Most names should stay stable. When a place must change, mark it with mut.

let counter := mut 1;
counter := counter + 1;
counter;

mut is a warning sign for the reader. It says, "This place may hold a different value later."

Think of a stable name like a printed receipt. Think of a mutable name like a whiteboard total that changes while people add items.

Use mutation for small local steps. For larger changes, prefer making a new value with a clear name.

Mutable places

Use mut when a name must be assigned again.

let count := mut 0;
count := count + 1;     -- 1
count := count + 1;     -- 2

The name count is a place. Each := after the first writes a new value into that place.

Prefer values first

Most code is easier to read when values do not change.

let count := 0;
let next := count + 1;      -- 1
let final := next + 1;      -- 2

Use mutation for loops, builders, and stateful algorithms. Use new names for simple transformations.