[swift-evolution] Auto Unwrapping Of Optionals

[Michael Peternell]

I thought about this too, and I think it would be convenient. It feels a bit like artificial intelligence though, and I'm not sure how easy it would be to implement this. It should be sharply defined what the compiler should infer and what not. This can be very hard.

For example, you write `print("Today is \(today)")`: should it print an Optional or should it print just the date, because it knows that it cannot be nil? I say it should print `.Optional(... 23:37)`. But what if it prints just the date? In this case `today` cannot be nil, so it would be safe to just print `23:37`. Now, I see the source code example for the first time. What thinking process do I have to go through to know if the variable is automatically unwrapped or not? I'll think "can this variable be nil at that place?" The compiler also thinks "can this variable be nil at that place?" Will the compiler and me always agree in this respect? Or do I have to think "Will the compiler think that the variable can be nil at this place? Can it decide that it can never be nil?"? In this case, the language user needs to have a mental model about what inferences the compiler is able to do, so the rules must be fixed and easily understandable. The model would probably be something along the lines of "NSDate() always returns a non-optional value, so `x ?? NSDate()` is non-optional too." But knowing that foo(3) will return a value because foo only returns nil if its argument is < 0 is probably out of scope, even if the foo-function is defined within the same source file and even if the `-O2` flag is turned on (because you don't want to have different semantics depending on how much optimization is turned on).

Therefore, I think it would be best to only apply implicit unwrapping in cases where the code would otherwise not compile.

E.g. now you have to write

    var x: Int? = someFunction()
    if(x == nil) { return 22 }
    return x!+2+(2*x!);

It should be possible to rewrite this as

    var x: Int? = someFunction()
    if(x == nil) { return 22 }
    return x+2+(2*x);

This should only work for variables defined within the same scope, so this should fail to compile:

    // type of self.foo is String
    if(self.foo == nil) { return 11 }
    // type of myFunc is String -> ()
    myFunc(self.foo)
  
because even though self.foo was non-nil 1 microsecond ago, it doesn't mean that it is non-nil now.

On the other hand, what happens if a variable is captured by a block? Do we know that the variable is not mutated from another thread? Should we disable all these rules as soon as there is a block somewhere that captures one of the variables and might mutate them?

I just started thinking... For all these reasons, I don't see how this is going to be implemented in a way that is consistent and that doesn't introduce all kinds of strange edge cases. And any solution should be comprehensible by a Swift-beginner IMHO. On the other hand, if I ever forget a "!" somewhere, and the compiler compiles it anyways because it knows that it is safe, I will not complain :)

-Michael

> Am 29.04.2016 um 16:37 schrieb Tod Cunningham via swift-evolution <swift-evolution at swift.org>:
> 
> I'm new to the swift evolution community, but I wanted to toss an idea out there to get some feedback on it. So here it goes...
> 
> Currently, if you assign a non-nil value to an optional and then want to access that optional later, in the same context, you need to manually unwrap the value.  This is usually done either by using "!" or by using something like "if let" or guard.
> 
> What would it be like if the compiler could auto unwrap, in cases where in knows the optional will have some value?  This would make the code "clean" and still be safe.
> 
> This concept of Auto Unwrapping of Optionals is similar to Implicitly Unwrapped Optionals, but is only applied when the compiler knows it is safe to do so.
> 
> Take the following example:
> 
> class Test {
>    var today: NSDate? = nil
>    func test() {
>        today = today ?? NSDate()
>        print("Today is \(today)")   // Would be printed as an optional
>        let timeInterval: NSTimeInterval = today!.timeIntervalSinceNow  // Requires ! or (if let) to unwrap
>        // ... do stuff with timeInterval ...
>    }
> }
> 
> With the above example, the compiler could known that today has a value after it's set in the test method.  So why couldn't the compiler auto unwrap it when accessed?  This would mean manual unwrapping would be unnecessary:
> 
> class Test {
>    var today: NSDate? = nil
>    func test() {
>        today = today ?? NSDate()
>        print("Today is \(today)")   // Would be printed as a value (not an optional)
>        let timeInterval: NSTimeInterval = today.timeIntervalSinceNow  // No ! required (auto unwrapped)
>        // ... do stuff with timeInterval ...
>    }
> }
> 
> If the value later gets set to an optional value, then it will no longer be auto unwrapable :
> 
> class Test {
>    var today: NSDate? = nil
> 
>    func optionalDay() -> NSDate? {
>        return NSDate()
>    }
> 
>    func test() {
>        today = today ?? NSDate()
>        print("Today is \(today)")   // Would be printed as a value (not an optional)
>        let timeInterval: NSTimeInterval = today.timeIntervalSinceNow    // No ! required (auto unwrapped)
>        let timeInterval2: NSTimeInterval = today!.timeIntervalSinceNow  // Explicit unwrapping would still be allowed
> 
>        // If today is assigned an optional value, we can no longer auto unwrap it
>        today = optionalDay()
>        print("Today is \(today)")   // Would be printed as an optional
>        let timeInterval3: NSTimeInterval = today!.timeIntervalSinceNow  // manual unwrapping would be required
>    }
> }
> 
> Note in the above example, explicit unwrapping would still be allow.  The variable is still an optional.  This allows for existing code to remain unchanged.
> 
> This change would encourage less use of forced unwrapping "!", generally require the developer to write less code, and would maintain code safety.  On the down side, it is performing some compiler “magic”.  It would be yet another thing to explain when trying to introduce people to swift and especially optionals.
> 
> What do you all think, would something like this be worth pursuing, what other pluses or minus would this introduce, has something like this already been discussed?
> 
> Thanks,
> Tod Cunningham
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution