Iterators and Errors - Rust Crash Course lesson 3

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Below are the solutions to the exercises from the last Rust Crash Course lesson, "Iterators and Errors."

This post is part of a series based on teaching Rust at FP Complete. If you're reading this post outside of the blog, you can find links to all posts in the series at the top of the introduction post. You can also subscribe to the RSS feed.

Exercise 1

The trick here is to "wrap" the Args data type with the Skip data type:

use std::env::{args, Args};
use std::iter::Skip;

fn main() {
    let args: Skip<Args> = args().skip(1);
    for arg in args {
        println!("{}", arg);
    }
}

You may have noticed that we didn't need to mark args as mutable. That's because we are moving the args value into the for loop, meaning that any mutation to it by the for loop cannot be seen in the main function.

Exercise 2

When we call parse in the context of that example, type inference tells us that the width and height results must be u32s, since they are used as fields in Frame. Rust is able to determine what implementation to use based on the needed return type. Cool!

Yet again, Haskellers are rolling their eyes and saying "old news."

Exercise 3

Complete source file:

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

#[derive(Debug)]
enum ParseError {
    TooFewArgs,
    TooManyArgs,
    InvalidInteger(String),
}

struct ParseArgs(std::env::Args);

impl ParseArgs {
    fn new() -> ParseArgs {
        ParseArgs(std::env::args())
    }


    fn require_arg(&mut self) -> Result<String, ParseError> {
        match self.0.next() {
            None => Err(ParseError::TooFewArgs),
            Some(s) => Ok(s),
        }
    }

    fn require_no_args(&mut self) -> Result<(), ParseError> {
        match self.0.next() {
            Some(_) => Err(ParseError::TooManyArgs),
            None => Ok(()),
        }
    }
}

fn parse_u32(s: String) -> Result<u32, ParseError> {
    match s.parse() {
        Err(_) => Err(ParseError::InvalidInteger(s)),
        Ok(x) => Ok(x),
    }
}

fn parse_args() -> Result<Frame, ParseError> {
    let mut args = ParseArgs::new();

    // skip the command name
    args.require_arg()?;

    let width_str = args.require_arg()?;
    let height_str = args.require_arg()?;
    args.require_no_args()?;
    let width = parse_u32(width_str)?;
    let height = parse_u32(height_str)?;

    Ok(Frame { width, height })
}

fn main() {
    println!("{:?}", parse_args());
}

Exercise 4

We want to ensure a minimum size for the width and height. First, let's add two more variants to the ParseError enum:

WidthTooSmall(u32),
HeightTooSmall(u32),

Then add the following to the parse_args function, just before the Ok:

if width < 2 {
    return Err(WidthTooSmall(width));
}
if height < 2 {
    return Err(HeightTooSmall(height));
}

Exercise 5

This is another perfect time to pull out our trailing question mark!

fn main () -> Result<(), self::parse_args::ParseError> {
    let frame = parse_args::parse_args()?;
    let mut game = Game::new(frame);
    let sleep_duration = std::time::Duration::from_millis(33);
    loop {
        println!("{}", game);
        game.step();
        std::thread::sleep(sleep_duration);
    }
}

Rust at FP Complete | Introduction