Rust for Lemmings Reading Club Week 1 [Project]

maegul (he/they)@lemmy.ml · 9 months ago

Rust for Lemmings Reading Club Week 1 [Project]

maegul (he/they)@lemmy.ml · edit-2 9 months ago

Thanks!!

Rng is a trait, which feature a lot in Rust, but is something I’ve had difficulty getting my head around. Where I’m at so far is they mostly provide convenience. The rand create provides a number of structures in its rngs module - ThreadRng, OsRng, SmallRng, and StdRng, depending on what you want to use to generate randomness. Instead of saying ThreadRng provides the gen_range() method, and OsRng provides the gen_range() method, etc, Rust just says they implement the Rng trait.

This makes a lot of sense actually. Thanks! It does lead to the awkward situation where you’d have to know that Rng is the underlying trait of the rest of the module and so import it. But like I said, I’m guessing that’s because this example is aiming only for easy high-level usage of rand. I’d guess there’s a lower level way of using the rand crate that would involve more direct imports, perhaps use rand::ThreadRng?

For the default behaviour of passing owned vs. borrowed variables, I guess it’s useful to explicitly state “I’m giving this variable to another function, because I don’t intend to use it anymore”, so that if you inadvertently do, the compiler can catch it and error.

Makes sense.

Sooo … is passing by value a thing in rust? Or does just about every method take only reference types as arguments?

freamon@endlesstalk.org · edit-2 9 months ago

I wondered the same, and tried use rand::rngs::ThreadRng but the compiler wouldn’t accept it, and suggest I use the trait instead. So it looks like the compiler can be helpful in identifying these things (and maybe traits are the first thing that Rust developers look for when reading the docs for a crate).

(wrongness edited out and hopefully corrected in new comment)

maegul (he/they)@lemmy.ml · 9 months ago

and maybe traits are the first thing that Rust developers look for when reading the docs for a crate

Yep. Makes a lot of sense. Probably gotta start thinking in terms of traits at some point. I haven’t spun up any LSP yet but hopefully that can help surface these sorts of things.

Still, at the moment, it does seem like a wrinkle in the usability of the language that you import something which implicitly underlies what you actually want to use.

Also, Thanks!

freamon@endlesstalk.org · 9 months ago

Sooo … is passing by value a thing in rust? Or does just about every method take only reference types as arguments?

I think this is an occasion where a vague familiarity with other languages ended up confusing me with Rust. The ‘&’ sign doesn’t mean ‘pass by reference’ in the same way as it does in C. Anything with a size that’s fixed at compile time is typically passed by value, whereas variables who’s size might change are passed by reference. The ‘&’ in Rust isn’t about that. For variables that are passed by reference, the ‘&’ is about whether the ownership of that memory address is transferred or not.

To illustrate:

fn abc(v: String) {
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');
    abc(v);

    // println!("v is {}", v);
}

works fine as it is, but will error if you uncomment the second println! The ‘v’ variable was passed by reference, but it’s ownership was transferred, so it can’t be referred to again.

morrowind@lemmy.ml · 9 months ago

The ‘&’ sign doesn’t mean ‘pass by reference’ in the same way as it does in C. Anything with a size that’s fixed at compile time is typically passed by value, whereas variables who’s size might change are passed by reference.

Are you sure? I noticed the rust book said this:

A reference is like a pointer in that it’s an address we can follow to access the data stored at that address; that data is owned by some other variable. Unlike a pointer, a reference is guaranteed to point to a valid value of a particular type for the life of that reference.

freamon@endlesstalk.org · 9 months ago

No, I’m not sure, tbh. It’s a concept I’m struggling with, and I reliant on others to correct/question me. I was trying to answer the question of whether things get passed by value or not, and I wanted to say yeah, loads of things do (anything who’s size is known at compile-time) and to caution against thinking too much in C terms.

Here’s where my thinking is now: a variable without a & in front is passed by value. A primitive type (i.e. something who’s size is known) will be copied. So if a=4 and you pass it to a function, you can still refer to a later. A variable-length type (e.g. a String) can’t be copied, so it is moved, and referring to it later will be an error.

A variable with a & in front is indeed a reference. It’s a memory address, so it’s of fixed size. For either a primitive or a variable-length type, the address can be copied when passed to a function, so it can be referred to again later without issue.

This feels more correct to me, so hopefully it is. If not, I’m sure someone will have a better answer soon (this community is growing well!).

maegul (he/they)@lemmy.ml · 9 months ago

On the whole primitive types and references thing, I find it helps me to remember that a reference/pointer (subtle but importance difference between rust and C where rust has more guarantees around a pointer to make it a "reference) is also basically just a number like a “primitive” i32 etc. And references/pointers obviously (?) have to get passed by value or copied (in order to “survive” their original stack frame right?), so passing any primitive by value or copying it really isn’t different from passing it by reference, apart from when you’re running a borrow checker for managing memory of course.

