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

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

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, a shark hacker from Christine's imagination. 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:

func foo() {}

fn foo() {}

Arguments

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

func foo(bar int) {}

fn foo(bar: i32) {}

Returns

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

func foo() int {
  return 2
}

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

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

fn foo() -> i32 {
  2
}

Hmm, what if I try to do something like this. Will this work?

fn foo() -> i32 {
    if some_cond {
        2
    }
    
    4
}

Let's find out! The compiler spits back an error:

error[E0308]: mismatched types
 --> src/lib.rs:3:9
  |
2 | /     if some_cond {
3 | |         2
  | |         ^ expected `()`, found integer
4 | |     }
  | |     -- help: consider using a semicolon here
  | |_____|
  |       expected this to be `()`

This happens because most basic statements in Rust can return values. The best way to fix this would be to move the 4 return into an else block:

fn foo() -> i32 {
    if some_cond {
        2
    } else {
        4
    }
}

Otherwise, the compiler will think you are trying to use that if as a statement, such as like this:

let val = if some_cond { 2 } else { 4 };

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.

import "errors"

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

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:

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:

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:

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 either an Option or a Result.

The Option type isn't shown in very much detail here, but it acts like a "this thing might not exist and it's your responsibility to check" container for any value. The closest analogue in Go is making a pointer to a value or possibly putting a value in an interface{} (which can be annoying to deal with in practice).

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

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:

println!("hello, world");

Variables

Variables are created using let:

var foo int
var foo = 3
foo := 3

let foo: i32;
let foo = 3;

Mutability

In Rust, every variable is immutable (unchangeable) by default. If we try to change those variables above we get a compiler error:

fn main() {
    let foo: i32;
    let foo = 3;
    foo = 4;
}

This makes the compiler return this error:

error[E0384]: cannot assign twice to immutable variable `foo`
 --> src/main.rs:4:5
  |
3 |     let foo = 3;
  |         ---
  |         |
  |         first assignment to `foo`
  |         help: make this binding mutable: `mut foo`
4 |     foo = 4;
  |     ^^^^^^^ cannot assign twice to immutable variable

As the compiler suggests, you can create a mutable variable by adding the mut keyword after the let keyword. There is no analog to this in Go.

let mut foo: i32 = 0;
foo = 4;

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. Lifetimes are how Rust calculates how "long" a given bit of data should exist in the program. Rust will then tell the compiled code to destroy the data from memory as soon as possible.

This is slightly inaccurate in order to make this simpler to explain and understand. It's probably more accurate to say that Rust calculates when to collect garbage at compile time, but the difference doesn't really matter for most cases

For example, this code will fail to compile because quo was moved into the second divide call:

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

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

To work around this you can pass a reference to the divide function:

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

Or even create a clone of it:

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

You can also get more fancy with explicit lifetime annotations, however as of Rust's 2018 edition they aren't usually required unless you are doing something weird. This is something that is also covered in more detail in The Rust Book.

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:

# Cargo.toml

[dependencies]
eyre = "0.6"

This depends on the crate eyre at version 0.6.x.

You can do much more with version requirements with cargo, see more here.

Dependencies can also have optional features:

# Cargo.toml

[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:

// go

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

This doesn't cover how the module system works, however the post I linked there covers this better than I can.

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 printer_fact = reqwest::get("https://printerfacts.cetacean.club/fact")
  .await?
  .text()
  .await?;
println!("your printer fact is: {}", printer_fact);

This will populate response with an amusing fact about everyone's favorite household pet, the printer.

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

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

This can then be called like this:

let printer_fact = get_text("https://printerfacts.cetacean.club/fact").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)

You can't get a perfect analog to pub(crate) in Go, but internal packages can get close to this behavior. Additionally you can have a lot more control over access levels than this, see here for more information.

Structures

Rust structures are created using the struct keyword:

type Client struct {
  Token string
}

pub 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>,
}

Strings

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

• &str, a slice reference to a String owned by someone else
• 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. A neat way to show them off (along with some other fancy features like the derivation system) is with the structopt crate. There is no easy analog for this in Go.

We've actually been dealing with enumerations ever since we touched the Result type earlier. Result and Option are implemented with enumerations.

#[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.

This code was borrowed from palisade in order to demonstrate this better. If you want to see these patterns in action, check this repository out!

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);
  }
  
  #[tokio::test] // needs tokio as a dependency
  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.

Alternatively, you can also use the .expect() method instead of .unwrap(). This lets you attach a message that will be shown when the result isn't Ok.

---

This is by no means comprehensive, see the rust book or Learn X in Y Minutes Where X = 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.