How I Learned?

Rust Workshop

• Rust Workshop Exercises: These are the exercises we do during the workshop.

• Slides Source: The slides of this presentation.

Rust References

• Official Learning Reference: Entry to all your needs.

• Rust Standard Library: The Rust standard library to search for common functionality.

• Rust Book: The extensive Rust book explaining topics in more (stm. too much) detail.

• Rust By Example: A summary about different Rust topics by small examples. This is good to get a first grasp of it.

• Rustling: More exercises. We also use some of these in our exercises.

• Library Best-Practices: For every use case there is a good library out there.

What the Heck is Rust 🦀

A Multi-Paradigm Language

• procedural like Python, i.e. functions 󰊕, loops 󰑙, …
• functional aspects, i.e. iterators 🏃, lambdas 󰡱 …
• object-oriented aspects but unlike Python ( its better ❤️)

What the Heck is Rust 🦀

A Compiled Language Unlike Python

• The Rust compiler rustc 🦀 will convert your code to machine-code ⚙️.
  Python is an interpreter.

• It has a strong type system (algebraic types: sum types, product types).

• It was invented in 2009 by Mozilla (Firefox) - Rust Foundation as the driver today.

Note: 10% of Firefox is in Rust for good reasons you will realize in the following.

Benefits You Get on the 🦀 Journey

A few selling points for python programmers.

Come on 🐨 show me syntax!

The syntax* is similar and as easy to read as in Python.

struct Apple {
  name: String
}

fn grow() -> Vec<Apple> {

  let apples = vec![Apple{"a"},
                    Apple{"b"}];

  for b in apples {
    println!("Apple: {b:?}");
  }

  return apples
}

@dataclass
class Apple:
  name: str

def grow() -> List[Apple]:

  apples = [Apple("a"),
            Apple("b")]

  for b in apples:
    print(f"Apple: {b.name}")


  return apples

*: 80% you will encounter is very readable (except macros etc.).

But Why?

More reasons why you should learn a compiled, statically typed language…

What Rust Promises 🤚

Pedal to The Metal

Rust Comes with a Warranty

Rust Comes with a Warranty

Rust Comes with a Warranty

Experience: “♥️ If it compiles, it is more often correct. ♥️”

Rust Comes with a Warranty

Experience: “♥️ If it compiles, it is more often correct. ♥️”

• No invalid syntax.
• Guaranteed thread safety.
• Model your business logic with struct and enums.

Performance

Why Should 🫵 Learn Rust?

Create a Project

cargo new hello-world

Cargo is the build tool for Rust. It includes a package manager, commands for initiating libraries/CLI etc.

cd hello-world
cargo run

Compiling hello-world v0.1.0
Finished dev [unoptimized + debuginfo] target(s) in 0.74s
Running `target/debug/hello-world`
Hello, world!

Computing a Simple Sum

fn main() {
    println!("sum(4) = 4 + 3 + 2 + 1 = {}", sum(4));
}

fn sum(n: u64) -> u64 {
    if n != 0 {
        n + sum(n-1)
    } else {
        n
    }
}
// Note: avoid recursion as you always can :)

Output:

sum(4) = 4 + 3 + 2 + 1 = 10

Variables (1)

fn main() {
    let some_x = 5;
    println!("some_x = {}", some_x);
    some_x = 6;
    println!("some_x = {some_x}");
}

Compiling hello-world v0.1.0
error[E0384]: cannot assign twice to immutable variable `some_x`
--> src/main.rs:4:5
2 |     let some_x = 5;
  |         ------
  |         |
  |         first assignment to `some_x`
  |         help: consider making this binding mutable: `mut some_x`
3 |     println!("some_x = {}", some_x);
4 |     some_x = 6;
  |     ^^^^^^^^^^ cannot assign twice to immutable variable

• Rust uses snake case (e.g. some_x) for variable names.
• The immutable (read-only) variable cannot be mutated in any way.

Variables (2)

fn main() {
    let mut some_x = 5;
    println!("some_x = {}", some_x);
    some_x = 6;
    println!("some_x = {}", some_x);
}

Compiling hello-world v0.1.0 (/home/teach-rs/Projects/hello-world)
Finished dev [unoptimized + debuginfo] target(s) in 0.26s
Running `target/debug/hello-world`
some_x = 5
some_x = 6

• Declare a mutable variable with mut to update

Declaring a Type of Variable

fn main() {
    let x: i32 = 20;
    //   ^^^^^---------- Type annotation. (as in python)
}

• Rust is strongly and strictly typed.
• Variables use type inference, so no need to specify a type (Henly-Millner Type System), (Understanding It).
• We can be explicit in our types (and sometimes have to be).

Primitives: Integers

let x = 42;
let y = 42u64; // decimal as u64
let z = 42_000; // underscore separator

let u = 0xff; // hexadecimal
let v = 0o77; // octal
let q = b'A'; // byte syntax (is u8)
let w = 0b0100_1101; // binary

• Rust prefers explicit integer sizes.
• Use isize and usize sparingly.

Primitives: Floating Points Numbers

fn main() {
    let x = 2.0;    // f64
    let y = 1.0f32; // f32
}

• f32: single precision (32-bit) floating point number.
• f64: double precision (64-bit) floating point number.
• f128: 128-bit floating point number.

Numerical Operations

fn main() {
    let sum = 5 + 10;
    let difference = 10 - 3;
    let mult = 2 * 8;
    let div = 2.4 / 3.5;
    let int_div = 10 / 3; // 3
    let remainder = 20 % 3;
}

Numerical Operations

• You cannot mix and match types, i.e.:

fn main() {
    let invalid_div = 2.4 / 5;          // Error!
    let invalid_add = 20u32 + 40u64;    // Error!
}

• Rust has your typical operations, just as with other python languages.

Primitives: Booleans and Operations

fn main() {
    let yes: bool = true;
    let no: bool = false;
    let not = !no;
    let and = yes && no;
    let or = yes || no;
    let xor = yes ^ no;
}

Comparison Operators

fn main() {
    let x = 10;
    let y = 20;
    x < y;  // true
    x > y;  // false
    x <= y; // true
    x >= y; // false
    x == y; // false
    x != y; // true
}

fn main() {
    3.0 < 20;      // invalid
    30u64 > 20i32; // invalid
}

Primitives: Characters

fn main() {
    let c: char = 'z'; // Note: the single-quotes ''.
    let z = 'ℤ';
    let heart_eyed_cat = '😻';
}

• A char is a 32-bit unicode scalar value (like in python).

Strings

    let s1 = String::from("Hello, 🌍!");
    //       ^^^^^^ Owned, heap-allocated string

Primitives: Tuples

fn main() {
  let tup: (i32, f32, char) = (1, 2.0, 'a');
}

fn main() {
  let tup = (1, 2.0, 'Z');
  let (a, b, c) = tup;
  println!("({}, {}, {})", a, b, c);

  let another_tuple = (true, 42);
  println!("{}", another_tuple.1);
}

Primitives: Arrays

fn main() {
    let arr: [i32; 3] = [1, 2, 3];
    println!("{}", arr[0]);

    let [a, b, c] = arr;
    println!("[{}, {}, {}]", a, b, c);
}

Control Flow

fn main() {
    let mut x = 0;
    loop {
        if x < 5 {
            println!("x: {}", x);
            x += 1;
        } else {
            break;
        }
    }

    let mut y = 5;
    while y > 0 {
        y -= 1;
        println!("y: {}", x);
    }

    for i in [1, 2, 3, 4, 5] {
        println!("i: {}", i);
    }
}

Functions

fn add(a: i32, b: i32) -> i32 {
    a + b // or: `return a+b`
}

fn returns_nothing() -> () {
    println!("Nothing to report");
}

fn also_returns_nothing() {
    println!("Nothing to report");
}

Statements

• Statements are instructions that perform some action and do not return a value.
• A definition of any kind (function definition etc.)
• The let var = expr; statement.
• Almost everything else is an expression.