Jayjader@jlai.lu · 9 months ago

This will make more sense once we (this community) get to the 4th week/session and properly get into ownership, but lemme try to explain anyways:

A variable without a & in front is moved into the [function’s] scope.

Upon exiting a scope, Rust automatically drops/de-allocates any variables that were owned by/moved into said scope.

This is why you need to either

pass by ref / have the scope borrow a reference to the variable, or
have your function return the variable/object/memory handle that was moved into it

when you want a variable/some data to “out-live” being passed as argument to a function call.

Most often you will use 1), but there are some cases where it can be much nicer to move things “into” a function call and store what you “get back out” (i.e. the return value). Using a “[Type-]State” pattern/approach is a good example of such a case (here’s a better explanation than I can give in a lemmy comment).

Example:

struct Unauthenticated;
struct Authenticated { name: String };

impl Unauthenticated {
    fn login(self, username: String) -> Authenticated {
        Authenticated { name: username }
    }
}

pub fn main() {
    let un_authed_user = Unauthenticated::new();
    let authed_user = un_authed_user.login("Alice"); // `un_authed_user` has effectively been moved into `authed_user`
}

Here, we as programmers don’t need to worry about making sure un_authed_user gets cleaned up before the program exits, and we don’t need to worry about data that could have been stored inside un_authed_user being freed too early (and thus not being available to authed_user).

Admittedly, this is a contrived example that doesn’t really need to worry about ownership, it’s just the bare minimum to illustrate the idea of moving data into and then back out of a function scope. I don’t know of a small enough “real-world” example to give here instead.

freamon@endlesstalk.org · 9 months ago

Thank you.

I think there’s more to it though, in that simple values aren’t moved, they’re always copied (with any & in front indicating whether it’s the value to copy or the address)

To illustrate:

fn how_many(a: u32, fruit: String) {
    println!("there are {} {}", a, fruit);
}


fn main() {
    let a=4;
    let fruit = String::from("Apples");
    how_many(a, fruit);

    println!("the amount was {}", a);         // this works
    println!("the fruit was {}", fruit);      // this fails
}

The ‘a’ was copied, and the ‘fruit’ was moved.

Jayjader@jlai.lu · 9 months ago

Correct! Values that can be copied are copied, and those that aren’t are moved.

All primitive data types are copyable (or "are Copy ", which will make more sense once we get to Traits). I think arrays of primitives are as well? Most everything else isn’t by default.

One of my favorite parts of Rust is that to make something copyable, you “just” implement the Copy trait for that thing.

maegul (he/they)@lemmy.ml · edit-2 9 months ago

Thanks!

Seems I gotta dig into the borrow checker before thinking too much about this!

Otherwise, compiling your code snippet is a nice illustration of how helpful the compiler tries to be … lots of tips in the output there! To anyone else, just try running rustc on this, with the second println! uncommented and see the output, which is half error half linting.

For variables that are passed by reference, the ‘&’ is about whether the ownership of that memory address is transferred or not.

Yea. So if the second println! were uncommented, how could we compile this? From what you’ve said, I’d guess that & means “borrow” (ie, not “move” ownership).

So if we alter abc to take a &String type and not String, and therefore only “borrow” the variable, and then pass in &v and not v to pass in a “borrowed” variable, it should compile.

fn abc(v: &String) {
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');
    abc(&v);

    println!("v is {}", v);
}

It seems to!

Of course, as the compiler suggests, we could instead just pass in v.clone() which presumably creates a new variable and effectively “passes by value”.

Digging in a bit more, what happens if abc (tries to) mutate the variable?

We can add v.push('X') to abc and see if we get different printouts. As the compiler would tell us, we would need to make the argument v mutable for this to work.

fn abc(mut v: String) {

    v.push('X');
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');
    abc(v.clone());

    println!("v is {}", v);
}

// OUTPUT:
// v is abcX
// v is abc

I’m not clear on why I don’t have to declare that the v.clone() is mutable in anyway though.

What about trying the same with a "borrowed’ variable?

Well we need mutable borrowed variables, so necessary adjustments to the types of abc and its call in main. And adding an additional mutation of v in main after abc is called, and we get two different println outputs, with each mutation applying to the same variable.

fn abc(v: &mut String) {

    v.push('X');
    println!("v is {}", v);
}

fn main() {
    let mut v=String::from("ab");
    v.push('c');

    abc(&mut v);

    v.push('Y');

    println!("v is {}", v);
}

// OUTPUT
// v is abcX
// v is abcXY

ericjmorey · 9 months ago

Seems I gotta dig into the borrow checker before thinking too much about this!

It’s covered in detail in chapter 4.

Rust for Lemmings Reading Club Week 1 [Project]

Rust for Lemmings Reading Club Week 1 [Project]

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