6. Mutability & Bindings

6.1 Deep Mutability Model

Zith uses deep mutability: a modifier on a binding flows into every nested field. Fields inside a struct inherit the mutability of the instance that holds them — no per-field mut annotation needed.

6.2 Binding Keywords

let/var control reassignability of the binding itself. Content mutability is handled separately through memory modifiers (§7).

// let/var control REBIND only. Content mutability comes from memory modifiers.
let x: mut Point;      // cannot reassign x; Point's fields are mutable (mut)
var y: Point;          // can reassign y; Point's fields are immutable (default, no mut)

// lend, unique, share, belong → imply mut
fn update(p: lend Point) { p.x += 1; }  // p is mutable (lend implies mut)
let r: unique Resource = acquire();     // r's fields are mutable (unique implies mut)

// view → implies immutable
fn read(c: view Config) { ... }         // c is read-only (view implies immutable)

const PI = 3.14159;
const COUNT: mut = 0;
COUNT += 1;   // valid at compile time only

6.3 Destructuring

let [x, y, z]: f32 = 1.0f;             // grouped same type, related semantics
let name: string; let age: i32; // individual unrelated
let [x,y,z] = | 5,4,'c'|;   // pack literal — see [§6.4](#64-pack-literals)

// If the loop never runs, 'or' supplies the fallback value
let r = for ([acc, i]: i32), (i in 0..n) {
            acc *= i + 1
        } or 0;

6.4 Pack Literals

Packs group heterogeneous values into a lightweight tuple-like structure. They are declared with | | and can be destructured with [ ]:

// Pack literal
let p = | 5, 4, 'c' |;

// Destructure
let [a, b, c] = p;

// Used in for loops with type annotation
let r = for ([acc, i]: i32), (i in 0..n) {
            acc *= i + 1
        } or 0;

Packs are like anonymous structs — the compiler extracts fields by order and passes them as function arguments. They have a concrete layout determined at compile time. They are primarily used for destructuring and as loop accumulators.

Zith Language Specification — Draft v0.9