Examples

fn my_fun() {
    println!("{}", 5);
}

let x = 10;

return 42;

let x = (let y = 10); // invalid

Expressions

fn main() {
    let y = {
        let x = 3;
        x + 1
    };
    println!("{}", y); // 4
}

Scope

• We just mentioned the scope braces ({ and }).
• Variable scopes are actually very important for how Rust works.

fn main() {
    println!("Hello, {}", name);  // invalid: name is not yet defined

    {
        let name = "world";  // from this point name is in scope
        println!("Hello, {}", name);
    } // name goes out of scope

    println!("Hello, {}", name);  // invalid: name is no more defined
}

Expressions - Control Flow

• Remember: A block/function can end with an expression, but it needs to have the correct type

• Control flow expressions as a statement do not
  need to end with a semicolon if they return unit (()).

fn main() {
    let y = 11;
    // if as an expression
    let x = if y < 10 {
        42    // missing ;
    } else {
        24    // missing ;
    };

    // if (control-flow expr.) as a statement
    if x == 42 {
        println!("Foo");
    } else {
        println!("Bar");
    } // no ; necessary
}

Expression - Control Flow

Quiz: Does this compile?

fn main() {
    if 2 < 10 {
        42
    } else {
        24
    }
}

fn main() {
    if 2 < 10 {
        42
    } else {
        24
    };
}

Expression - Control Flow

Quiz: Does this compile?

fn main() {
    let a = if if 1 != 2 { 3 } else { 4 } == 4 {
        2
    } else {
        1
    };

    println!("{}", a)
}

Scope (more)

When a scope ends, all variables for that scope become “extinct” (deallocated/removed from the stack).

fn main() { // nothing in scope here
  let i = 10; // i is now in scope

  if i > 5 {
      let j = 20; // j is now in scope
      println!("i = {}, j = {}", i, j);
  } // j is no longer in scope

  println!("i = {}", i);
} // i is no longer in scope

def main():
  i = 10;

  if i > 5:
      j = 20
      print(f"i = {j}, j = {i}")


  print(i, j) # 💩: j is STILL in scope

Note: This is very different from python.

Printing & Formatting Values

With the format! or println! macros (later) you can format or print variables to stdout of your application:

fn main() {
  let x = 130;
  let y = 50;

  println!("{} + {}", x, y);
  println!("{x} + {y}");

  let s: String = format!("{x:04} + {0:>10}", y);
  println!("{s}")
}

Output:

130 + 50
130 + 50
0130 +         50

Exercise Time (1)

Approx. Time: 20-45 min.

Do the following exercises:

• basic-syntax: all, 09 (optional: macros, read here: https://doc.rust-lang.org/book/ch19-06-macros.html)

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Memory

• A computer program consists of a set of instructions.
• Those instructions manipulate some memory.
• How does a program know what memory can be used?

Program Execution

An executable binary (a file on your disk) will be loaded first into the memory.

• The machine instructions are loaded into the memory.

• The static data in the binary (i.e. strings etc) is also loaded into memory.

• The CPU starts fetching the instructions from the RAM and will start to execute the machine instructions.

• Two memory mechanisms are at play when executing: the stack and the heap

Further Technical Videos: A program is not a process, Why is the heap so slow, Why is the stack so fast.

Stack

Continuous areas of memory for local variables in functions.

• It is fixed in size.

• Stack grows and shrinks, it follows function calls. Each function has its own stack frame for all its local variables.

• Values must have fixed sizes known at compile time. (If the compiler doesn’t know it cannot compute the stack frame size)

• Access is extremely fast: offset the stack pointer.

• Great memory locality  CPU caches.

Stack - Example

fn foo() { // Enter 2. stack frame.
    let a: u32 = 10; // `a` points on the stack containing 10.
    println!("a address: {:p}", &a);

    let b: u32 = a;  // Copy `a` to `b`.
    println!("b address: {:p}", &b);
} // `a,b` out of scope, we leave the stack frame.

fn main() { // Enter 1. stack frame.
  foo()
}

Stack frame for foo needs at least \(2 \cdot 32\) bits = \(2 \cdot 4\) bytes = \(8\) bytes.

a address: 0x7ffdb6f09c08
b address: 0x7ffdb6f09c0c  // 08 + 4bytes = 0c

The Heap (1)

The heap is just one big pile of memory for dynamic memory allocation.

The Heap (2)

The memory management on the heap depends on the language you write.

Mechanics 🦀

• Full Control and Safety: Rust - Via compile time enforcement of correct memory management.

  • It does this with an explicit ownership concept.
  • It tracks life times (of references).

Variable Scoping (recap)

fn main() {
    let i = 10; // `i` in scope.

    if i > 5 {
        let j = i;
    }  // `j` no longer in scope.

    println!("i = {}", i);
} // i is no longer in scope

• Types of i and j are examples of a Copy types.
• What if copying is too expensive?

Ownership (1)

// Create a variable on the stack.
let a = 5;

// Create an owned, heap allocated string
let a = String::from("hello");

Ownership (2)

fn foo() {
  let a = 5;
  let b = a;
}

fn foo() {
  let a = String::from("hello");
  let b = a;
}

Ownership (3)

fn foo() {
  let a = String::from("hello");
  let b = a;
  println!("{}, world!", a);
  //                     ^--- Nope!! ❌
}

error[E0382]: borrow of moved value: `a`
--> src/main.rs:4:28
  |
2 |     let a = String::from("hello");
  |         - move occurs because `a`
  |           has type `String`, which
  |           does not implement the `Copy`
  |           trait
  |
3 |     let b = a;
  |             - value moved here
4 |     println!("{}, world!", a);
  |                            ^
  |            value borrowed here
  |            after move

Ownership - The Rules

Ownership - Move into Function

fn main() {
  let a = String::from("hello");

  let len = calculate_length(a);
  println!("Length of '{}' is {}.",
           a, len);
}

fn calculate_length(s: String) -> usize {
  s.len()
}

error[E0382]: borrow of moved value: `a`
--> src/main.rs:4:43
  |
2 | let a = String::from("hello");
  |     - move occurs because `a`
  |       has type `String`,
  |       which does not implement the
  |       `Copy` trait
  |
3 | let len = calculate_length(a);
  |           value moved here -
  |
4 | println!("Length of '{}' is {}.",
  |          a, len);
  |          ^
  |          value borrowed here after move

Ownership - Moving Out of Function

We can return a value to move it out of the function

fn main() {
    let a = String::from("hello");
    let (len, a) = calculate_length(a);

    println!("Length of '{}' is {}.", a, len);
}

fn calculate_length(s: String) -> (usize, String) {
    (s.len(), s)
}

Compiling playground v0.0.1
Finished dev ...
Running `target/debug/playground`

Length of 'hello' is 5.

Clone

fn main() {
    let x = String::from("hellothisisaverylongstring...");
    let len = get_length(x.clone());
    println!("{}: {}", x, len);
}

fn get_length(arg: String) -> usize {
    arg.len()
}

Clone Explicitness vs. Python

In contrast to Rust, python hides when stuff gets copied or referenced:

# Python
a = 1
b = a
b += 1
print(a) # `a` is unchanged
         #  -> `int` is a copy-type.

# Python
a = {'a': 1}
b = a    # `b` is a reference to `a`.
b['a'] += 1
print(a) # `a` is 2
         #  -> dict is reference-counted.

Exercise Time (2)

Approx. Time: 20-30 min.

