Nil-Coalescing Operator without changing value [duplicate] - swift

Question

This question already has answers here:
Perform assignment only if right side is not nil
(7 answers)
Closed 5 years ago.
Nil-Coalescing Operators are one of my favorite things about Swift. Since becoming quite familiar with Swift, I've run into a few different special cases. One is where I want to assign an Optional value to a variable if it exists, otherwise, do nothing. I currently see two ways of doing this:
var a : String?
var b : String?
// Possibly assign a non-nil value to a and/or b
/* First Way */
a = b ?? a
/* Second Way */
if let b = b {
a = b
}
In this context, it seems like the first way is probably fine, but when variables get much longer names like mapViewController.destinationCardTitle, the first way can get pretty long. I also can't just assign the value of a to nil or an empty String because if it already contains a non-nil value and b = nil, I don't want to change the value of a.
I'm wondering if there is a way to basically do the following without writing a twice.
a = b ?? a

Answer 1

You can design your own infix operator:
infix operator ?=
func ?=<T>(lhs: inout T, rhs: T?) {
lhs = rhs ?? lhs
}
var a = "a"
var b: String? = "b"
a ?= b
print(a) // "b\n"

Answer 2

Since your concern is with long variable names, let's look at your possibilities with long variable names:
nil-coalescing:
aLongVariableName = bCrazyLongVariableName ?? aLongVariableName
if-let:
if let x = bCrazyLongVariableName {
aLongVariableName = x
}
if-nil:
if bCrazyLongVariableName != nil {
aLongVariableName = bCrazyLongVariableName
}
Of those three possible options, if let is the shortest amount of code since each long variable name is only shown once. But you have the overhead of if let x = and the curly braces. So you need to have variable names that are long enough to counter that extra syntax.
Personally, readability is more important than length. And copy and paste means far less typing. For this reason, using ?? is the much better solution. Its intent is clearer.
You asked about a way to avoid the 2nd a using ??. You can't. There is no other syntax that provides the real shortcut you are looking for.
You could write a function that takes a as an inout parameter and b as a normal parameter.
func x(_ a: inout String?, _ b: String?) {
if b != nil {
a = b
}
}
var a = String?
var b = String?
x(&a, b)

Answer 3

It sure is interesting - and this is when you have a golden opportunity to make the most out of Swift. Use the big hammer on this one:
var a: String?
var b: String?
func attempt_To_Set_A_New_Value_To_A(using b: Any?) -> String {
if b != nil {
guard b is String else {
// OH no! It's not a string!!!
return ""
}
// Just to make sure..
if let bString = b as? String {
return bString
}
}
// WARNING - b is definitely nil!
return ""
}
a = attempt_To_Set_A_New_Value_To_A(using b: b).isEmpty() ? a : attempt_To_Set_A_New_Value_To_A(using b: b)
If you want to have less fun, or you're simply having a bad day, I'd probably write it as such:
if b != nil { a = b }