Swift optionals - Why does var a:Int? a? = 4 return nil

Question

Why does print(a) in the following code print nil?

var a:Int?
a? = 4
print(a)  //prints nil

var b:Int? = 4
print(b) //prints optional(4) 
         


        

Answer 1

Instead of ......... as as? is nil the entire statement won't run

a? = 4       ==   nil = 4

do

a = 4