Do the following exercises:

• move-semantics: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Ownership

We previously talked about ownership:

Moving Out of a Function

We have previously seen this example:

fn main() {
    let a = String::from("hello");
    let len = calculate_length(a);

    println!("Length of '{}' is {}.", a, len);
}

fn calculate_length(s: String) -> usize {
    s.len()
}

• Does not compile ⇒ ownership of a is moved into calculate_length ⇒ no longer available in main.
• We can use Clone to create an explicit copy.
• We can give ownership back by returning the value.

Question: Are there other options?

Moving Out of a Function (🐍)

In Python we have this:

def main() {
    a = "hello";
    l = calculate_length(a);

    print(f"Length of '{a}' is {l}.");
}

def calculate_length(s: str) -> int {
    return len(s)
}

Question: To what memory does s refer to? Is it a copy?

Borrowing

• Analogy: if somebody owns something you can borrow it from them, but eventually you have to give it back.

• If a value is borrowed, it is not moved and the ownership stays with the original owner.

• To borrow in Rust, we create a reference

fn main() {
    let x = String::from("hello");
    let len = get_length(&x); // borrow with &

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

fn get_length(s: &String) -> usize {
    s.len()
}

Shared References

Create a shared (read-only or immutable) reference &:

fn main() {
    let s = String::from("hello");
    change(&s);

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

fn change(s: &String) {
    s.push_str(", world");
}

error[E0596]:
    cannot borrow `*s` as mutable,
    as it is behind a `&` reference
 --> src/main.rs:8:5
8 | fn change(s: &String) {
  |              -------
  |     help: consider changing this to
  |            be a mutable reference:
  |           `&mut String`
9 |     s.push_str(", world");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |     `s` is a `&` reference, so the data
  |     it refers to cannot be borrowed
  |     as mutable

For more information about this error,
try `rustc --explain E0596`.

Exclusive References

Create an exclusive (writable or mutable) reference &mut:

fn main() {
    let mut s = String::from("hello");
    change(&mut s);

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

fn change(s: &mut String) {
    s.push_str(", world");
}

Compiling playground v0.0.1 (/playground)
Finished dev target(s) in 2.55s
Running `target/debug/playground`
hello, world

Rules for Borrowing and References

To any value, you can either have at the same time:

These rules are enforced by the borrow checker.

Borrowing and Memory Safety

• The ownership model does guarantee no:
  null pointer dereferences, data races, dangling pointers, use after free.

• 🦺 Rust is memory safe without any runtime garbage collector.

• 🚀 Performance of a language that would normally let you manage memory manually.

Borrow Checker’s Scope

• There are different facilities in Rust to work around some limitations of the borrow checker (Interior Mutability (later)).

Reference Example

fn main() {
    let mut s = String::from("hello");
    let a = &s;
    let b = &s;
    let c = &mut s;
    println!("{} - {} - {}", a, b, c);
}

error[E0502]: cannot borrow `s` as mutable
              because it is also
              borrowed as immutable
 --> src/main.rs:5:14
  |
3 |     let a = &s;
  |              - immutable borrow occurs
  |                here
4 |     let b = &s;
5 |     let c = &mut s;
  |              ^^^^^ mutable borrow occurs here
  |
6 |     println!("{} - {} - {}", a, b, c);
  |                              ^
  |  immutable borrow later used here

For more information about this error,
try `rustc --explain E0502`.

Returning References - Quiz

Question: Does the following work? Link

fn give_me_a_ref() -> &String {
  let s = String::from("Ups");
  &s
}

error[E0106]: missing lifetime specifier
1 | fn give_me_a_ref() -> &String {
  |                       ^ expected named
  |                         lifetime parameter
  = help: this function's return type
        contains a borrowed value,
          but there is no value for it to be borrowed from
help: consider using the `'static` lifetime,
      but this is uncommon unless you're returning a
      borrowed value from a `const` or a `static`
1 | fn give_me_a_ref() -> &'static String {
  |                        +++++++
help: instead, you are more likely
      to want to return an owned value
1 | fn give_me_a_ref() -> String {

❗Note: Returning a reference to a stack value (e.g. s) is not possible.

Returning References - Quiz

The following is the correct signature:

fn give_me_a_value() -> String {
    let s = String::from("Hello, world!");
    s
}

Returning References

You can however pass a reference through the function:

fn give_me_a_ref(input: &(String, i32)) -> &String {
    &input.0
}

• Rust annotates each reference with a lifetime.
• How to use lifetimes?  later!

Exercise Time (3)

Approx. Time: 20-30 min.

Do the following exercises:

• borrowing: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Types Redux

We have seen so far:

Borrowing will make more sense with some more complex data types.

Structuring data

Rust has two important ways to structure data

• struct : product type
• enums : sum type
• unions

Structs (tuple structs)

A tuple struct is similar to a tuple but it has an own name:

struct ControlPoint(f64, f64, bool);

fn main() {
  let cp = ControlPoint(10.5, 12.3, true);
  println!("{}", cp.0); // prints 10.5
}

Structs

More common (and preferred) - structs with named fields:

struct ControlPoint {
  x: f64,
  y: f64,
  enabled: bool,
}

• Each member has a proper identifier.

fn main() {
  let cp = ControlPoint {
    x: 10.5,
    y: 12.3,
    enabled: true,
  };

  println!("{}", cp.x); // prints 10.5
}

Enumerations

One other powerful type is the enum. It is a sum type:

enum Fruit {
  Banana,
  Apple,
}

fn main() {
  let fruit = Fruit::Banana;
}

• An enumeration has different variants, python analogy:

  variant: str | int | List[str]

• Each variant is an alternative value of the enum, pick a value on creation.

Enumeration - Mechanics

enum Fruit {
  Banana, // discriminant: 0
  Apple, // discriminant: 1
}

Enumerations - Data (1)

Enums are very powerful: each variant can have associated data

enum Fruit {
  Banana(u16, u16), // discriminant: 0
  Apple(f32, f32), // discriminant: 1
}

fn main() {
  let 🍌 = Fruit::Banana(3, 2);
  let 🍎 = Fruit::Apple(3.0, 4.);
}

Mix Sum and Product Types

Combining sum-type with a product-type:

struct Color {
  rgb: (bool, bool, bool)
}

enum Fruit {
  Banana(Color),
  Apple(bool, bool)
}

fn main() {
  let 🍌 = Fruit::Banana(Color{rgb: (false,true,false)});
  let 🍎 = Fruit::Apple(false, true);
}

The type Fruit has \((2\cdot 2 \cdot 2) + (2\cdot 2) = 12\) possible states.

Enumerations - Discriminant

You can control the discriminant like:

#[repr(u32)]
enum Bar {
    A, // 0
    B = 10000,
    C, // 10001
}

fn main() {
    println!("A: {}", Bar::A as u32);
    println!("B: {}", Bar::B as u32);
    println!("C: {}", Bar::C as u32);
}

Exercise Time (4)

Approx. Time: 20-30 min.

Do the following exercises:

• composite-types: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Extracting Data from enum

• We must ensure we interpret enum data correctly.
• Use pattern matching to do so.

Pattern Matching

Using the if let [pattern] = [value] statement:

fn accept_banana(fruit: Fruit) {
  if let Fruit::Banana(a, _) = fruit {
    println!("Got a banana: {}", a);
  } else {
    println!("not handled")
  }
}

Match Statement

Pattern matching is very powerful if combined with the match statement:

fn accept_fruit(fruit: Fruit) {
  match fruit {
    Fruit::Banana(3) => {
      println!("Banana is 3 months old.");
    },
    Fruit::Banana(v) => {
      println!("Banana: age {:?}.", v)
    }
    Fruit::Apple(true, _) => {
      println!("Ripe apple.");
    },
    _ => {
      println!("Wrong fruit...");
    },
  }
}

enum Fruit {
  Banana(u8),
  Apple(bool, bool)
}

Match Expression

The match statement can even be used as an expression:

fn get_age(fruit: Fruit) {
  let age = match fruit {
    Fruit::Banana(a) => a,
    Fruit::Apple(_) => 1, // _ matches the tuple.
  };

  println!("The age is: {}", age);
}

• All match arms must return the same type.
• No catch all (_ =>) arm -> all cases are handled.

