How exactly does the “let” keyword work in Swift?

Question

I\'ve read this simple explanation in the guide:

The value of a constant doesn’t need to be known at compile time, but you must assign it a value exa

Answer 1

Let

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Swift uses two basic techniques to store values for a programmer to access by using a name: let and var. Use let if you're never going to change the value associated with that name. Use var if you expect for that name to refer to a changing set of values.

let a = 5  // This is now a constant. "a" can never be changed.
var b = 2  // This is now a variable. Change "b" when you like.

The value that a constant refers to can never be changed, however the thing that a constant refers to can change if it is an instance of a class.

let a = 5
let b = someClass()
a = 6  // Nope.
b = someOtherClass()  // Nope.
b.setCookies( newNumberOfCookies: 5 )  // Ok, sure.

Let and Collections

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When you assign an array to a constant, elements can no longer be added or removed from that array. However, the value of any of that array's elements may still be changed.

let a = [1, 2, 3]
a.append(4)  // This is NOT OK. You may not add a new value.
a[0] = 0     // This is OK. You can change an existing value.

A dictionary assigned to a constant can not be changed in any way.

let a = [1: "Awesome", 2: "Not Awesome"]
a[3] = "Bogus"             // This is NOT OK. You may not add new key:value pairs.
a[1] = "Totally Awesome"   // This is NOT OK. You may not change a value.

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That is my understanding of this topic. Please correct me where needed. Excuse me if the question is already answered, I am doing this in part to help myself learn.