What

As a challenge between contracts (I’m currently a consultant) I’ve decided to learn rust. I’ve been following “the book” and using clion as an IDE. Whilst learning the language I’ve been writing this blog post about things I’ve either found interesting or a little difficult.

Delights

Here are the things I really enjoyed from my first day of rust.

Nice error messages

The compiler’s errors are generally very helpful. For example if I miss out a possibility in a match expression:

error[E0004]: non-exhaustive patterns: `Equal` not covered
   --> src/main.rs:18:11
    |
18  |     match guess.cmp(&secret_number) {
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^ pattern `Equal` not covered

Immutable by default

I like being able to control where mutation happens so defaulting to immutable makes a lot of sense to me.

If I create a string and then try and mutate it like this

let s = String::from("hello");
s.push_str(" again");

then the compiler will raise the following (very helpful) error:

error[E0596]: cannot borrow `s` as mutable, as it is not declared as mutable
 --> src/main.rs:4:5
  |
2 |     let s = String::from("hello");
  |         - help: consider changing this to be mutable: `mut s`
3 | 
4 |     s.push_str(" again");
  |     ^ cannot borrow as mutable

because I’ve tried to mutate something that was immutable. It provides a solution if I want to make it mutable but it has to be done thoughtfully.

This also includes function arguments that are references so the following code will also error because I try and mutate a reference:

fn add_woop(s: &String) {
    s.push_str("woop")
}

instead, it would need to be:

fn add_woop(s: &mut String) {
    s.push_str("woop")
}

Result types combined with pattern matching

I’ve worked in the past with elixir/erlang so pattern matching was already something I was familiar with, I like how expressive this is:

let guess: u32 = match guess.trim().parse() {
    Ok(num) => num,
    Err(_) => continue,
 };

Surprises

Following on from the delights I also found a few things either chalenging or at least surprising on my first day.

Variable name shadowing

In other languages re-using a variable name is something I would usually avoid, but for rust variable shadowing appears early on in the tutorial.

let mut guess = String::new();

io::stdin()
    .read_line(&mut guess)
    .expect("Failed to read line");

let guess: u32 = guess.trim().parse().expect("Please type a number!");

My worry here is that I will accidentally change something rather than doing it on purpose.

A semicolon between an expression and a statement

// This returns x + 1
fn plus_one(x: i32) -> i32 {
    x + 1
}

// vs

// this "implicitly returns `()` as its body has no tail or `return` expression"
fn plus_one(x: i32) -> i32 {
    x + 1;
}

In the case of the later though the compiler does return a very helpful error message:

error[E0308]: mismatched types
 --> src/main.rs:7:24
  |
7 | fn plus_one(x: i32) -> i32 {
  |    --------            ^^^ expected `i32`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
8 |     x + 1;
  |          - help: consider removing this semicolon

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `functions`

To learn more, run the command again with --verbose.

Delightful surprises

I’m really enjoying the way rust deals with memory management. There’s something I want to try and write about ownership and borrowing. I’ve not thought about how to express it yet though, so I’ll save this for another post.