Complex Match Statements

fn main() {
    let input = 'x';
    match input {
        'q'                       => println!("Quitting"),
        'a' | 's' | 'w' | 'd'     => println!("Moving around"),
        '0'..='9'                 => println!("Number input"),
        key if key.is_lowercase() => println!("Lowercase: {key}"),
        _                         => println!("Something else"),
    }
}

• | means or.
• 1..=9 is an inclusive range (later!).

Quiz: Why not if key.is_lowercase() after => ?

Answer: That would never print Something else.

Implementing Member Functions

To associate functions to structs and enums, we use impl blocks

fn main() {
  let x = "Hello";
  x.len();
}

• Syntax x.len() similar to field access in structs.

The impl Block (2)

struct Banana
{
  size: f64;
}

impl Banana {
  fn get_volume(&self) -> f64 {
    return self.size * self.size * 1.5;
  }
}

fn main() {
  let b = Banana{size: 4};
  let v = b.get_volume();
}

The self & Self: Implementation

• self parameter: the receiver on which a function is defined.
• Self type: shorthand for the type of current implementation.

struct Banana { size: f64; }

impl Banana {
    fn new(i: f64) -> Self { Self { size: i } }

    fn consume(self) -> Self {
        Self::new(self.size - 5.0)
    }
    // Take read reference of `Banana` instance.
    fn borrow(&self) -> &f64 { &self.size }
    // Take write reference of `Banana` instance.
    fn borrow_mut(&mut self) -> &mut f64 {
        &mut self.size
    }
}

The self & Self: Application

fn main () {
  let mut f = Banana::new();
  println!("{}", f.borrow());

  *f.borrow_mut() = 10;

  let g = f.consume();
  println!("{}", g.borrow());
}

Generics

structs become more powerful with generics:

struct PointFloat(f64, f64);
struct PointInt(i64, i64);

This is repeating data types 🤨. Is there something better?

struct Point<T>(T, T);

fn main() {
  let float_point: Point<f64> = Point(10.0, 10.0);
  let int_point: Point<i64> = Point(10, 10);
}

Generics are much more powerful (later more!)

The Option Type

A quick look into the standard library of Rust:

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

fn main() {
  let some_int = Option::Some(42);
  let no_string: Option<String> = Option::None; // You need the type here!
}

Error Handling

What would we do when there is an error?

fn divide(x: i64, y: i64) -> i64 {
  if y == 0 {
    // what to do now?
  } else {
    x / y
  }
}

Error Handling

What would we do when there is an error?

fn divide(x: i64, y: i64) -> i64 {
  if y == 0 {
    panic!("Cannot divide by zero");
  } else {
    x / y
  }
}

Error Handling with panic!

• Only use panic! in small programs if normal error handling would also exit the program.

• ❗Avoid using panic! in library code or other reusable components.

Error Handling with Option<T>

We could use an Option<T> to handle the error:

fn divide(x: i64, y: i64) -> Option<i64> {
  if y == 0 {
    None
  } else {
    Some(x / y)
  }
}

Error Handling with Result<T,E>

The Result<T,E> is a powerful enum for error handling:

enum Result<T, E> {
  Ok(T),
  Err(E),
}

enum DivideError {
  DivisionByZero,
  CannotDivideOne,
}

fn divide(x: i64, y: i64) -> Result<i64, DivideError> {
  if x == 1 {
    Err(DivideError::CannotDivideOne)
  } else if y == 0 {
    Err(DivideError::DivisionByZero)
  } else {
    Ok(x / y)
  }
}

Handling Results

Handle the error at the call site:

fn div_zero_fails() {
  match divide(10, 0) {
    Ok(div) => println!("{}", div),
    Err(e) => panic!("Could not divide by zero"),
  }
}

Handling Results

When prototyping you can use unwrap or expect on Option and Result:

fn div_zero_fails() {
  let div = divide(10, 0).unwrap();
  println!("{}", div);

  div = divide(10, 0).expect("should work!");
  println!("{}", div);
}

Handling Results

Rust has lots of helper functions on Option and Result:

fn div_zero_fails() {
  let div = divide(10, 0).unwrap_or(-1);
  println!("{}", div);
}

Besides unwrap, there are some other useful utility functions

The Magic ? Operator

There is a special operator associated with Result, the ? operator

See how this function changes if we use the ? operator:

fn can_fail() -> Result<i64, DivideError> {
  let num: i32 = match divide(10, 1) {
    Ok(v) => v,
    Err(e) => return Err(e),
  };

  match divide(num, 2) {
    Ok(v) => Ok(v * 2),
    Err(e) => Err(e),
  }
}

The Magic ? Operator

fn can_fail() -> Result<i64, DivideError> {
  let num: i32 = match divide(10, 1) {
    Ok(v) => v,
    Err(e) => return Err(e),
  };

  match divide(num, 2) {
    Ok(v) => Ok(v * 2),
    Err(e) => Err(e),
  }
}

fn can_fail() -> Result<i64, DivideError> {
  let num = divide(10, 1)?;




  Ok(divide(num, 2)? * 2)



}

• The ? operator does an implicit match:

  • on Err(e)  e is immediately returned (early return).

  • on Ok(v)  the value v is extracted and it contiues.

Exercise Time (5)

Approx. Time: 20-1.5h min.

Do the following exercises:

• options (short)
• error-handling (longer)
• error-propagation (longer)

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Vec<T>: Storage for Same Type T

The Vec<T> is an array of types T that can grow.

• Compare this to the array, which has a fixed size:

  fn main() {
    let arr = [1, 2];
    println!("{:?}", arr);
  
    let mut nums = Vec::new();
    nums.push(1);
    nums.push(2);
  
    println!("{:?}", nums);
  }

Vec: Constructor Macro

Vec is common type. Use the macro vec! to initialize it with values:

fn main() {
  let mut nums = vec![1, 2];
  nums.push(3);

  println!("{:?}", nums);
}

Vec: Memory Layout

How can a vector grow? Things on the stack need to be of a fixed size.

• A Vec allocates its contents on the heap (a [i64; 4] is on the stack).

• Quiz: What happens if the capacity is full and we add another element.

  • The Vec reallocates its memory with more capacity to another memory location  Lots of copies 🐌 ⏱️.

Vectors and Arrays

Lets write a sum function for arrays [i64; 10]:

fn sum(data: &[i64; 10]) -> i64 {
  let mut total = 0;

  for val in data {
    total += val;
  }

  total
}

Vectors and Arrays

Or one for just vectors:

fn sum(data: &Vec<i64>) -> i64 {
  let mut total = 0;

  for val in data {
    total += val;
  }

  total
}

Slices

There is better way.

Slices are typed as [T], where T is the type of the elements in the slice.

