Variables

Defining variables is done with the let, const and static keywords.

let

let defines a mutable variable. The value inside the variable itself is always mutable, but the variable can be changed.

let a = 5;
a = 10; // Valid
a = 3.14; // Invalid, `a` is implicitly an i32

const

const defines an immutable variable. The value is still mutable, but the variable cannot be changed.

const a = 5;
a = 10; // Invalid

struct Pair<V> {
    key: str,
    val: V
}

const my_pair = new Pair<i32>{key: "Hello", val: 3};
my_pair.key = "World"; // Valid

static

static is used for values you never want to get garbage-collected. They cannot be defined in functions or the main block.

static PI = 3.14;

main {
    PI = 4; // Invalid
    let PI = 4; // Invalid
    const PI = 5; // Invalid
}

Type hinting in variables

let someOption = none;  // Incorrect

Here, the compiler doesn't know what else can someOption be other than none, so it throws an error. You need to specify the other possible type.

let someOption: i32? = none; // Correct!

Deconstructing structs or tuples

let my_tuple = [1, 2, 3, 4, 5];
let [firstElement, secondElement] = my_tuple;
print(firstElement, secondElement); // 1, 2

struct Student {
    grades: Map<str, Grade>
    favorite_subject: str
}

const { favorite_subject } = new Student { grades: Map::create(), favorite_subject: "programming" };
print(favorite_subject); // "programming"