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% Rust Workshop

Rust Romandie meetup + Hackers at EPFL

Dimiter Petrov & Romain Ruetschi

November 12, 2014

Hello, Rust

Rust is a fairly new systems programming language focused on safety and speed.

fn main() {
    println!("Hello, world!");
}
> Hello, world!

History

  • Started as a personal project in 2006 by Graydon Hoare
  • First compiler written in OCaml
  • Mozilla took over the project 3 years later
  • Compiler rewritten in Rust itself in 2010

Why Rust at Mozilla?

  • New experimental, highly parallel browser engine: Servo
  • Browsers evolve in a hostile, highly concurrent environment
  • Most security problems arise from memory safetey and concurrency issues
  • Thus a safer, saner systems language is needed, with built-in concurrency

Why Rust, for you?

  • High-level features
  • Low-level control
  • Powerful Foreign Function Interface (FFI)
  • Direct memory control
  • Guaranteed memory safety
  • Concurrency without data races
  • Zero cost abstractions

High-level features

  • Type inference
  • Lambdas
  • Closures
  • Iterators
  • Algebraic data types
  • Pattern matching
  • Traits (= type classes)
  • Macros
  • Modules

Low-level control

  • Ability to trade compiler-enforced safety for lower-level control
  • Eg. dereferencing raw pointer, pointer arithmetic, calling external code
  • Such unsafe code must be surrounded by an unsafe block
  • Thus easy to spot during code review or when debugging

Powerful FFI

  • Call Rust code from other languages (C, C++, or even GCed languages like Ruby)
  • Create bindings for libraries exposing a C interface

Direct memory control

  • No garbage collector
  • No runtime overhead
  • No unpredictable pauses in execution
  • Stack and heap allocations

But first, the basics

  • Primitive types
  • Variable bindings
  • Functions
  • Printing stuff
  • Control structures
  • Strings
  • Arrays, vectors and slices
  • Structs and methods

Numeric types

Name Type Example
Signed integer int 5i
Unsigned integer uint 5u
8-bit uint u8 5u8
16-bit uint u16 5i
etc.
32-bit float f32 3.14f32
64-bit float f64 3.14_f64

Primitive types

Name Type Example
Unit () ()
Boolean bool true | false
Array [T] [1, 2, 3] fixed size, can be allocated on either the stack or the heap
Slice &[T] &[1, 2, 3] 'view' into an array, doesn't own the data it points to, but borrows it
Tuple (A, B, C...) ("Rust", 2006i, 'r') 32-bit unsigned word

Textual types

Name Type Example
Unicode scalar value char 'a' 32-bit unsigned word
Unicode scalar array [char] ['a', 'b', 'c'] ~ UTF-32 string
Unicode string str "rust is cool" array of 8-bit unsigned bytes
~ sequence of UTF-8 codepoints

Variable bindings

let x: int = 5;

// type annotations are (usually) optional
let x = 5i;

let mut y = 2i;
y += 1;

Mutability

let x = 5i;

x = x + 1; // error

Mutability

let mut x = 5i;

x = x + 1; // ok

Mutability

let x = 5i;
let mut y = x;

y = y + 1;

Functions

fn main() {
    let res = do_stuff(2, 3);
    println!("result is {}", res);
}

fn do_stuff(a: int, b: int) -> int {
    let c = a + b;

    return c * 2;
}

Functions

You can omit return if you want to return the last expression.

fn main() {
    let res = do_stuff(2, 3);
    println!("result is {}", res);
}

fn do_stuff(a: int, b: int) -> int {
    let c = a + b;

    c * 2 // ! no semicolon
}

But you also need to omit the last semicolon, otherwise the function will return ().

Printing stuff

fn main() {
    let name = "John";
    let age = 42;
    
    println!("{} is {}", name, age);
}

println!() is actually a macro, not a function.

Printing stuff

You can re-order arguments by putting their index inside the braces.

fn main() {
    let name = "John";
    let age = 42;
    
    println!("{1} is {0}", age, name);
}

Conditionals

if age > 16 {
    println!("Have a beer!");
}
else {
    println!("Sorry, no beer for you.");
}

No parenthesis around the condition.

