Immutable Variables (let)

A variable introduced with let is immutable: once initialized, it cannot be reassigned. Immutability is the default in Rux — reach for let first and switch to var only when mutation is genuinely needed.

let x = 42;                 // type inferred as int
let name: Slice<char8> = "Rux";  // explicit type annotation

x = 10;   // error: cannot assign to an immutable binding

Why Prefer let

Defaulting to let makes the few values that do change stand out, and it lets the compiler reject accidental writes. A binding you never intend to reassign should say so, so that readers — and the compiler — can rely on it.

Immutability Extends to Fields

For a value type such as a struct, let makes the whole value immutable, not just the name. You can neither reassign the variable nor modify its fields:

let p = Point { x: 1.0, y: 2.0 };

p = Point { x: 0.0, y: 0.0 };  // error: cannot reassign a let binding
p.x = 5.0;                     // error: cannot modify a field of an immutable struct

See Mutability of Structs for the complete rules.

See Also

• Mutable Variables (var) — when a value needs to change
• Mutability of Structs — how let propagates into fields
• Constants — immutable values known at compile time