Properties

Slices

The catch with size known at compile time:

fn sum(data: [i64]) -> i64 {
  let mut total = 0;

  for val in data {
    total += val;
  }

  total
}

fn main() {
  let data = vec![10, 11, 12];
  println!("{}", sum(data));
}

error[E0277]: the size for values of type `[i64]`
              cannot be known at
              compilation time
 --> src/main.rs:1:8
  |
1 | fn sum(data: [i64]) -> i64 {
  |        ^^^^ doesn't have a size known
                at compile-time
  |
  = help: the trait `Sized` is not
          implemented for `[i64]`
help: function arguments must have a
      statically known size, borrowed types
      always have a known size

Slices

fn sum(data: &[i64]) -> i64 {
  let mut total = 0;

  for val in data {
    total += val;
  }

  total
}

fn main() {
  let data = vec![10, 11, 12];
  println!("{}", sum(data));
}

Compiling playground v0.0.1 (/playground)
Finished dev [unoptimized + debuginfo] target(s) in 0.89s
 Running `target/debug/playground`

Slices - Memory Layout

Creating Slices

One cannot create slices out of thin air, they have to be located somewhere. Three possibilities:

Creating Slices - Borrowing

Using a borrow:

fn sum(data: &[i32]) -> i32 { /* ... */ }

fn main() {
  let v = vec![1, 2, 3, 4, 5, 6];
  let total = sum(&v);

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

Creating Slices - Ranges

Using ranges:

fn sum(data: &[i32]) -> i32 { /* ... */ }

fn main() {
  let v = vec![0, 1, 2, 3, 4, 5, 6];
  let all = sum(&v[..]);
  let except_first = sum(&v[1..]);
  let except_last = sum(&v[..5]);
  let except_ends = sum(&v[1..5]);
}

Creating Slices

From a literal:

fn sum(data: &[i32]) -> i32 { todo!("Sum all items in `data`") }

fn get_v_arr() -> &'static [i32] {
    &[0, 1, 2, 3, 4, 5, 6]
}

fn main() {
  let all = sum(get_v_arr());
}

• Interestingly get_v_arr works, but looks like it would only exist temporarily.

• Literals actually exist during the entire lifetime of the program.

• &'static indicates: this slice exist for the entire lifetime of the program (later more on lifetimes).

Strings

We have already seen the String type being used before, but let’s dive a little deeper

• Strings are used to represent text.

• In Rust they are always valid UTF-8.

• Their data is stored on the heap.

• A String is similar to Vec<u8> with extra checks to prevent creating invalid text.

Strings

Let’s take a look at some strings

fn main() {
  let s = String::from("Hello world 🌏");

  println!("{:?}", s.split_once(" "));

  println!("{}", s.len());

  println!("{:?}", s.starts_with("Hello"));

  println!("{}", s.to_uppercase());

  for line in s.lines() {
    println!("{}", line);
  }
}

String Literals

Constructing some strings

fn main() {
  let s1 = "Hello world 🌏";
  let s2 = String::from("Hello world");
}

• s1 is a slice of type &str: a string slice.

String Literals

Constructing some strings

fn main() {
  let s1: &str = "Hello world";
  let s2: String = String::from("Hello world");
}

The String Slice - &str

Its possible to get only a part of a string. But what is it?

Types str, String, [T; N], Vec

String or str ?

When to use String and when str?

fn string_len1(data: &String) -> usize {
  data.len()
}

fn string_len1(data: &str) -> usize {
  data.len()
}

Exercise Time (6)

Approx. Time: 30-50 min

Do the following exercises:

• slices:

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Recap 1 - References

• Shared References &T: read-only, many possible.
• Exclusive Reference &mut T: write, only a single at any point in the program.

Recap 1 - Option & Result & ?

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

enum Result<R, E> {
  Ok(R),
  Err(E),
}

• Magic Operator

Recap - Borrow Checker

• The borrow Checkers Scope.

Quiz - Does that Compile?

#[derive(Debug)]
enum Color {None, Blue}
struct Egg { color: Color }

fn colorize(egg: &mut Egg) -> &Color {
  egg.color = Color::Blue;
  return &egg.color
}

fn main() {
  let mut egg = Egg {color: Color::None};
  let color: &Color;
  {
    let egg_ref = &mut egg;
    color = colorize(egg_ref)
  }
  println!("color: {color:?}")
}

Quiz - Addendum

The colorize method is basically a method on Egg which takes a exclusive reference &mut self only for the duration of that function.

#[derive(Debug)]
enum Color {None, Blue}
struct Egg { color: Color }

impl Egg {
  fn colorize(&mut self) -> &Color {
    self.color = Color::Blue;
    return &self.color
  }
}

fn main() {
  let mut egg = Egg {color: Color::None};
  let color: &Color;
  {
    color = egg.colorize()
  }
  println!("color: {color:?}")
}

Put it in a Box

Box<T> will allocate a type T on the heap and wrap the pointer underneath:

fn main() {
  // Put an integer on the heap
  let boxed_int: Box<i64> = Box::new(10);
}

• 🧰 Boxing: Store a type T on the heap.

Boxing

Reasons to box a type T on the heap:

Exercise Time (7)

Approx. Time: 40-50 min.

Do the following exercises:

• exercise: boxed-data: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

The Problem

fn add_u32(l: u32, r: u32) -> u32 { /*...*/}

fn add_i32(l: i32, r: i32) -> i32 { /*...*/ }

fn add_f32(l: f32, r: f32) -> f32 { /*...*/ }

No-one likes repeating themselves. We need generic code!

Generic code

An example

fn add<T>(a: T, b: T) -> T { /*...*/}

Or, in plain English:

Bounds on Generic Code

We need to provide information to the compiler:

The trait Keyword

Describe what the type can do but not specifying what data it has:

trait Add {
    fn add(&self, other: &Self) -> Self;
}

This is similar in other languages:

from abc import ABC, abstractmethod
class Add(ABC):
    @abstractmethod
    def add(self, other: Self):
        pass

type Add interface {
    func add(other Add)
}

::: notes Traits are not types! :::

Implementing a trait

Describe how the type does it

impl Add for u32 {
    fn add(&self, other: &Self) -> Self {
      *self + *other
    }
}

Using a trait

// Import the trait
use my_mod::Add

fn main() {
  let a: u32 = 6;
  let b: u32 = 8;
  // Call trait method
  let result = a.add(&b);
  // Explicit call
  let result = Add::add(&a, &b);
}

Trait Bounds

fn add_values<T: Add>(this: &T,
                      other: &T) -> T {
  this.add(other)
}
// or, equivalently
fn add_values<T>(this: &T,
                 other: &T) -> T
  where T: Add
{
  this.add(other)
}
// or shorthand with `impl Trait`
fn add_values(this: &impl Add,
              other: &impl Add) -> impl Add
{
  this.add(other)
}

Limitations of Add

What happens if…

• We want to add two values of different types?
• Addition yields a different type?

Making Add Generic

Generalize on the input type O:

trait Add<O> {
    fn add(&self, other: &O) -> Self;
}

impl Add<u16> for u32 {
    fn add(&self, other: &u16) -> Self {
      *self + (*other as u32)
    }
}

We can now add a u16 to a u32.

Defining Output of Add

• Addition of two given types always yields one specific type of output.
• Add associated type for addition output.

trait Add<O> {
  type Out;
  fn add(&self, other: &O) -> Self::Out;
}

impl Add<u16> for u32 {
  type Out = u64;

  fn add(&self, other: &u16) -> Self::Out) {
    *self as u64 + (*other as u64)
  }
}

Trait std::ops::Add

The way std does it

pub trait Add<Rhs = Self> {
    type Output;

    fn add(self, rhs: Rhs) -> Self::Output;
}

• Default type of Self for Rhs

Implementation of std::ops::Add

use std::ops::Add;
pub struct BigNumber(u64);

impl Add for BigNumber {
  type Output = Self;

  fn add(self, rhs: Self) -> Self::Output {
      BigNumber(self.0 + rhs.0)
  }
}

fn main() {
  // Call `Add::add`
  let res = BigNumber(1).add(BigNumber(2));
}

Quiz: What’s the type of res?  BigNumber(u64)

Implementation std::ops::Add (2)

pub struct BigNumber(u64);

impl std::ops::Add<u32> for BigNumber {
  type Output = u128;

  fn add(self, rhs: u32) -> Self::Output {
      (self.0 as u128) + (rhs as u128)
  }
}

fn main() {
  let res = BigNumber(1) + 3u32;
}

Quiz: What’s the type of res?  u128

Type Parameter vs. Associated Type

impl Add<u32> for u32 {/* */}
impl Add<i64> for u32 {/* */}

impl Add<u32> for u32 {
  // Addition of two u32's is always u32
  type Out = u32;
}

Example - Associated Types

trait Distance {
    type Scalar;

    fn distance(&self, a: &Self::Scalar, b: &Self::Scalar) -> bool;
    fn first(&self) -> &Self::Scalar;
    fn last(&self) -> &Self::Scalar;
}

// Implement Distance for some types here ....
impl Distance for ...

