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Rust for Lemmings Reading Club Week 1 [Project]

Hi All! Welcome to the Reading Club for Rust's "The Book" ("The Rust Programming Language"). This is week 1 (the beginning!!).

Have a shot at going through "the reading" and post any thoughts, confusions or insights here

"The Reading"

The Twitch Stream

What's Next Week?

  • Chapters 3 and 4
  • Start thinking about challenges or puzzles to try as we go in order to get some applied practice!
    • EG, Advent of Code
    • Maybe some basic/toy web apps such as a "todo"
18 comments

  • Even though The Book is a bit verbose in these first few sections and really only touches on the basics of the language, I enjoyed going through it! Once you've gone to the end of just chapter 2, you've touched on project management with cargo, compiling with rustc or cargo, match statements, and some of the other syntactical details of the language. It's definitely enough to get you started if you're new to the language!

    For me, my outstanding questions, which come from the final exercise that builds a "guessing game" (code extracted below for easy reference):

    • Why are macros covered so much later in the book (ch 19)? ... not for mere mortals?
    • I don't think I really know at all what an object like Ordering actually is and what mechanically happened when it was used in the match statement.
    • Why did I import/use rand::Rng but then write rand.thread_rng().gen_range(). That I'm importing something not explicitly used in the code (Rng is never used) feels off to me. I can only guess that this was a shortcut to get to use a higher level interface. But what is Rng?
    • This will probably come up when we cover "Ownership" ... but it strikes me now that we were passing variables by reference (eg &secret_number and &secret_number. Given rust's concern with ownership and the "borrow checker" as a means of preventing memory safety issues ... why isn't it the default behaviour that a variable is passed by reference? Why do we have to explicitly pass the reference ourselves (with the ampersand syntax &secret_number)? 
     rust
        
use std::io;
use std::cmp::Ordering;
use rand::Rng;

fn main() {

    println!("Guess the number!");

    let secret_number = rand::thread_rng().gen_range(1..=100);
    // println!("Secret number is: {secret_number}");

    loop {

        println!("Please input your guess.");

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

        println!("you guessed: {guess}");

        match guess.cmp(&secret_number) {
            Ordering::Less => println!("Too small!"),
            Ordering::Greater => println!("Too big!"),
            Ordering::Equal => {
                println!("You win");
                break;
            }
        }
    }
}

    • This is just my attempt at answering (I'm learning too):

      Macros are easy to use, allowing beginners to write 'hello world' for example, but hide a bunch of complicated stuff we're unlikely to understand until later. (I vaguely remember from Java that just writing something to the console was a whole bunch statements strung together.)

      I found it useful to document what each line was doing in the example, to get my head around the terminology.
      std is a crate, io is a module in the std crate, and that module provides methods like stdin()
      std is a crate, cmp is a module in the std crate, and that module provides enums like Ordering
      rand is a crate, Rng is a trait from the rand crate, and that trait provides methods like thread_rng()

      Ordering is a enum - a type with a list of variants with developer-friendly names. In the same way that a 'DaysOfTheWeek' enum would have 'Monday', 'Tuesday' ..., etc, Ordering has Less, Greater, or Equal. match statements work with enums, in a 'for this, do that' kind of way.

      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. Since Rng provides the gen_range() method, it means that everything that implements it will provide it.

      thread_rng() returns a ThreadRng structure, which provides gen_range() via the Rng trait, but we need to bring that trait into scope with the use keyword in order to be able to call it.

      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.

      • 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?

    • The Enum Ordering provides compile-time safety. For example, if cmp() takes a string or int, the compiler can't catch invalid inputs ("less", "equal", -123, ...) at compile time and crash at runtime.

      • Hmm, not sure I’m entirely with you.

        If the argument to cmp is of an incorrect or incompatible type (where AFAIU the parent object and argument have to be the same type, eg u32), that alone will be surfaced at compile time no?

        If so, then Ordering is actually relatively trivial. It’s an enum, with variants for each possible outcome of a comparison on orderable variables (eg numbers).

        And the output of cmp is an Ordering type, which is nice for match statements, as, AFAIU, it forces us to address all possible scenarios (each being a variant of the Ordering enum).

        But the compile time safety will come from basic type checking on the argument to cmp.

        Am I off base here?

18 comments