For me, the biggest things to take away from these chapters were:

• Enums and pattern matching
• Borrow checker concerns emerging from these new data structures
• Derivable traits

Enums and pattern matching for the win

• That the pattern matching facility is powerful, and that enums can have associated data, structured independently for each variant ... really provides a (relatively straight forward and versatile "happy path" in rust IMO.
  • I tried to hack together a file differ in rust a couple of months ago (see my post on my solution here, mostly in the comments) and found myself just leaning into enums + pattern matching and rather enjoying the process.
• So much so that major (and celebrated?) features of the language such as Option and Result types are really just applications of enums (along with rust's good type system)

The example in the book of the IP Address enum type is quite a nice demonstration I think:

enum IpAddr {
    V4(u8, u8, u8, u8),
    V6(String),
}

let home = IpAddr::V4(127, 0, 0, 1);
let loopback = IpAddr::V6(String::from("::1"));

We're still learning "The Borrow Checker"

• Ownership and borrowing concerns are still alive here in their application to structs and enums and what best practices arise out of it all.

Match statements

• In match statements, the concerns are relatively straight forward (I think). Match arms take ownership of the variables they "use/touch" (I'm still unclear on the details there!) ...
• so if you want a variable to live beyond the match statement, match on a reference.

EG:

let opt: Option<String> = Some(String::from("Hello world"));

match &opt {
    Some(s) => println!("Some: {}", s),
    None => println!("None!")
};

println!("{:?}", opt);

• There's a slightly tricky thing that happens implicitly here:
  • Though the match is on &opt, the s in the pattern Some(s) is also a reference because rust implicitly "pushes down" the reference from the outer enum to the inner field or associated data.
  • Seems tricky, but also ergonomically sensible.

Borrowing self in methods

Probably the trickiest and most relevant part of the two chapters

• the self in methods, like any other variable, can be one of three types in terms of ownership:
  • Owned by the method, like a plain variable
  • A reference (&self)
  • A mutable reference (&mut self)

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }

    fn set_width(&mut self, width: u32) {
        self.width = width;
    }

    fn max(self, other: Rectangle) -> Rectangle {
        Rectangle { 
            width: self.width.max(other.width),
            height: self.height.max(other.height),
        }
    }
}

• What's tricky about this is that a method's signature for self has consequences that both reach back to the initial type of the root object (ie, is it mutable or not) and forward to what can be done with the root type afterward.

  • EG, a method that takes &mut self can't be used on a variable that isn't initially mutable.
  • EG, a method that takes ownership of self effectively kills the root object, making it unusable after the method is called!!

• I'm sure there are a bunch of patterns that emerge out of this (anyone with some wisdom here?) ...

• But the simple answer seems to borrow self, and if necessary, mutably borrow.

• Taking ownership of self is an interesting way to enforce a certain kind of usage and behaviour though.

• As the object dies, the natural return of an owning method would be a new object, probably of the same type.

• Which leads into a sort of functional "pipe-line" or "method chaining" style of usage, not unlike the "Faux-O" idea in Cory Bernhardt's talk Boundaries. It's likely not the most performant, but arguably has some desirable qualities.

Derivable Traits

• We haven't gotten to traits yet, but they come up here.
• Turns out rust has kinda has a system of inheritance for structs where a trait can be easily implemented for a struct "automagically": #[derive(Debug)]

EG:

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

• This particular trait, Debug, allows for the printing of a struct's full makeup with println!.

• All of the "Derivable" traits (from the std lib) are listed in Appendix C of The Book

• There aren't that many, but they're useful:

  • Copy and Clone enable a struct to be copied without having to worry about ownership (though you have to be careful about the types of the fields, as its their copy methods that are ultimately relied on)
  • Four traits that implement methods for comparison and equality operators
  • Hash for hashing an object
  • Default for defining default values

• Of course, when we cover traits we'll learn how to implement them ourselves for our custom types, but these seem to be fundamental features of the language, and easy enough to use right away.

I want to highlight one of the more subtle-yet-important clarifications made in these 2 chapters: associated functions vs methods, and how method calls are just syntactic sugar for function calls.

Unlike in many other languages, there is no formal distinction (e.g. via separate keywords) between methods vs constructors vs property getters. The first parameter as well as the return type determine if a given associated function is "actually" a constructor or a method (etc.).

Personally, I find this incredibly elegant; it's a form of "less is more" that gets out of my way when I'm coding while still allowing me to use all of the existing patterns that I know from elsewhere.

Also, on the topic of "having references as fields in structs" ... see this conversation we had here about that.

That conversation started from a post on the users . rust-lang forum, where the ultimate pithy and harsh conclusion about doing this was:

You’re not allowed to use references in structs until you think Rust is easy. They’re the evil-hardmode of Rust that will ruin your day.

😉

Use Box or Arc to store things in structs “by reference”. Temporary borrows don’t do what you think they do.