:= and = do different jobs. := writes a value somewhere. = compares two values and produces a Boolean.

Bind a name

Use let total := 42; when you introduce a name.

let port := 8080;
let host := "localhost";

The name on the left receives the value on the right. This is not a comparison.

Assign a mutable place

Use mut when the place must change later. Use := again to write the new value.

let port := mut 8080;
port := port + 1;

The second line reads the old port, adds one, and writes the result back into port.

Use = to ask whether two values match. Use /= to ask whether they differ.

let actual := 8081;
let isDefault := actual = 8080;
let changed := actual /= 8080;

isDefault is .False. changed is .True.

Do not use = when a name needs a value.

- let port = 8080;
+ let port := 8080;

Use = only when the result should be a Bool value.