How can we check if b is in fact nil at line #4?

Question

Running the below piece of code on Xcode 9 playground, I noticed that nil does not equal nil. How can we check if b is in fact nil at line #4?

let s: String? = nil
var a: Optional<Any> = .some(s as Any)
if let b = a {
    if b == nil {
        print("b equals nil")
    } else {
        print("b doesn't equal nil. b is \(b)")
    }
}

UPDATE 1:

I do understand why the behavior is so. What I am looking for is how to check if b is nil since comparing it with nil doesn't work here.

UPDATE 2:

To avoid confusion, I changed the var name to b at line 3 (if let b = a)

UPDATE 3:

The answer turns out to be like this:

let s: String? = nil
var a: Optional<Any> = .some(s as Any)
if let b = a {
    if Mirror(reflecting: b).descendant("Some") as? String == nil {
        print("b equals nil")
    } else {
        print("b doesn't equal nil. b is \(b)")
    }
}

I think you are overlooking the fact that `a` is actually a constant now in line #4. Its no longer the `a` defined in line #2, since you are unwrapping it with the same name if line #3 i.e `if let a = a` — GoodSp33d, Oct 26 '17 at 05:52
`a` can't be `nil` on line 4. That's the whole point of `if let`. — rmaddy, Oct 26 '17 at 06:27
@maddy: That’s not true. `let x: Int? = nil; let xx: Int?? = .some(x); if let a = xx { print(a) }` prints `nil`. — zoul, Oct 26 '17 at 06:34
The way I see it, `a` _is_ `nil` on line 4 – `Optional.none`, to be precise. What doesn’t work as expected is the comparison to `nil` due to hiding the type behind `Any`. — zoul, Oct 26 '17 at 06:49
Related: https://stackoverflow.com/questions/34644128/why-non-optional-any-can-hold-nil — Martin R, Oct 26 '17 at 07:03
I have updated the question to make it clearer what I am asking. — Thanh Pham, Oct 26 '17 at 07:05
@zoul: The `a` on line 4 (now called `b`) has type `Any` and is not an optional. Therefore `if b == nil` always fails. — Martin R, Oct 26 '17 at 07:06
Agreed. (It’s still `nil`, though? It’s an optional, but seen as `Any` by the compiler.) Do you have an idea why the compiler even allows comparing `Any` to anything else? — zoul, Oct 26 '17 at 07:09
@zoul: The compiler "auto-wraps" `b` into an optional in the comparison to `if Optional(b) == nil { ... }` — Martin R, Oct 26 '17 at 07:13
@ThanhPham Don't edit your question to include the answer. Instead, post an actual answer below. — rmaddy, Oct 26 '17 at 18:09

Hexfire · Answer 1 · 2017-10-26T06:03:44.143

2

When you write:

let s: String? = nil

You basically create a generic enum value: s: Optional<String> = .none

enum Optional<T> {
    case .some(T)
    case .none
}

Then you wrap that value into a new enum as associated value for .some :

var a: Optional<Any> = .some(s as Any)

which is the same as typing:

var a: Optional<Any> = .some(Optional<String>.none as Any)

so a by itself is not nil. It contains .some value wrapped. When you unwrap it, you still get nested wrapped Optional. This is why you pass further than line #4. But inherent value of Optional is still nil. This is why you see it printed.

edited Oct 26 '17 at 06:03

answered Oct 26 '17 at 05:49

Hexfire

5,945
8
32
42

1

this should be the correct answer. as it explains that 'a' by itself is not nil but a generic Optional object with a 'Wrapped' value in it, thats why the if let statement succeeds. – Rishabh Oct 26 '17 at 05:54
what is the result if you assign as `let a:Any? = s as Any` – Jaydeep Vyas Oct 26 '17 at 06:00
IMHO the _real_ question here is why the comparison with `nil` in `a == nil` fails, and that’s because of the `Any` type. That’s what’s surprising about the code – simply nesting a `nil` inside another optional (`.some(Optional.none)`) would work as expected, printing `a equals nil`. – zoul Oct 26 '17 at 06:07
@zoul This is not the case, because here `a` is not `nil` in `a == nil` comparison. It's an enum value with the wrapped optional, which, in turn, is nil. – Hexfire Oct 26 '17 at 06:10
Try using `dump` just before the comparison, the outer optional was stripped by the `if let` assignment and the inner optional is `nil`. – zoul Oct 26 '17 at 06:12

Rishabh · Answer 2 · 2017-10-26T05:26:53.990

1

As the warning states:

Comparing non-optional of type Any to nil always returns false. Here the if let statement make 'a' variable a non-optional type and hence you always get false.

As for answering the original question, you have done it the right way thats how you can check for a nil value:

if a == nil {}

or like this

if (!a) {}

For more on how swift represents optional values check the answer in this question-

Why non optional Any can hold nil?

edited Oct 26 '17 at 05:26

answered Oct 26 '17 at 05:23

Rishabh

465
5
14

So how do we check if a is in fact nil at line 4? – Thanh Pham Oct 26 '17 at 05:25
sorry, i missed that. updated my answer now. – Rishabh Oct 26 '17 at 05:27

score 0 · Answer 3 · answered Oct 26 '17 at 05:36

0

I don't know why you are creating another variable a.

But I tried your code like this and it worked:

let s: String? = nil
let a: Any? = s
//if let a = a {
    if a == nil {
        print("a equals nil")
    } else {
        print("a doesn't equal nil. a is \(String(describing: a))")
    }
//}

answered Oct 26 '17 at 05:36

Amit

4,837
5
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I am getting `a equals nil` – Amit Oct 26 '17 at 05:38
@JaydeepVyas : Don't downvote immediately. At least try the code man. – Amit Oct 26 '17 at 05:40
you directly assign s replace with `s as Any` and it will not work – Jaydeep Vyas Oct 26 '17 at 05:41
@JaydeepVyas : Dude now seriously. I am getting the output in console man. – Amit Oct 26 '17 at 05:42
do you know the difference between `let a: Any? = s` And `var a: Optional = .some(s as Any)` – Jaydeep Vyas Oct 26 '17 at 05:52

score 0 · Answer 4 · answered Oct 26 '17 at 05:42

Nullable types present in swift as (<YourTypy>, nil). For example, String? presents as (String, nil). So, a type of myVar: String? can be equal String or nil.

When you use if let a = a structure, it moves into true branch only if type of a equals YourType, not nil. And in true branch you will have variable a with type YourType, not YourType?. So, a in true branch will never equal nil.

You should write notification about nil in false branch of if let

How can we check if b is in fact nil at line #4?

4 Answers4