// The caller specifying a bound does not need to
// specify the `Scalar`.
fn distance<T: Distance>(container: &T) -> bool {
    container.distance(container.first(), container.last())
}

If we would have chosen trait Distance<Scalar> instead, the type Scalar need to be provided everywhere its used.

Derive a Trait

#[derive(Clone, Debug)]
struct Dolly {
  num_legs: u32,
}

fn main() {
  let dolly = Dolly { num_legs: 4 };
  let second_dolly = dolly.clone();

  println!("Dolly: {:?}", second_dolly)
}

Orphan Rule

Coherence: There must be at most one implementation of a trait for any given type

Compiling Generic Functions

impl Add for i32 {/* ... */}
impl Add for f32 {/* ... */}

fn add_values<T: Add>(a: &T, b: &T) -> T
{
  a.add(b)
}

fn main() {
  let sum_one = add_values(&6, &8);
  let sum_two = add_values(&6.5, &7.5);
}

Code is monomorphized:

Exercise Time (8)

Approx. Time: 40-50 min.

Do the following exercises:

• generics: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Operator Overloading

std::ops::Add<T> et al.

• Shared behavior

use std::ops::Add;
pub struct BigNumber(u64);

impl Add for BigNumber {
  type Output = Self;
  fn add(self, rhs: Self) -> Self::Output {
      BigNumber(self.0 + rhs.0)
  }
}
fn main() {
  // Now we can use `+` to add `BigNumber`s!
  let res: BigNumber = BigNumber(1) + BigNumber(2);
}

• Others: Mul, Div, Sub, ..

Markers

std::marker::Sized

• Marker traits

/// Types with a constant size known at compile time.
/// [...]
pub trait Sized { }

Others:

• Sync: Types of which references can be shared between threads.
• Send: Types that can be transferred across thread boundaries.

Default Values

std::default::Default

pub trait Default: Sized {
    fn default() -> Self;
}

#[derive(Default)] // Derive the trait
struct MyCounter {
  count: u32,
}

// Or, implement it (if you really need to)
impl Default for MyCounter {
  fn default() -> Self {
    MyCounter { count: 1 }
  }
}

Duplication

std::clone::Clone & std::marker::Copy

pub trait Clone: Sized {
    fn clone(&self) -> Self;

    fn clone_from(&mut self, source: &Self) {
      *self = source.clone()
    }
}

pub trait Copy: Clone { } // That's it!

Conversions

Into<T> & From<T>

pub trait From<T>: Sized {
    fn from(value: T) -> Self;
}

pub trait Into<T>: Sized {
    fn into(self) -> T;
}

impl <T, U> Into<U> for T
  where U: From<T>
{
    fn into(self) -> U {
      U::from(self)
    }
}

Reference Conversion

AsRef<T> & AsMut<T>

pub trait AsRef<T: ?Sized>
{
    fn as_ref(&self) -> &T;
}

pub trait AsMut<T: ?Sized>
{
    fn as_mut(&mut self) -> &mut T;
}

• Provide flexibility to API users.
• T need not be Sized, e.g. slices [T] can implement AsRef<T>, AsMut<T>

Reference Conversion (2)

AsRef<T> & AsMut<T>

fn move_into_and_print<T: AsRef<[u8]>>(slice: T) {
  let bytes: &[u8] = slice.as_ref();
  for byte in bytes {
    print!("{:02X}", byte);
  }
}

fn main() {
  let owned_bytes: Vec<u8> = vec![0xDE, 0xAD, 0xBE, 0xEF];
  move_into_and_print(owned_bytes);

  let byte_slice: [u8; 4] = [0xFE, 0xED, 0xC0, 0xDE];
  move_into_and_print(byte_slice);
}

Have user of move_into_and_print choose between stack local [u8; N] and heap-allocated Vec<u8>

Destruction: std::ops::Drop

pub trait Drop {
    fn drop(&mut self);
}

• Called when owner goes out of scope.

Destruction:std::ops::Drop

struct Inner;
struct Outer { inner: Inner }

impl Drop for Inner {
  fn drop(&mut self) {
    println!("Dropped inner");
  }
}
impl Drop for Outer {
  fn drop(&mut self) {
    println!("Dropped outer");
  }
}

fn main() {
  // Explicit drop
  std::mem::drop(Outer { inner: Inner });
}

Dropped outer
Dropped inner

// Implementation of `std::mem::drop`
fn drop<T>(_x: T) {}

More Std-Traits

There is more:

• Comparision PartialEq and Eq etc.
• Read and BufRead for abstraction over u8 sources.
• Display and Debug to format types.
• Iterator to provide an iterator for your type.

Std-Traits and the Orphan Rule

When you provide a type, always implement the basic traits from the standard if they are appropriate, e.g.

Other traits for later:

• Send : Auto trait: A value T can safely be send across thread boundary.
• Sync : Auto trait: A value T can safely be shared between threads.
• Sized : Marker trait to denote that type T is known at compile time

Exercise Time (9)

Approx. Time: 40-50 min.

Do the following exercises:

• traits
• std-traits
• extension-traits: a very good one (moderate to hard)!
• local-storage-vec: ⚠️ Hardcore exercise (spare for later)

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

What Lifetime?

Example - Lifetime Scopes

fn main() {
    let r;

    {
        let x = 5;
        r = &x;
    }

    println!("r: {r}");
}

Question: Will this compile?

Example - Lifetime Scopes (2)

Variable r lives for lifetime 'a and x for 'b.

fn main() {
    let r;                // ---------+- 'a
                          //          |
    {                     //          |
        let x = 5;        // -+-- 'b  |
        r = &x;           //  |       |
    }                     // -+       |
                          //          |
    println!("r: {r}");   //          |
}                         // ---------+

Answer: No, r points to x which is dropped in L6.

Example - Lifetime in Function (1)

Question: Will this compile?

/// Return reference to longest of `a` or `b`.
fn longer(a: &str, b: &str) -> &str {
    if a.len() > b.len() {
        a
    } else {
        b
    }
}

Example - Lifetime in Function (2)

Answer: No. rustc needs to know more about a and b.

/// Return reference to longest
/// of `a` and `b`.
fn longer(a: &str, b: &str) -> &str {
  if a.len() > b.len() {
      a
  } else {
      b
  }
}

error[E0106]: missing lifetime specifier
 --> src/lib.rs:2:32
  |
2 | fn longer(a: &str, b: &str) -> &str {
  |              ----     ----     ^
  | expected named lifetime parameter
  |
  = help: this function's return type contains
    a borrowed value, but the signature does
    not say whether it is borrowed from `a` or `b`
help: consider introducing a named lifetime parameter
2 | fn longer<'a>(a: &'a str, b: &'a str) -> &'a str {
  |          ++++     ++          ++          ++

For more information about this error,
try `rustc --explain E0106`.

Lifetime Annotations

Solution: Provide a constraint with a lifetime parameter 'l:

fn longer<'l>(a: &'l str, b: &'l str) -> &'l str {
    if a.len() > b.len() {
        a
    } else {
        b
    }
}

Validating Boundaries

• Lifetime validation is done within function boundaries (and scopes e.g. {...}).
• No information of calling context is used.

