4 ms·

Once I discovered ‘static was the subtype of all lifetimes rather than the super things began clicking for me.

by almost_usual 3y ago

andyferris 3y ago
Could you elaborate? I find this unintuitive.
- csjh 3y ago
  Since static lifetimes last for the entirety of the program, it can sub in for any other lifetime (as it is guaranteed to exist until the end of the other lifetime)
- raincole 3y ago
  I am not familiar with the formal type theory, but this one is intuitive if you view it this way: 1. When A is a subtype of B, it means A can be used as B (a Teacher can be used by any function that accepts a Human). 2. static lifetime lives longer any other lifetimes, so it can be used as other lifetimes.
cwzwarich 3y ago
`static is actually the superlifetime of all lifetimes, but & is contravariant in its lifetime parameter (and covariant in its type parameter).
- almost_usual 3y ago
  Mind expanding on this? The nomicon describes &’a T where both ‘a’ and ‘T’ are covariant. I thought I had a clear picture but now I’m confused. https://doc.rust-lang.org/nomicon/subtyping.html#variance https://doc.rust-lang.org/nomicon/subtyping.html#variance
  - cwzwarich 3y ago
    Here is the original GitHub issue on the question: https://github.com/rust-lang/rust/issues/15699 https://github.com/rust-lang/rust/issues/15699 And an RFC by some people that felt frustrated by this arguably implementation-centric view that kind of lost steam: https://github.com/rust-lang/rfcs/issues/391 https://github.com/rust-lang/rfcs/issues/391 Intuitively, the bottom lifetime should the one that is uninhabited, which would be a lifetime with no extent rather than 'static.
    - almost_usual 3y ago
      Interesting, thanks for sharing!
  - Georgelemental 3y ago
    `rustc` used to use different terminology than the nomicom, but this is no longer the case: https://github.com/rust-lang/rust/pull/107339 https://github.com/rust-lang/rust/pull/107339
- Georgelemental 3y ago
  This is not true anymore: https://github.com/rust-lang/rust/pull/107339 https://github.com/rust-lang/rust/pull/107339

Once I discovered ‘static was the subtype of all lifetimes rather than the super things began clicking for me.

andyferris 3y ago

Could you elaborate? I find this unintuitive.

csjh 3y ago
Since static lifetimes last for the entirety of the program, it can sub in for any other lifetime (as it is guaranteed to exist until the end of the other lifetime)
raincole 3y ago
I am not familiar with the formal type theory, but this one is intuitive if you view it this way: 1. When A is a subtype of B, it means A can be used as B (a Teacher can be used by any function that accepts a Human). 2. static lifetime lives longer any other lifetimes, so it can be used as other lifetimes.

cwzwarich 3y ago

`static is actually the superlifetime of all lifetimes, but & is contravariant in its lifetime parameter (and covariant in its type parameter).

almost_usual 3y ago
Mind expanding on this? The nomicon describes &’a T where both ‘a’ and ‘T’ are covariant. I thought I had a clear picture but now I’m confused. https://doc.rust-lang.org/nomicon/subtyping.html#variance https://doc.rust-lang.org/nomicon/subtyping.html#variance
- cwzwarich 3y ago
  Here is the original GitHub issue on the question: https://github.com/rust-lang/rust/issues/15699 https://github.com/rust-lang/rust/issues/15699 And an RFC by some people that felt frustrated by this arguably implementation-centric view that kind of lost steam: https://github.com/rust-lang/rfcs/issues/391 https://github.com/rust-lang/rfcs/issues/391 Intuitively, the bottom lifetime should the one that is uninhabited, which would be a lifetime with no extent rather than 'static.
  - almost_usual 3y ago
    Interesting, thanks for sharing!
- Georgelemental 3y ago
  `rustc` used to use different terminology than the nomicom, but this is no longer the case: https://github.com/rust-lang/rust/pull/107339 https://github.com/rust-lang/rust/pull/107339
Georgelemental 3y ago
This is not true anymore: https://github.com/rust-lang/rust/pull/107339 https://github.com/rust-lang/rust/pull/107339