[swift-evolution] Auto Unwrapping Of Optionals

[David Sweeris]

> On Apr 30, 2016, at 5:42 PM, Rod Brown <rodney.brown6 at icloud.com> wrote:
> 
> Re-sent for Swift Evolution. Response at end.
> 
> On 1 May 2016, at 6:31 AM, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
>>> On Apr 30, 2016, at 7:18 AM, Rod Brown via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> I think this specific proposal asking for compiler magic to auto-unwrap invisibly and only in very limited cases, as this proposal suggests, ends up breaking a lot more than it fixes. I can only see circumstances of this working with variables in the current scope, as anything like a property could be updated by other methods, threads etc, and the compiler couldn't be certain of state.
>>> 
>>> I think a language feature like you describe would be a lot more helpful, but I'd love to hear others' views on that.
>>> 
>>> - Rod
>> 
>> 
>> Yeah, auto-unwrapping "wherever it might be possible" seems too magical to me. I wouldn’t object to the compiler auto-unwraping optionals within a well defined code block, though:
>> //foo is T?
>> if foo != nil {
>> //foo is T within this set of curly braces
>> }
>> But even that invokes a bit of compiler magic, in that for this one type of enum (`Optional`), the compiler knows that if it isn’t one case, it must be the other. I’d prefer a more general solution…
>> 
>> What if the “is” keyword could function as a kind of incomplete switch?
>> var foo: UnicodeDecodingResult
>> ...
>> if foo is .Result {
>>     //since we know foo is a result, `foo` refers to foo's associated or raw value within this set of curly braces
>> }
>> This allows the language feature (and relevant compiler code paths) to be used with any enum, not just Optionals. The “optional unwrapping behavior" could then be written like this:
>> var bar = 4 as Int?
>> ...
>> if bar is .Some {
>>     //bar is 4 within this set of curly braces
>> }
>> 
>> - Dave Sweeris
> 
> I think your idea makes a lot more sense in respect to ensuring we don't have as much magic.
> 
> That said, I still wonder about the implications for thread safety etc. While it isn't a focus of Swift 3, it's something to think about whether this promotes a paradigm that cannot be supported in a threaded environment, specifically accessing properties.
> 
> The if-let paradigm is a lot stronger for this set of actions. It gains a separate reference or copy to the internal value, and allows you to action it safely. Should the property change in the meantime, it isn't relevant, because you have you own reference/copy, and then you have the right to re-set the property as required.
> 
> This, however, would theoretically add in an invisible ! for you. This leaves you unable to handle the situation should the variable have been changed by another thread between your check and your subsequent action.
> 
> Unless I'm missing something, I worry about the behaviour of such a "feature" in a multithreaded environment. I think the previous "inout" idea actually held a lot more weight in this regard - at least then you can act on the copy, and have the change propagate to the main declaration, and overwrite any changes made on another thread.


I think it would have the same resiliency as if-let, since I was envisioning this to just be syntactic sugar for a switch statement. That is, this:
if foo is .Result { //`foo` refers to foo's associated or raw value within the following code block
    //code block
}
would get rewritten to this, for enums with associated values:
switch foo {
case .Result(let foo): //we get a local copy of `foo` (the associated value) for the following code block
    //code block
default: break
}
or this, for enums with raw values:
switch foo {
case .Result:
    let _foo = foo.rawValue //the compiler substitutes `_foo` for `foo`
    //code block
default: break
}

There’d have to be some more auto-generated code to copy assigned values back into the original `foo`, but I don’t think it’d be hard to do.

- Dave Sweeris
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