Question: Why no calling context?

Answer: Because its only important to know the lifetime relation between input & output - the constraint.

Example - Validating Boundaries

fn main() {
  let x = 3;                       // ------------+- 'a
  {                                //                 |
    let y = 10;                    // ------+--- 'b   |
    let r: &i64 = longest(&x, &y); // --+- 'c     |   |  'l := min('a,'b) => 'l := 'b
    println!("longest: {r}")       //   |         |   |
  }                                // --+---------+   |
}                                  // ----------------+

Borrow checker checks if r’s lifetime fulfills <= 'b  'c <= 'b  ✅.

Lifetime Annotations in Types

If references are used in structs, it needs a life-time annotation:

/// A struct that contains a reference.
pub struct ContainsRef<'r> {
  ref: &'r i64
}

English:

Lifetime Elision

Question: “Why haven’t I come across this before?”

Answer: “Because of lifetime elision!”

Lifetime Elision

Rust compiler has heuristics for eliding lifetime bounds:

Lifetime Elision Examples

fn print(s: &str);                                      // elided
fn print<'a>(s: &'a str);                               // expanded

fn debug(lvl: usize, s: &str);                          // elided
fn debug<'a>(lvl: usize, s: &'a str);                   // expanded

fn substr(s: &str, until: usize) -> &str;               // elided
fn substr<'a>(s: &'a str, until: usize) -> &'a str;     // expanded

fn get_str() -> &str;                                   // ILLEGAL (why?)

fn frob(s: &str, t: &str) -> &str;                      // ILLEGAL (why?)

fn get_mut(&mut self) -> &mut T;                        // elided
fn get_mut<'a>(&'a mut self) -> &'a mut T;              // expanded

Exercise Time

Approx. Time: 15-30 min.

Do the following exercises:

• lifetimes: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Closures

fn main() {
    let z = 42;
    let compute = move |x, y| x + y + z; // Whats the type of this?

    let res = compute(1, 2);
}

• Closures or lambda expressions are anonymous (unnamed) functions.

What is a Closure?

The closure |x: i32| x * x can mechanistically be mapped to the following struct

struct SquareFunc {}

impl SquareFunc {
  fn call(&self, x: i32) {
    x * x
  }
}

❗The “struct with fields” is a mental model how |x: i32| x * x is mechanistically implemented by the compiler.

What is a Fn Closure?

For a closure which captures (“closes-over”) variables from the environment:

let z = 43;
let square_it = |x| x * x + z;  // => Fn(i32) -> i32     (LSP)
                                // compiler opaque type: approx `SquareIt`.
square_it(10);

approx. maps to:

struct SquareIt<'a> {
  z: &'a i32;
}

impl SquareIt {
  fn call(&self, x: i32) {
    x * x + self.z
  }
}

let z = 43;
let square_it = SquareIt{z: &z};
square_it.call(10);

What is a FnMut Closure?

A closure with some mutable state:

fn main() {
  let mut total: i32 = 0;

  let mut square_it = |x| {
      // => FnMut(i32) -> i32  (LSP)
      total += x * x;
      x * x
  };

  square_it(10);
  assert_eq!(100, total);
}

struct SquareIt<'a>' {
  total: &'a mut i32
}

impl SquareIt {
  fn call(&mut self, x: i32) {
    self.total += x * x;
    x * x
  }
}

Capture by Value

Capture by value with move:

fn main() {
  let mut total: i32 = 0; // Why `mut` here?

  let mut square_it = move |x| {
      // => FnMut(i32) -> i32  (LSP)
      total += x * x;
      x + x
  };

  square_it(10);
  assert_eq!(0, total)
}

struct SquareIt {
  total: i32
}

impl SquareIt {
  fn call(&mut self, x: i32) {
    self.total += x * x;
    x * x
  }
}

Example - Quiz

Does it compile? Does it run without panic?

fn main() {
    let mut total: i32 = 0;

    let mut square_it = |x| { // => FnMut(i32) -> i32
        total += x * x;
        x * x
    };
    total = -1;

    square_it(10);
    assert_eq!(-1, total)
}

Answer: It does not compile as total is mut. borrowed on L8.
Move square_it before total = -1.

Closure Traits

Fn, FnMut and FnOnce are traits which implement different behaviors for closures. The compiler implements the appropriate ones!

Quiz - What is it?

let mut s = String::from("foo");
let t     = String::from("bar");

let func = || {
    s += &t;
    s
};

struct Closure<'a> {
    s : String,
    t : &'a String,
}

impl<'a> FnOnce<()> for Closure<'a> {
    type Output = String;
    fn call_once(self) -> String {
        self.s += &*self.t;
        self.s
    }
}

Closure and Functional Programming

Useful when working with iterators, Option and Result:

let numbers = vec![1, 3, 4, 10, 29];

let evens: Vec<_> = numbers.into_iter()
                           .filter(|x| x % 2 == 0)
                           .collect();

Closures and Functional Programming (2)

Useful when generalizing interfaces, e.g. visitor pattern

struct Graph{ nodes: Vec<i32>; };

impl Graph {

  fn visit(&self, visitor: impl FnOnce(i32)) {
    // Remember: All closure at least implement `FnOnce`.
    for n in self.nodes {
      visitor(n) // Call visitor function for each node.
    }
  }

}

Exercise Time (10)

Approx. Time: 20-60 min.

Do the following exercises:

• closures: all

Build/Run/Test:

just build <exercise> --bin 01
just run <exercise> --bin 01
just test <exercise> --bin 01
just watch [build|run|test|watch] <exercise> --bin 01

Recap 2 - Closures

fn main() {
    let numbers = vec![1, 2, 5, 9];
    let smaller_than_5 = |x: i32| -> bool { x < 5 };

    let res = filter(&numbers, smaller_than_5);

    print!("Result: {res:?}")
}

• Question: What is the type of smaller_than_9?
• Question: How to write filter to be most generic.

Trait… Object?

• We learned about traits.
• We learned about generics and monomorphization.

There’s more to this story though…

Question: What was monomorphization again?

Monomorphization (recap)

The Add trait.

impl Add for i32 {/* ... */}
impl Add for f32 {/* ... */}

fn add_values<T: Add>(l: &T, r: &T) -> T
{
  l.add(r)
}

fn main() {
  let sum_one = add_values(&6, &8);
  let sum_two = add_values(&6.5, &7.5);
}

Dynamic Dispatch

What if don’t know the concrete type implementing the trait at compile time?

use std::io::Write;
use std::path::PathBuf;

struct FileLogger { log_file: PathBuf }
impl Write for FileLogger { /* ... */}

struct StdOutLogger;
impl Write for StdOutLogger { /* ... */}

fn log<L: Write>(logger: &mut L, msg: &str) {
  write!(logger, "{}", msg);
}

fn main() {
  let file: Option<PathBuf> = // args parsing...

  let mut logger = match file {
      Some(f) => FileLogger { log_file: f },
      None => StdOutLogger,
  };

  log(&mut logger, "Hello, world!🦀");
}

Error!

error[E0308]: `match` arms have incompatible types
  --> src/main.rs:19:17
   |
17 |       let mut logger = match log_file {
   |  ______________________-
18 | |         Some(log_file) => FileLogger { log_file },
   | |                           -----------------------
   | |                           this is found to be of
   | |                           type `FileLogger`
19 | |         None => StdOutLogger,
   | |                 ^^^^^^^^^^^^ expected struct `FileLogger`,
   | |                              found struct `StdOutLogger`
20 | |     };
   | |_____- `match` arms have incompatible types

What’s the type of logger?

