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Flattening Rust’s learning curve

(corrode.dev)
452 points birdculture | 9 comments | 13 May 25 22:25 UTC | HN request time: 0.278s | source | bottom
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Animats ◴[14 May 25 00:22 UTC] No.43979394[source]▶
It's like reading "A Discipline of Programming", by Dijkstra. That morality play approach was needed back then, because nobody knew how to think about this stuff.

Most explanations of ownership in Rust are far too wordy. See [1]. The core concepts are mostly there, but hidden under all the examples.

    - Each data object in Rust has exactly one owner.
      - Ownership can be transferred in ways that preserve the one-owner rule.
      - If you need multiple ownership, the real owner has to be a reference-counted cell. 
        Those cells can be cloned (duplicated.)
      - If the owner goes away, so do the things it owns.

    - You can borrow access to a data object using a reference. 
      - There's a big distinction between owning and referencing.
      - References can be passed around and stored, but cannot outlive the object.
        (That would be a "dangling pointer" error).
      - This is strictly enforced at compile time by the borrow checker.
That explains the model. Once that's understood, all the details can be tied back to those rules.

[1] https://doc.rust-lang.org/book/ch04-01-what-is-ownership.htm...

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1. psychoslave ◴[14 May 25 05:15 UTC] No.43981064[source]▶
This explanation doesn't expose anything meaningful to my mind, as it doesn't define ownership and borrowing, both words being apparently rooted in an analogy with financial asset management.

I'm not acquainted with Rust, so I don't really know, but I wonder if the wording plays a role in the difficulty of concept acquisition here. Analogies are often double edged tools.

Maybe sticking to a more straight memory related vocabulary as an alternative presentation perspective might help?

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2. Renaud ◴[14 May 25 05:41 UTC] No.43981185[source]▶
I find it strange that you relate borrowing and ownership to financial asset management.

From that angle, it indeed doesn’t seem to make sense.

I think, but might be completely wrong, that viewing these actions from their usual meaning is more helpful: you own a toy, it’s yours to do as tou please. You borrow a toy, it’s not yours, you can’t do whatever you want with it, so you can’t hold on to it if the owner doesn’t allow it, and you can’t modify it for the same reasons.

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3. ithkuil ◴[14 May 25 06:57 UTC] No.43981643[source]▶
Analogies often leak.

1. In real life I can borrow a toy from you and while I have that toy in my hands, the owner can exchange ownership with somebody else, while the object is borrowed by me. I.e. in real life the borrowing is orthogonal to ownership. In rust you can't do that.

2. Borrowing a toy is more akin to how mutable references work in rust. Immutable references allow multiple people to play with the same toy simultaneously, provided they don't change it.

Analogies are just analogies

4. throwaway81523 ◴[14 May 25 07:11 UTC] No.43981730[source]▶
If you've worked inside of CPython or other programs with manual reference counting, the idea of borrowing shows up there, where you receive a reference from another part of the program and then mess with the object without tweaking the reference count, "borrowing" an existing reference because any copies you've of the address will be short lived. The term shows up throughout CPython.
5. arnsholt ◴[14 May 25 07:32 UTC] No.43981880[source]▶
The way I think about it is more or less in terms of how a C program would work: if you assume a heap allocated data structure, the owner is the piece of code that is responsible for freeing the allocation at the appropriate time. And a reference is just a pointer with some extra compile time metadata that lets the borrow checker prove that the reference doesn’t outlive the referent and that there’s no mutable aliasing.
6. imtringued ◴[14 May 25 10:17 UTC] No.43982812[source]▶
You think borrowing a lawn mower or owning a power drill is financial asset management?
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7. psychoslave ◴[14 May 25 10:48 UTC] No.43982958[source]▶
What do you mean with usual sense? Maybe it's "financial" that put the interpretation out of the track, but financial comes fidus, that is trust, as in trust that outcomes of reality will meet some expectation of a mental representation.¹

"You own a toy" is the first thing a child is teached as wrong assumption by reality if not by careful social education, isn't it? The reality is, "you can play with the toy in some time frame, and sharing with others is the only way we can all benefit of joyful ludic moment, while claims of indefinite exclusive use of the toy despite limited attention span that an individual can spend on it is socially detrimental."

Also memory as an abstract object pragmatically operate on very different ground than a toy. If we could duplicate any human hand grabbable object as information carried by memory holding object, then any economy would virtually be a waste of human attention.

¹ edit: actually I was wrong here, I have been in confusion with "fiduciary". Finance instead comes from french "fin"(end), as in "end of debt".

8. psychoslave ◴[14 May 25 12:44 UTC] No.43983853[source]▶
On the most abstract level, even "I" and "think" are misleading notions of what’s passing through current attention. So "borrowing" and "owning" are not really great starting point notions to "think" in that sense. But on the more mundane level of mentally handling stuffs, that’s an analogy that can have its own merits (and flaws, of course).
9. xnickb ◴[14 May 25 13:23 UTC] No.43984243[source]▶
Many people can borrow your toy to have look at it, but only one person can borrow it and play with it. And they are only allowed to play while no one is watching. And if you want to modify your toy with some tool it's not your's anymore, it yas moved and now belongs to the tool.

I guess I'm trying to say that analogy is of limited use here.