[swift-evolution] Propagating Optionals

William Sumner prestonsumner at me.com
Sun Sep 25 18:26:46 CDT 2016

> On Sep 25, 2016, at 2:19 PM, Trans via swift-evolution <swift-evolution at swift.org> wrote:
> As I've been learning Swift recently, one aspect of the language
> jumped out at me with a "code smell". Namely, the way Optionals are
> handled. For starters, just consider how long this chapter is:
> https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/OptionalChaining.html
> That's pretty beefy for something on the surface is pretty simple.
> More concretely, consider the example given:
>   class Person {
>       var residence: Residence?
>   }
>   class Residence {
>       var numberOfRooms = 1
>   }
>   let john = Person()
>   let roomCount = john.residence.numberOfRooms
>   // error: value of optional type 'Residence?' not unwrapped; did
> you mean to use '!' or '?'?
> As general rule of thumb, whenever I get an error and the system tells
> me what I probably meant, that is a pretty good sign the system isn't
> doing all it can for me and making me jump through an unnecessary
> hoop.
> Basically "john.residence.numberOfRooms" is a completely wasted
> expression -- it's meaningless. You have to put a `?` or `!` in there
> to get anything useful. I can't see any good reason for that.
> "john.residence.numberOfRooms" could just behave one way or the other,
> either as if the `?` were there, or the `!`. And of the two, the
> obvious choice is `?` because... I already told the program it the was
> optional in "var residence: Residence?".  I said it was optional, and
> yep I meant that. (Reminds me of the old retort "did I stutter?")
> Thus, if I try to assign it to something else it too should be
> optional. If I want it to be otherwise I'd add the `!`.
> Making this change would just simplify a whole mess of code and about
> half that chapter would all but vanish.

You have to be explicit about how to handle nullability because you’re attempting to access the property of an optional property.

> In addition, seeing that `!` acts a short-circuit to error, it would
> be nice to have something equivalent for fallback value. We can't use
> `??` b/c it doesn't chain, though maybe it could be made to? And I'd
> rather not reuse `?.` here  (for reasons I can explain later). Maybe
> `:` is a good choice? In any case, exact syntax aside,
>   let homelessShelter = Residence()
>   let roomCount = john.residence:homelessShelter.numberOfRooms
> Now, roomCount will not be optional, because there is a guaranteed value.
> I think simplifying Optionals this way would be a good fit for Swift,
> making this part of the language a whole lot cleaner and clearer.

You can accomplish this using parenthesis:

let roomCount = (john.residence ?? homelessShelter).numberOfRooms


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