Conditionals

if/else is an expression:

let message = if health > 0 {
    "Good job"
} else {
    "Game over"
};

println!("{}", message);

While

while x < 10 {
    // do stuff
}

Loop

Instead of

while true {
    // do stuff
}

use

loop {
    // do stuff
}

Rust's control-flow analysis treats this construct differently than a while true, which leads to better generated code.

For

for x in range(0i, 10i) {
    // do stuff
}

For

for var in expression {
    code
}

expression must be an Iterator (we'll talk about it later).

Strings

  • Guaranteed to be validly-encoded UTF-8 sequences
  • Not null-terminated (can contain null bytes)
  • Can be constant strings (&str) or growable strings (String)

String slices (&str)

A pointer and a length. String slices are a 'view' into already allocated strings like string literals:

let hello = "Hello, world!";

Growable strings (String)

String can grow if defined as mutable.

let mut hello = "Hello".to_string();

hello.push_str(", world!");

// hello now contains "Hello, world!"

Conversions between the two

  • Call as_slice() on a String to convert it to a &str
  • Call to_string() on a &str to convert it to a String
let s1: String = "Hello".to_string();
let s2: &str = s1.as_slice();

Converting String to &str is cheap, converting &str to String involves an allocation.

Arrays, vectors and slices

  • An array is a fixed-sized list of elements of the same type
  • A vector is a dynamic, growable array
  • A slice is a reference to an array.

Arrays

An array is a fixed-sized list of elements of the same type.

// Only the first item needs a type suffix
let a = [1i, 2, 3];

// which you can omit if you specify the type
let a: [int, ..3] = [1, 2, 3];

println!("a has {} elements", a.len());
println!("the first element of a is {}", a[0]);

Vectors

A vector is a dynamic, growable array

let mut participants = vec!["Bob", "Bill"];

participants.push("Joe");

// participants[2] == "Joe";

Slices

A slice is a reference to an array.

A slice allows safe and efficient access to a portion of an array without copying.

let a: [int, ..5] = [0, 1, 2, 3, 4];
let middle: &[int] = a.slice(1, 4); // just the elements [1, 2, 3]

let a = [0i, 1, 2, 3, 4];
let middle = a.slice(1, 4); // just the elements [1, 2, 3]

Structs and methods

Structs

struct Rectangle {
    width: f32,
    height: f32
}

Methods

Methods (static and members) are defined inside an impl block.

struct Rectangle {
    width: f32,
    height: f32
}

impl Rectangle {
    // Methods here.
}

Static methods

Static methods are defined this way, and can be called with Type::method_name().

struct Rectangle { width: f32, height: f32 }

impl Rectangle {
    fn new(width: f32, height: f32) -> Rectangle {
        Rectangle { width: width, height: height }
    }
}

fn main() {
    let rect = Rectangle::new(1.2, 4.9);
    println!("{}", react.area());
}

Member methods

Member methods take a &self parameter, which is a reference to the struct the method is called on.

struct Rectangle { width: f32, height: f32 }

impl Rectangle {
    fn area(&self) -> f32 {
        self.width * self.height
    }
}

fn main() {
    let rect = Rectangle::new(1.2, 4.9);
    let area = react.area();

    println!("{}", area);
}

Generics

struct PairOfInts { first: int, second: int }

fn swap_pair(p: PairOfInts) -> PairOfInts {
  PairOfInts { first: p.second, second: p.first }
}
struct Pair<T> { first: T, second: T }

fn swap_pair<T>(p: Pair<T>) -> Pair<T> {
  Pair { first: p.second, second: p.first }
}

fn main() {
  let p = Pair { first: 1i, second: 2i };
  let swapped = swap_pair(p);
}

Stack and heap allocations

By default, values are allocated on the stack.

let rect = Rectangle {
    width: 1.0,
    height: 1.0
};

rect is of type Rectangle

Stack and heap allocations

You can allocate a value on the heap with the box keyword.

