readme_en.md

WTF Cairo: 4. Variable Mutability

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In this chapter, we introduce variable mutability in Cairo, including shadowing and the let, mut, and const keywords.

Immutable Variables

For safety reasons, variables in Cairo are immutable by default, similar to Rust. Once a variable is assigned a value, it cannot be changed afterwards.

// In Cairo, variables are immutable by default
let x_immutable = 5;
// The following code will result in an error
// x_immutable = 10

Mutable Variables

Mutability can be very useful and can make code more convenient to write. Like in Rust, you can use the mut keyword to declare mutable variables:

// Use the `mut` keyword to declare mutable variables
let mut x_mutable = 5;
x_mutable = 10;

Constants

Similar to immutable variables, constants are values bound to a name and are not allowed to change. However, there are a few differences between constants and variables:

1. Constants are declared with the const keyword, not let.
2. The type of the value must be annotated.
3. Constants can only be declared and assigned in the global scope (within a contract and outside of functions).
4. You can't use mut with const.

const CONST_NUMBER: felt252 = 888;

#[external(v0)]
fn mutable_and_const(self: @ContractState) {
    // You can assign a const to a variable
    let y_immutable = CONST_NUMBER + 2;
}

Shadowing

In Cairo, you can declare a new variable with the same name as a previous variable, effectively shadowing the previous one. This is different from using mut, as you are creating a new variable when you use the let keyword again. This allows you to change the type or mutability of the value while reusing the same name.

#[external(v0)]
fn shadow(self: @ContractState) -> felt252 {
    // Shadowing: you can declare a new variable with the same name as previous ones.
    let x_shadow = 5;
    // You can change the data type or mutability with shadowing
    let x_shadow = 10_u8;
    let mut x_shadow = 15;
    return x_shadow;
}

Summary

In this chapter, we delved into variable mutability in Cairo, including shadowing and the let, mut, and const keywords.