xesite/blog/TLDR-rust-2020-09-19.markdown

title	date	tags
TL;DR Rust	2020-09-19	rust go golang

TL;DR Rust

Recently I've been starting to use Rust more and more for larger and larger projects. As things have come up, I realized that I am missing a good reference for common things in Rust as compared to Go. This post contains a quick high-level overview of patterns in Rust and how they compare to patterns in Go. This will focus on code samples. This is no replacement for the Rust book, but should help you get spun up on the various patterns used in Rust code.

Also I'm happy to introduce Mara to the blog!

Hey, happy to be here! I'm Mara, I'll interject with side information, challenge assertions and more! Thanks for inviting me!

Let's start somewhere simple: functions.

Making Functions

Functions are defined using fn instead of func:

// go

func foo() {}

// rust

fn foo() {}

Arguments

Arguments can be passed by separating the name from the type with a colon:

// go

func foo(bar int) {}

// rust

fn foo(bar: i32) {}

Returns

Values can be returned by adding -> Type to the function declaration:

// go

func foo() int {
  return 2
}

// rust

fn foo() -> i32 {
  return 2;
}

In Rust values can also be returned on the last statement without the return keyword or a terminating semicolon:

// rust

fn foo() -> i32 {
  2
}

Functions that can fail

The Result type represents things that can fail with specific errors. The eyre Result type represents things that can fail with any error. For readability, this post will use the eyre Result type.

The angle brackets in the Result type are arguments to the type, this allows the Result type to work across any type you could imagine.

// go

import "errors"

func divide(x, y int) (int, err) {
  if y == 0 {
    return 0, errors.New("cannot divide by zero")
  }
  
  return x / y, nil
}

// rust

use eyre::{eyre, Result};

fn divide(x: i32, y: i32) -> Result<i32> {
  match y {
    0 => Err(eyre!("cannot divide by zero")),
    _ => Ok(x / y),
  }
}

Huh? I thought Rust had the Error trait, shouldn't you be able to use that instead of a third party package like eyre?

Let's try that, however we will need to make our own error type because the eyre! macro creates its own transient error type on the fly.

First we need to make our own simple error type for a DivideByZero error:

// rust

use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct DivideByZero;

impl fmt::Display for DivideByZero {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "cannot divide by zero")
    }
}

impl Error for DivideByZero {}

So now let's use it:

// rust

fn divide(x: i32, y: i32) -> Result<i32, DivideByZero> {
  match y {
    0 => Err(DivideByZero{}),
    _ => Ok(x / y),
  }
}

However there is still one thing left: the function returns a DivideByZero error, not any error like the error interface in Go. In order to represent that we need to return something that implements the Error trait:

// rust

fn divide(x: i32, y: i32) -> Result<i32, impl Error> {
    // ...
}

And for the simple case, this will work. However as things get more complicated this simple facade will not work due to reality and its complexities. This is why I am shipping as much as I can out to other packages like eyre or anyhow. Check out this code in the Rust Playground to mess with this code interactively.

Pro tip: eyre (via color-eyre) also has support for adding custom sections and context to errors similar to Go's fmt.Errorf %w format argument, which will help in real world applications. When you do need to actually make your own errors, you may want to look into crates like thiserror to help with automatically generating your error implementation.

The ? Operator

In Rust, the ? operator checks for an error in a function call and if there is one, it automatically returns the error and gives you the result of the function if there was no error. This only works in functions that return a Result.

// go

func doThing() (int, error) {
  result, err := divide(3, 4)
  if err != nil {
    return 0, err
  }
  
  return result, nil
}

// rust

use eyre::Result;

fn do_thing() -> Result<i32> {
  let result = divide(3, 4)?;
  Ok(result)
}

If the second argument of divide is changed to 0, then do_thing will return an error.

And how does that work with eyre?

It works with eyre because eyre has its own error wrapper type called Report, which can represent anything that implements the Error trait.

Macros

Rust macros are function calls with ! after their name:

// rust

println!("hello, world");

Variables

Variables are created using let:

// go
var foo int
var foo = 3
foo := 3

// rust
let foo: i32;
let foo = 3;

Mutability

In Rust, every variable is immutable (unchangeable) by default. To create a mutable variable, add the mut keyword after the let keyword. There is no analog to this in Go.

let mut i: i32 = 0;
i = i + 5;

This is slightly a lie. There's more advanced cases involving interior mutability and other fun stuff like that, however this is a more advanced topic that isn't covered here.