Stack and heap allocations

{
    let x = box 5i;
    println!("{}", *x); // Prints 5
}
{
    int *x = (int *)malloc(sizeof(int));
    if (!x) abort();
    *x = 5;
    printf("%d\n", *x);
    free(x);
}

Boxes

let rect = box Rectangle {
    width: 1.0,
    height: 1.0
};

rect is now type Box<Rectangle>

Boxes

Although it's of type Box<Rectangle>, you can still call Rectangle methods on rect.

let rect = box Rectangle::new(1.0, 1.0);
let area = rect.area();

println!("area = {}", area);

Guaranteed memory safety

  • No buffer overflows
  • No dangling pointers

Concurrency without data races

  • Only immutable data can be shared
  • Shared data must remain live as long as any task might access it

Zero cost abstractions

Memory safety is enforced during compilation, there is no runtime overhead.

But how?

Rust is built around 3 concepts:

  • Lifetime
  • Ownership
  • Borrowing

Lifetime

Time span during which a resource (value) is valid.

Lifetime

  • You can think of it as a scope.
  • At the end of its lifetime, a value will be deallocated.

Lifetime

{
    int *x = malloc(sizeof(int));

    *x = 5;

    free(x);
}

Lifetime

{
    let x = box 5i;
    
    // x gets deallocated here
}

Move & Copy

When you pass data to a function (or even assign it to a variable), that data can be copied, moved, or borrowed (more about it soon).

Copy

fn main() {
    let a = 5;
    let b = add_one(a);
    println!("{}", a);
}

fn add_one(x: int) -> int {
    x + 1
}

Move

struct Person {
    name: String,
    age: uint
}

fn main() {
    let john = Person { name: "John".to_string(), age: 42 };
    show(john);
    
    // `john` has already been deallocated
}

fn show(person: Person) {
    println!("{} is {}", person.name, person.age);
    
    // `john` will be deallocated here
}

Move

struct Person {
    name: String,
    age: uint
}

fn main() {
    let john = Person { name: "John".to_string(), age: 42 };
    show(john);
    show(john);
}

fn show(person: Person) {
    println!("{} is {}", person.name, person.age);
    
    // `john` will be deallocated here
}

Move

error: use of moved value: `john`
     show(john);
          ^
note: `john` moved here because it has type `Person`, which is non-copyable
     show(john);
          ^
error: aborting due to previous error

Move

struct Person { name: String, age: uint }

fn main() {
    let john = Person { name: "John".to_string(), age: 42 };
    
    // `john` moves into show here
    show(john);
    // `john` is deallocated by now
    // so the next line doesn't compile
    show(john);
}

fn show(person: Person) {
    println!("{} is {}", person.name, person.age);
    
    // `john` will be deallocated here
}

Move (assignment)

Assigning a value to a variable is similar to passing it to a function.

struct Person { name: String, age: uint }

fn main() {
    let john = Person { name: "John".to_string(), age: 42 };
    
    // `john` moves into `john_bis` here
    let john_bis = john;
    
    // so the next line won't compile
    show(john);
}

fn show(person: Person) {
    println!("{} is {}", person.name, person.age);
    
    // `john` will be deallocated here
}

Ownership & Borrowing

When you create a resource, you're the owner of that resource.

Being an owner gives you some privileges:

  • You control when that resource is deallocated.
  • You may lend that resource, immutably, to as many borrowers as you'd like.
  • You may lend that resource, mutably, to a single borrower.

Ownership & Borrowing

But it also comes with some restrictions:

  • If someone is borrowing your resource (either mutably or immutably), you may not mutate the resource or mutably lend it to someone.
  • If someone is mutably borrowing your resource, you may not lend it out at all (mutably or immutably) or access it in any way.

