Five minutes Rust

Rust concise 5 Minutes cheat-sheet for each concepts.

---

Hello, Rust!

fn main() {
    println!("Hi There!{}", "Ankur Anand");
}

Important println! is not an function. It's a macro in rust. (macro end with !)

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Macros!

• Macros are like functions, but they're named with ! at the end.
• Can do generally very powerful stuff.
  • They actually generate code at compile time!
  • Think macro in c but more hygienic. More later!
• Call and use macros like functions.
• You can define your own with macro_rules! macro_name blocks.
  • These are very complicated. More later!
• Because they're so powerful, a lot of common utilities are defined as macros.
• println! macro

print! & println!

• Print stuff out. Yay.
• Use {} for general string interpolation, and {:?} for debug printing.
  • Some types can only be printed with {:?}, like arrays and Vecs.

print!("{}, {}, {}", "foo", 3, true);
// => foo, 3, true
println!("{:?}, {:?}", "foo", [1, 2, 3]);
// => "foo", [1, 2, 3]

---

format!

• Uses println!-style string interpolation to create formatted Strings.

let fmted = format!("{}, {:x}, {:?}", 12, 155, Some("Hello"));
// fmted == "12, 9b, Some("Hello")"

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Variable Bindings

How to store a value and refer it later ?

In rust we can decalare a variable with let keyword. i.e Variables are bound with let

let name = "ankur";

• Bindings are implicitly-typed: the compiler infers based on context.
• The compiler can't always determine the type of a variable, so sometimes you have to add type annotations.

let x: i16 = 9;

All the variables are immutable by default in Rust. i.e Once the variable has been decalared and initialized, you cannot assign some other value to it by default.

Trying to reassign will throw you an error at compile time.

fn main() {
    let x_immutable = 10;
    println!("x value : {}", x_immutable);
    x_immutable = 20;
}

error[E0384]: cannot assign twice to immutable variable `x_immutable`
 --> var.rs:4:5
  |
2 |     let x_immutable = 10;
  |         ----------- first assignment to `x_immutable`
3 |     println!("x value : {}", x_immutable);
4 |     x_immutable = 20;
  |     ^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable

So if We want to reassign variable to some other value, add mut keyword before it.

fn main() {
    let mut y_mutable = 20;
    println!("y mut {}", y_mutable);
    y_mutable = 30;
    println!("y mut {}", y_mutable);
}

• Bindings may be shadowed:

let x = 17;
let y = 53;
let x = "Shadowed!";
// x is not mutable, but we're able to re-bind it

• The shadowed binding for x above lasts until it goes out of scope.

• Above, we've effectively lost the first binding, since both xs are in the same scope.

• Patterns may also be used to declare variables:

let (a, b) = ("foo", 12);

---

Expressions

Expression are something that returns a value

• Almost everything is an expression: in Rust. Except variable bindings

• The "nothing" type is called "unit", which is written ().

  • The type () has only one value: ().
  • () is the default return type.

• Discard an expression's value by appending a semicolon. Now it returns ().

  • Hence, if a function ends in a semicolon, it returns ().

fn foo() -> i32 { 5 }
fn bar() -> () { () }
fn baz() -> () { 5; }
fn qux()       { 5; }

• Because everything is an expression, we can bind many things to variable names:

let x = -5;
let y = if x > 0 { "greater" } else { "less" };
println!("x = {} is {} than zero", x, y);

• Aside: "{}" is Rust's (most basic) string interpolation operator
  • like printf's "%s" in C/C++.