they're so unproven. But I do find novel designs fascinating. Some even match my own aesthetic preferences.

> But the point is that all that is done at compile time, which is also the time when all more specialised features are checked.

> ...

> Again, everything is checked at compile-time. Once it compiles it will work just like generics.

No. My compile-time when using a library with a comptime type in Zig is not guaranteed to work because my user experience could depend on if the library writer tested with the types (or compile-time input) that I am using.[1] That’s not a problem in Java or Haskell: if the library works for Mary it will work for John no matter what the type-inputs are.

> That's fine and expected. I believe that most language preferences are aesthetic, and there have been few objective reasons to prefer some designs over others, and usually it's a matter of personal preference or "extra-linguistic" concerns, such as availability of developers and libraries, maturity, etc..

Please don’t retreat to aesthetics. What I brought up is a concrete and objective user experience tradeoff.

[1] based on https://strongly-typed-thoughts.net/blog/zig-2025#comptime-i...

> No. My compile-time when using a library with a comptime type in Zig is not guaranteed to work because my user experience could depend on if the library writer tested with the types (or compile-time input) that I am using.[1] That’s not a problem in Java or Haskell: if the library works for Mary it will work for John no matter what the type-inputs are.

What you're saying isn't very meaningful. Even generics may impose restrictions on their type parameters (e.g. typeclasses in Zig or type bounds in Java) and don't necessarily work for all types. In both cases you know at compile-time whether your types fit the bounds or not.

It is true that the restrictions in Haskell/Java are more declarative, but the distinction is more a matter of personal aesthetic preference, which is exactly what's expressed in that blog post (although comptime is about as different from C++ templates as it is from Haskell/Java generics). Like anything, and especially truly novel approaches, it's not for everyone's tastes, but neither are Java, Haskell, or Rust, for that matter. That doesn't make Zig's approach any less novel or interesting, even if you don't like it. I find Rust's design unpalatable, but that doesn't mean it's not interesting or impressive, and Zig's approach -- again, like it or not -- is even more novel.