Borrow

You may lend that resource, immutably, to as many borrowers as you'd like.

struct Person { name: String, age: uint }

fn main() {
    let john = Person { name: "John".to_string(), age: 42 };
    
    // `john` moves into `show`.
    show(&john);
    
    // `show` hands us `john` back.
    show(&john);
    // the previous line will thus compile.
}

fn show(person: &Person) {
    println!("{} is {}", person.name, person.age);
}

Borrow

John is 42
John is 42

Mutable borrow

You may lend that resource, mutably, to a single borrower.

struct Person { name: String, age: uint }

fn main() {
    let mut john = Person { name: "John".to_string(), age: 42 };
    
    grow_older(&mut john);
    
    show(&john); // John is 43
}

fn grow_older(person: &mut Person) {
    person.age += 1;
}

Mutable borrow

The following will compile too, as grow_older gives us john back:

fn main() {
    let mut john = Person { name: "John".to_string(), age: 42 };
    show(&john); // John is 42
    grow_older(&mut john);
    grow_older(&mut john);
    show(&john); // John is 44
}

Mutable borrow

But this won't:

fn main() {
    let mut john = Person { name: "John".to_string(), age: 42 };

    let mut john_bis = &mut john;

    grow_older(&mut john);
    
    show(&john);
}

Mutable borrow

error: cannot borrow `john` as mutable more than once at a time
    grow_older(&mut john);
                    ^
note: previous borrow of `john` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `p` until the borrow ends
    let mut john_bis = &mut john;
                         ^
note: previous borrow ends here
fn main() {
...
}
^
error: aborting due to previous error

Mutable borrow

fn main() {
    let mut john = Person { name: "John".to_string(),
                            age: 42 };

    // first borrow of `john` as mutable
    let mut john_bis = &mut john;

    // `john` cannot be mutably borrowed again, won't compile
    grow_older(&mut john);
    
    show(&john);
}

Lifetime annotations

Let's say we want to return a reference to a struct's member field.

struct Point { x: f64, y: f64 }

fn get_x(point: &Point) -> &f64 {
    &point.x
}

Lifetime annotations

Until a few weeks ago, get_x() had to be written this way:

fn get_x<'a>(point: &'a Point) -> &'a f64 {
    &point.x
}

But in that case, those annotations are now optional.

'a represents the lifetime of point. We hereby specify that the reference we return must have the same lifetime as point.

Lifetime annotations

This means that code such as

let mut p = Point { x: 2.0, y: 3.0 };
let x = get_x(&p);

p = Point { x: 4.0, y: 6.0 };

will not compile, because x outlives the value it is reference from.

Lifetime annotations

struct Line { start: &Point, end: &Point }

fn make_line(p1: &Point, p2: &Point) -> Line {
    Line { start: p1, end: p2 }
}

fn main() {
    let p1 = Point { x: 1.0, y: 2.3 };
    let p2 = Point { x: 2.0, y: 5.9 };

    let line = make_line(&p1, &p2);
}

error: missing lifetime specifier

Lifetime annotations

struct Line<'a> {
    start: &'a Point,
    end: &'a Point
}

fn make_line<'b>(p1: &'b Point, p2: &'b Point) -> Box<Line<'b>> {
    box Line { start: p1, end: p2 }
}

fn main() {
    let p1 = Point { x: 1.0, y: 2.3 };
    let p2 = Point { x: 2.0, y: 5.9 };

    let line = make_line(&p1, &p2);
}

Net result

  • You never have to malloc or free
  • You never have to retain or release
  • Rust will deallocate a value when the current owner is done with it
  • The compiler will guarantee that borrowed values are not stolen

Net result

  • In low-level languages, we allocate and free memory, close sockets, etc.
  • In high-level languages, we never free memory, but we routinely close sockets and files, and release locks.
  • In Rust we do neither :)

High-level features

Lambdas

    let xs = vec!(1i, 2i, 3i);
    let ys = xs.map(|x| x * 2);

    // ys == [2i, 4i, 6i]

Lambdas

fn add_one(x: int) -> int {
    x + 1
}

fn apply(x: int, adder: |int| -> int) -> int {
    adder(x)
}

Closures

    let factor = 8;
    let xs = vec!(1i, 2i, 3i);
    let ys = xs.map(|x| x * factor);
    
    // ys == [8i, 16i, 24i]

Traits

  • No inheritance in Rust :)
  • No interfaces either…
  • Instead Rust provides a mechanism called traits.

Traits

A trait is a sort of interface that defines some behavior. If a type implements a trait, that means that it supports and implements the behavior the trait describes.