Heterogeneous Collections

What if we want to create collections of different types implementing the same trait?

trait Render {
  fn paint(&self);
}

struct Circle;
impl Render for Circle {
  fn paint(&self) { /* ... */ }
}

struct Rectangle;
impl Render for Rectangle {
  fn paint(&self) { /* ... */ }
}

fn main() {
  let circle = Circle{};
  let rect = Rectangle{};

  let mut shapes = Vec::new();
  shapes.push(circle);
  shapes.push(rect);

  shapes.iter()
        .for_each(|s| s.paint());
}

Error Again!

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:20:17
   |
20 |     shapes.push(rect);
   |            ---- ^^^^ expected struct `Circle`,
   |                      found struct `Rectangle`
   |            |
   |            arguments to this method are incorrect
   |
note: associated function defined here
  --> /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/alloc/src/vec/mod.rs:1836:12

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

What is the type of shapes?

Dynamically Sized Types (DST)

Rust supports Dynamically Sized Types (DSTs): types without a statically known size or alignment.

On the surface, this is a bit nonsensical: rustc always needs to know the size and alignment to compile code!

Examples of Sized vs. !Sized

Trait Objects dyn Trait

fn main() {
  let log_file: Option<PathBuf> = // ...

  // Create a trait object that implements `Write`
  let logger: &mut dyn Write = match log_file {
    Some(log_file) => &mut FileLogger { log_file },
    None => &mut StdOutLogger,
  };
}

Quiz - Instantiate a Trait?

struct A{}
trait MyTrait { fn show(&self) {}; }
impl MyTrait for A {}

fn main() {
  let a: MyTrait = A{};
  let b: dyn MyTrait = A{};
}

Question: Does that compile?

Generics and Sized : How?

fn generic_fn<T: Eq>(x: T) -> T { /*..*/ }

Generics and Sized

• All generic type parameters are implicitly Sized by default (everywhere structs, fns etc.):

  For example:

  fn generic_fn<T: Eq + Sized>(x: T) -> T { // Sized is obsolete here.
    //...
  }

  or

  fn generic_fn<T>(x: &T) -> u32
  where
      T: Eq + Sized // Sized is obsolete here.
  {
    // ...
  }

Generics and ?Sized

Sometimes we want to opt-out of Sized: use ?Sized:

fn generic_fn<T: Eq + ?Sized>(x: &T) -> u32 { ... }

Generics and ?Sized - Quiz

Does that compile? Why?/Why not?

fn generic_fn<T: Eq + ?Sized>(x: &T) -> u32 { 42 }

fn main() {
  generic_fn("hello world")`
}

Generics and ?Sized - Quiz

Does that compile? Why?/Why not?

// removed the reference ------- v
fn generic_fn<T: Eq + ?Sized>(x: T) -> u32 { 42 }

fn main() {
  generic_fn("hello world");
}

Generics and ?Sized - Quiz (Tip)

How to print the type T?

fn generic_fn<T: Eq>(x: T) -> u32 {
    42
}

fn main() {
    generic_fn("hello world");
}

fn generic_fn<T: Eq + std::fmt::Display>(x: T) -> u32 {
    println!(
      "x: {} = '{x}'",
      std::any::type_name::<T>());

    42
}

fn main() {
    generic_fn("hello world");
}

x: &str = 'hello world'

Dynamic Dispatch on the Stack

/// Same code as last slide
fn main() {
  let log_file: Option<PathBuf> = //...

  // Create a trait object that implements `Write`
  let logger: &mut dyn Write = match log_file {
    Some(log_file) => &mut FileLogger{log_file},
    None => &mut StdOutLogger,
  };

  log(&mut logger, "Hello World!");
}

Dynamic Dispatch on the Heap

/// Same code as last slide
fn main() {
  let log_file: Option<PathBuf> = //...

  // Create a trait object on heap that impl. `Write`
  let logger: Box<dyn Write> = match log_file {
    Some(log_file) => Box::new(FileLogger{log_file}),
    None => Box::new(StdOutLogger),
  };

  log("Hello, world!🦀", &mut logger);
}

Fixing Dynamic Logger

• Trait objects &dyn Trait, Box<dyn Trait>, … implement Trait!

// L no longer must be `Sized`, so to accept trait objects.
fn log<L: Write + ?Sized>(entry: &str, logger: &mut L) {
    write!(logger, "{}", entry);
}

fn main() {
    let log_file: Option<PathBuf> = // ...

    // Create a trait object that implements `Write`
    let logger: &mut dyn Write = match log_file {
        Some(log_file) => &mut FileLogger { log_file },
        None => &mut StdOutLogger,
    };
    log("Hello, world!🦀", logger);
}

And all is well! Live Stack Dyn. Dispatch, Live Heap Dyn. Dispatch.

Forcing Dynamic Dispatch

If one wants to enforce API users to use dynamic dispatch, use &mut dyn Write on log:

fn log(entry: &str, logger: &mut dyn Write) {
    write!(logger, "{}", entry);
}

fn main() {
    let log_file: Option<PathBuf> = // ...

    // Create a trait object that implements `Write`
    let logger: &mut dyn Write = match log_file {
        Some(log_file) => &mut FileLogger { log_file },
        None => &mut StdOutLogger,
    };

    log("Hello, world!🦀", &mut logger);
}

Heterogeneous Collection on the Heap

fn main() {
  let mut shapes = Vec::new();


  let circle = Circle;
  shapes.push(circle);

  let rect = Rectangle;
  shapes.push(rect);

  shapes.iter()
        .for_each(|s| s.paint());
}

fn main() {
  let mut shapes: Vec<Box<dyn Render>>
    = Vec::new();

  let circle = Box::new(Circle);
  shapes.push(circle);

  let rect = Box::new(Rectangle);
  shapes.push(rect);

  shapes.iter()
        .for_each(|s| s.paint());
}

All set!

Heterogeneous Collection on the Stack 🍭

fn main() {
    let shapes: [&dyn Render; 2] = [&Circle {}, &Rectangle {}];
    shapes.iter().for_each(|shape| shape.paint());
}

All set!

Trait Object Limitations

• Pointer indirection cost.

• Harder to debug.

• Type erasure (you need a trait).

• Not all traits work:

  Traits need to be Object Safe

Object Safety

A trait is object safe when it fulfills:

• Trait T must not be Sized: Why?
• If trait T: Y, thenY must be object safe.
• No associated constants allowed.
• No associated types with generic allowed.
• All associated functions must either be dispatchable from a trait object, or explicitly non-dispatchable:
  • e.g. function must have a receiver with a reference to Self

Details in The Rust Reference. Read them!

These seem to be compiler limitations.

Non-Object Safe Trait (😱)

trait Fruit {
  fn create(&self) -> Self;
  fn show(&self) -> String;
}

struct Banana { color: i32 }

impl Fruit for Banana {
  fn create(&self) -> Self { Banana {} }

  fn show(&self) -> String {
      return format!("banana: color {}", self.color).to_string();
  }
}

fn main() {
    let obj: Box<dyn Fruit> = Box::new(Banana { color: 10 });
    println!("type: {}", obj.show())
}

Non-Object Safe Traits (💩)

The trait `Fruit` cannot be made into an object

22 |     println!("type: {}", obj.show())
   |                          ^^^^^^^^^^ `Fruit` cannot be made into an object

note: for a trait to be "object safe" it needs to
      allow building a vtable to allow the call to be
      resolvable dynamically; for more information
      visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

1  | trait Fruit {
   |       ----- this trait cannot be made into an object...
2  |     fn create(&self) -> Self;
                             ^^^^ ...because method `create`
                                  references the `Self` type in its return type

Trait Object Summary

• Trait objects allow for dynamic dispatch and heterogeneous containers.
• Trait objects introduce pointer indirection.
• Traits need to be object safe to make trait objects out of them.