Hacker News Reader

Comment by jll29 | Hacker News Reader

...and Ada, too. I like the idea of attributes of data objects, to access the size of x simple write x'Size (also for types e.g. Natural'Size).

The Wirth languages (from which Ada is also a descendant) were so much more readable than C, yet relatively capable for systems programming, as demonstrated by systems like TeX, MacOS, Wirth's Modula compilers and the OS for the Lilith workstation he co-designed from scratch.

View on HN

Never used Ada but I think you can define range types so int range 0...11. Which I feel is something that you really want in embedded and applications level programming.

>> Never used Ada but I think you can define range types so int range 0...11.

Yes. Ada supports integral types with custom ranges:

https://learn.adacore.com/courses/intro-to-ada/chapters/stro...

In the medium-long term I want to do this for Rust as "Pattern types" because the thing I actually want (custom types with niches) is gated on Pattern types, as the way to explain to the type system where the niche goes is a Pattern. I was persuaded that we can't/ shouldn't just say we'll half ass it, we must do it properly if we're doing it.

e.g. I don't necessarily have a use for an integer from 0 to 11, but I do see a use for BalancedI8, a one byte type with values -127 to +127 via 0, thus omitting -128. I reckon lots of people don't need -128, whereas a niche is very useful. Rust provides NonZeroI8, which has -128 through +127 but no zero, but I find that's less often what you want, and it's not today possible to make your own in stable Rust (and in nightly Rust you need a not-for-mortals perma-unstable attribute today).

You can make it on stable Rust by using an enum with a missing variant. The compiler will give you a niche automatically.

Of course that doesn't scale to larger types as even with a macro the compile speed will be horrible.

Yeah, "enum with a missing variant" is like, awful but maybe viable for BalancedI8, but it's clearly insane for BalancedI32 let alone BalancedI64 and those seem at least as useful.

Today, in practice, if you want BalancedI32 what you'd do is swizzle NonZeroI32, so that a trivial CPU operation converts between the two types and you can deliver almost the same optimisations in practice. But, I think that's also pretty ugly, and Rust clearly could do the nicer thing here, indeed it does in my crate that only builds on nightly and is using not-for-public-use compiler internal attributes. So that's what I want, just apparently not enough to spend my vacation time last month working on it. Maybe next month.

Reply on news.ycombinator.com