Traits

You can think of them as (better) Java interfaces. They are in fact very similar to Haskell's typeclasses.

One of the main differences is that you can define a trait implementation separately from the struct definition, even in another module. This means that you can eg. implement a trait you defined yourself for a type provided by a library.

They're also much more powerful (but won't get into too much detail here).

Traits

struct Rectangle { width: f32, height: f32 }

trait HasArea {
    fn area(&self) -> f32;
}

impl HasArea for Rectangle {
    fn area(&self) -> f32 {
        self.width * self.height
    }
}

Traits

fn main() {
    let rect = Rectangle {
        width: 1.2,
        height: 4.9
    };
    println!("{}", react.area());
}

Traits

struct Circle { radius: f32 }

impl HasArea for Circle {
    fn area(&self) -> f32 {
         std::f32::consts::PI * (self.radius * self.radius)
    }
}

Traits

fn print_area<T>(shape: T) {
    println!("This shape has an area of {}", shape.area());
}

error: type T does not implement any method in scope named area

Traits

fn print_area<T: HasArea>(shape: T) {
    println!("This shape has an area of {}", shape.area());
}

Traits

fn print_area<T: HasArea>(shape: T) {
    println!("This shape has an area of {}", shape.area());
}

fn main() {
    let c = Circle { radius: 1.0 }
    let r = Rectangle { width: 3.0, height: 2.0 }

    print_area(c);
    print_area(r);
}

This shape has an area of 3.141592654
This shape has an area of 6.0

Traits

print_area(10i);

error: failed to find an implementation of trait main::HasArea for int

Traits

We can implement traits for any type. So this would work, even if it makes no sense:

impl HasArea for int {
    fn area(&self) -> f64 {
        println!("this is silly");
        *self as f64
    }
}

fn main() {
    10i.area();
    print_area(10i);
}

Implementing traits for primitive types should generally be avoided.

Traits

One restriction:

Either the trait or the type you're writing the impl for must be inside your crate (i.e. your library).

A few built-in traits

trait ToString {
    fn to_string(&self) -> String;
}

trait ToJson {
    fn to_json(&self) -> Json;
}

trait Equiv<T> {
    fn equiv(&self, other: &T) -> bool;
}

Enums

  • Also called sum types in the literature.
  • Similar to Haskell's data and Scala's case class.
enum Boolean {
    True,
    False
}

Rust's bool type is not implemented this way as it is a primitive.

Enums

enum Boolean {
    True,
    False
}

let b: Boolean = True;

A value of type Boolean can be either True or False.

Enum

From the standard library (almost):

enum Ordering { Less, Equal, Greater }

fn cmp(a: int, b: int) -> Ordering {
    if      a < b { Less }
    else if a > b { Greater }
    else          { Equal }
}

let ordering = cmp(x, y);

if ordering == Less {
    println!("less");
} else if ordering == Greater {
    println!("greater");
} else if ordering == Equal {
    println!("equal");
}

Pattern matching

Rust provides pattern matching, which lets you rewrite this:

if ordering == Less {
    println!("less");
} else if ordering == Greater {
    println!("greater");
} else if ordering == Equal {
    println!("equal");
}

Pattern matching

as this:

match ordering {
    Less    => println!("less"),
    Greater => println!("greater"),
    Equal   => println!("equal")
}

Pattern matching

It also works with primitives:

let i = 5i;

match i {
    0 => println!("zero"),
    1 => println!("one"),
    _ => println!("> 1")
}

Pattern matching

Patterns must be exhaustive:

match ordering {
    Less    => println!("less"),
    Greater => println!("greater")
}

error: non-exhaustive patterns: Equal not covered [E0004]

Pattern matching

There's a "catch-all" pattern: _.

match ordering {
    Less => println!("less"),
    _    => println!("not less") 
}

error: non-exhaustive patterns: Equal not covered [E0004]

Back to enums

Enums can also store data. One simple example is the built-in Option type. Here's how it is defined in the standard library:

enum Option<T> {
    None,
    Some(T),
}

The type Option<T> represents an optional value of type T.

