Why can you assign non-optional values to optional types in Swift?

Question

Why do assignments involving Swift optionals type check? For example in,

 var foo : Int? = 0
 foo = foo!

foo and foo! do not have the same

Answer 1

After reading rickster's answer, I came up with a simple laymen terms answer. To me the whole whole gist of his answer is

Since an optional indicates the presence or absence of a value, you shouldn't have to do anything special to indicate the presence of a value other than provide one

An optional is an enum. Which has 2 cases a or b.

                                      String?
                                         |
                                An enum with 2 cases
                                         |                               
                                 a               b
                                 |               |
                                Set            notSet
                                 |               |
                        any value like "hi"     nil

So you could do either of the things when you want to assign to an optional.

Say the value is either:

• a: it's set to "hi"
• b: it's not set, so it's nil
• c: it just equals to another enum ie another optional

code:

var str : String?
var anotherOptional : String?

str = nil // nil <-- this is like case b
str = "hi" // "hi" <-- this is like case a

str = anotherOptional // nil <-- this is like case c

anotherOptional = "hi again"
str = anotherOptional // "hi again" <-- this is like case c