size_of and align_of [LWN.net]

size_of and align_of

Posted Jul 28, 2024 20:10 UTC (Sun) by tialaramex (subscriber, #21167)
In reply to: size_of and align_of by NYKevin
Parent article: Rust 1.80.0 released

I think for my purpose the need to repeat the lambda everywhere is a problem, or at least a gross inconvenience. So that's why I think Lazy is important and I had not found Once adequate.

Basically instead of 4, a single byte with the pattern 00000100 in it, my four is: Two num::BigUints (a numerator and denominator) to form a rational magnitude, then a sign flag, a sum type which says whether we're also multiplying this by a fixed irrational value, and the potentially complicated arithmetic (with cached state, potentially very large) for the said irrational value if we are.

I would like to be able for the type of this data structure to provide something similar to a constant like FOUR and ZERO rather than (as I do in the current prototype) making such values each time they're needed.

This data structure is of course horribly unwieldy as a replacement for i32 or f64 - but it's perfectly reasonable for a desktop calculator type application, where the performance loss is more than amply compensated by knowing when there's an exact answer or it's in terms of a well known irrational such as Pi or e.

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size_of and align_of

Posted Jul 29, 2024 2:58 UTC (Mon) by NYKevin (subscriber, #129325) [Link] (1 responses)

I don't see why. You can just write this:

fn make_small_bigint(value: i64) -> BigNum{
// Do whatever simple logic you need here.
}

// Can also have e.g. make_small_rational(numerator, denominator), etc.

// Global scope:
static OnceLock<BigNum> FOUR = OnceLock::new();

// Callsite:
let four = FOUR.get_or_init(|| make_small_bigint(4));

LazyLock just means you can write let four = *FOUR; instead of let four = FOUR.get_or_init(|| make_small_bigint(4));. You still have to write the function make_small_bigint() either way, because you will need to pass exactly the same lambda to the LazyLock at construction time.

size_of and align_of

Posted Aug 1, 2024 15:02 UTC (Thu) by tialaramex (subscriber, #21167) [Link]

Yeah, exactly. You're correct that it's not *impossible* or even *actually difficult*, it's just messy and annoying and that's not enough improvement over my current situation, whereas Lazy is cleaner.

Don't worry, I was never under the impression it was *impossible* even when I started (long before 1.70) I was just expecting to need a third party crate to make this tidy and now probably I won't.

There's a lot more work to do to properly approach the vision from the Java API but in a nicer language, but I think I'm closing on the point where it's a useful alternative to tools like the Unix "basic calculator" bc or the typical GUI calculator on your desktop OS.