Lifetimes

Rust does garbage collection at compile time. It also passes ownership of memory to functions as soon as possible. For example:

let quo = divide(4, 8)?;
let other_quo = divide(quo, 5)?;

// Fails compile because quo was given to divide to create other_quo
let yet_another_quo = divide(quo, 4)?;

To work around this you need to either clone the value or pass a reference:

let other_quo = divide(quo.clone(), 5);
let yet_another_quo = divide(quo, 4)?;

To pass a reference to a function, use the & character:

let something = do_something(&quo)?;

Passing Mutability

Sometimes functions need mutable variables. To pass a mutable reference, add &mut before the name of the variable:

let something = do_something_to_quo(&mut quo)?;

Project Setup

Imports

External dependencies are declared using the Cargo.toml file:

[dependencies]
anyhow = "1.0"

This depends on the crate anyhow at version 1.0.x.

Dependencies can also have optional features:

[dependencies]
reqwest = { version = "0.10", features = ["json"] }

This depends on the crate reqwest at version 0.10.x with the json feature enabled (in this case it enables reqwest being able to automagically convert things to/from json using Serde).

External dependencies can be used with the use statement:

import "github.com/foo/bar"

use foo; //      -> foo now has the members of crate foo behind the :: operator
use foo::Bar; // -> Bar is now exposed as a type in this file

use anyhow::{anyhow, Result}; // exposes the anyhow! and Result members of anyhow

Async/Await

Async functions may be interrupted to let other things execute as needed. This program uses tokio to handle async tasks. To run an async task and wait for its result, do this:

let response = reqwest::get("https://printerfacts.cetacean.club/fact")
  .await?
  .text()
  .await?;

This will populate response with the HTML source of https://within.website.

To make an async function, add the async keyword before the fn keyword:

async fn get_html(url: String) -> Result<String> {
  reqwest::get(&url)
    .await?
    .text()
    .await?
}

This can then be called like this:

let within_website_html = get_html("https://within.website").await?;

Public/Private Types and Functions

Rust has three privacy levels for functions:

• Only visible to the current file (no keyword, lowercase in Go)
• Visible to anything in the current crate (pub(crate), internal packages in go)
• Visible to everyone (pub, upper case in Go)

This project will mostly use pub(crate) as none of this code is intended to be consumed by other programs (though this may change in the future).

Structures

Rust structures are created using the struct keyword:

type Client struct {
  Token string
}

pub(crate) struct Client {
  pub token: String,
}

If the pub keyword is not specified before a member name, it will not be usable outside the Rust source code file it is defined in:

type Client struct {
  token string
}

pub(crate) struct Client {
  token: String,
}

Encoding structs to JSON

serde is used to convert structures to json. The Rust compiler's derive feature is used to automatically implement the conversion logic.

type Response struct {
  Name        string  `json:"name"`
  Description *string `json:"description,omitempty"`
}

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
pub(crate) struct Response {
  pub name: String,
  pub description: Option<String>, // Option means that there can either be something or nothing there
}

Strings

Rust has a few string types that do different things. You can read more about this here, but at a high level this project only uses two of them:

• String, an owned UTF-8 string
• PathBuf, a filepath string (encoded in whatever encoding the OS running this code uses for filesystems)

The strings are different types for safety reasons. See the linked blogpost for more detail about this.

Enumerations / Tagged Unions

Enumerations, also known as tagged unions, are a way to specify a superposition of one of a few different kinds of values in one type. The main place they are used in this project is for command line parsing with structopt. There is no easy analog for this in Go.

#[derive(StructOpt, Debug)]
#[structopt(about = "A simple release management tool")]
pub(crate) enum Cmd {
    /// Creates a new release for a git repo
    Cut {
        #[structopt(flatten)]
        common: Common,
        /// Changelog location
        #[structopt(long, short, default_value="./CHANGELOG.md")]
        changelog: PathBuf,
    },

    /// Runs releases as triggered by GitHub Actions
    GitHubAction {
        #[structopt(flatten)]
        gha: GitHubAction,
    },
}

Enum variants can be matched using the match keyword:

match cmd {
    Cmd::Cut { common, changelog } => {
        cmd::cut::run(common, changelog).await
    }
    Cmd::GitHubAction { gha } => {
        cmd::github_action::run(gha).await
    }
}

All variants of an enum must be matched in order for the code to compile.

Testing

Test functions need to be marked with the #[test] annotation, then they will be run alongside cargo test:

mod tests { // not required but it is good practice
  #[test]
  fn math_works() {
    assert_eq!(2 + 2, 4);
  }
  
  #[test]
  async fn http_works() {
    let _ = get_html("https://within.website").await.unwrap();
  }
}

Avoid the use of unwrap() outside of tests. In the wrong cases, using unwrap() in production code can cause the server to crash and can incur data loss.

---

This is by no means comprehensive, see the rust book or Learn X in Y Minutes Where y = Rust for more information. This code is written to be as boring and obvious as possible. If things don't make sense, please reach out and don't be afraid to ask questions.