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Rust 101 - 4: Exercises for module A1 (diode.zone)
submitted 2 years ago by freamon@futurology.today to c/learningrustandlemmy@lemmy.ml
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cross-posted from: https://diode.zone/videos/watch/9766d1f1-6018-48ec-ad67-e971758f8a3a

Going through some exercises on basic Rust syntax and ownership.

Links:

Rust 101 is a series of videos explaining how to write programs in Rust. The course materials for this series are developed by tweede golf. You can find more information at https://github.com/tweedegolf/101-rs and you can sponsor the work at https://github.com/sponsors/tweedegolf . They are released under the Creative Commons Attribution Share Alike 4.0 International license.

This series of videos is copyright 2023 Andy Balaam and the tweede golf contributors and is released under the Creative Commons Attribution Share Alike 4.0 International license.

These videos are roughly on track with the Reading Club apparently, so this video belongs here this week, I think.

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[-] maegul@lemmy.ml 1 points 2 years ago

Ok, so the first exercise here 1-basic-syntax/01.rs throws me a little bit. With the compiler error it's easy enough to fix, but conceptually I didn't actually have this down.

fn multiply(a: i32, b: i32) {
    a * b
}

This function is invalid. The crucial error from the compiler is:

error[E0308]: mismatched types
 --> src/bin/01.rs:6:5
  |
5 | fn multiply(a: i32, b: i32) {
  |                            - help: try adding a return type: `-> i32`
6 |     a * b
  |     ^^^^^ expected `()`, found `i32`

So the quick fix it needs is a return type. That's fine.

The bits that throw me are:

  • The default return type is an empty tuple? (whatever is meant by ()).
  • There is no type inference on the return type? For let v = a * b; rust is happy to infer that v: i32, why not a function's return? I'd guess some policy of ensuring function declarations are clean with respect to types and reaping the compiling and safety features that follow that.
[-] Jayjader@jlai.lu 2 points 2 years ago

I think I have some answers for you if you haven't resolved them yet:

  • the default return type is not an empty tuple, but the unit type which basically represents a unit of computation being performed, without any data as output (i.e. purely side-effects are allowed). It is kinda the equivalent of void from java- and c-style languages. Here is a decent explanation on StackOverflow if you prefer that sort of thing.
  • there absolutely is inference on the return type (I've used it in other rust projects before), but I can't remember if it only, or also takes into account how the function is externally called. In any case, this function's sole use is on the line
    println!("{}", multiply(10, 20));

And "sadly" (for us here), this doesn't explicitly say "I am expecting an int/i32 as output of multiply", it only says "I am expecting something that implements the Display trait". I think that in such cases it defaults to assuming the unit type () because it is the "simplest"/"most conservative" guess the compiler can make in such cases.

[-] maegul@lemmy.ml 2 points 2 years ago

Thanks!! Very helpful!

[-] Jayjader@jlai.lu 2 points 2 years ago

I found a more precise answer re: type inference: https://stackoverflow.com/questions/24977365/differences-in-type-inference-for-closures-and-functions-in-rust

As I understand it, if a function is public you will need to explicit the return type. If it's private/local/not visible from outside the module wherein it is defined, then inference can kick in.

But generally, you'll want to specify the return type to make the compiler that much more aware of your intent.

[-] maegul@lemmy.ml 1 points 2 years ago

That makes a lot of sense. Intuitively I figured it would be something like that.

Still, as a compilation error, it’s confusing to see an error on the unit type being inconsistent when the issue is really an untyped return. Though now I know what the unit type is it’s a bit clearer what such a compiler error is about.

this post was submitted on 14 Feb 2024
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