An Option is either

  • Some, and contains a value of type T
  • None, and does not contain anything

Option

let opt: Option<int> = Some(5);

let opt_plus_two = opt + 2;

error: binary operation + cannot be applied to type core::option::Option<int>

Option

To make use of the value inside an Option, we must pattern-match on it:

let opt: Option<int> = Some(5);

let opt_plus_two = match opt {
    Some(x) => x + 2,
    None    => -1
}

This forces us to handle the case where there might be no value.

Option

Options have a number of uses in Rust:

  • Initial values
  • Return values for functions that are not defined over their entire input range (partial functions)
  • Return value for otherwise reporting simple errors, where None is returned on error
  • Optional struct fields
  • Optional function arguments
  • Nullable pointers

Option

Option also provides a few convenience methods:

  • fn is_some(&self) -> bool
  • fn is_none(&self) -> bool
  • fn unwrap_or(self, def: T) -> T
  • fn unwrap(self) -> T Use with caution

Option

A couple more:

  • fn map<U>(self, f: |T| -> U) -> Option<U>
  • fn and_then<U>(self, f: |T| -> Option<U>) -> Option<U>

Option

fn get_name() -> Option<String> { /* ... */ }

let name: Option<String> = get_name();

let display_name = name.map(|n| format!("My name is {}", n))
                       .unwrap_or("I don't have a name");

println!(display_name);

If get_name() returns Some("Marie"), this will print My name is Marie, and if it returns None, this will print I don't have a name.

Iterators

for x in range(0i, 10i) {
    // do stuff
}

This works because range(0, 10) returns an Iterator<int>.

Iterator<T> provides a next() function that we can call repeatedly to get a sequence of values, each wrapped in Some. When no more values are available, next() returns None.

Iterators

The for loop on the previous slide can be written like this:

let mut range = range(0i, 10i);

loop {
    match range.next() {
        Some(x) => {
            println!("{}", x);
        },
        None => { break }
    }
}

Iterators

Vectors can be iterated over too. Vec<T> provides an iter() method which returns an Iterator<&T> that we can use to iterate over the elements.

let nums = vec![1i, 2i, 3i];

for num in nums.iter() {
    println!("{}", num);
}

Iterators

Iterator provides methods such as map, filter, take, and friends.

Consumers

Iterators are not collections, they just allow to iterate over a (potentially infinite) sequence of elements.

It is possible to turn an Iterator into a collection with the collect method.

let a = [1i, 2, 3, 4, 5];
let iter = a.iter().map(|&x| x + 1);

let a_plus_one: Vec<int> = iter.collect();
// a_plus_one = [2i, 3, 4, 5, 6];

The type annotation is mandatory. let a_plus_one = iter.collect(); would throw an error.

Macros

You have already seen them. println!() is a macro. They're distinguishable by the ! at the end of the function name.

We won't get into too much detail here for lack of time.

Crates and modules

A "crate" in Rust is what you'd call a "package" or "library" in other languages. A crate contains modules (which can contain other modules).

If we have a greetings crate that contains a public module english that defines a public method hello(), we'd use it like this:

extern crate greetings;

fn main() {
  greetings::english::hello();
}

Modules

The use keyword imports names in the local scope.

extern crate greetings;
use greetings::english;

fn main() {
  english::hello();
}

Cargo

Cargo is Rust's package manager. It:

  • gets and compiles external dependencies
  • builds your projects
  • runs your tests

Use Cargo for all your projects.

Starting a project

$ cargo new hello_world --bin

The --bin flag indicates you're making a binary, not a library.

This generates a package description file, Cargo.toml, where you declare the project's dependencies and metadata.

[package]

name = "hello_world"
version = "0.0.1"
authors = ["Your Name <you@example.com>"]

Compiling your project

src/main.rs:

fn main() {
    println!("Hello, world!")
}

cargo build to build or cargo run to build and run your project

$ cargo run
   Compiling hello_world v0.0.1 (file:///path/to/hello_world)
     Running `target/hello_world`
Hello, world!

Thank you!

Questions?

References