A dedicated function for evaluating Void returning closures when Optional instance is not nil

[wadetregaskis]

@mattpolzin

If we are representing

if let number = number { ... }

with either

if number != nil { ... // number is not optional in this scope } 

or

if let number { ... // number is not optional in this scope }

Then the second option is my preference for a few reasons

1. if let is established enough to make the leap intuitive.
2. We don't accidentally eliminate any code from being possible. If we magically turn an optional into a non-optional within the scope of a conditional then there might be interesting (maybe uncommon) situations where we break code that relies on the variable retaining its optional type within the scope. I could try to come up with an example maybe later.
3. It feels less magical to me, which I guess is really just a corollary to point number 1. Rebinding based on a nil-check is established in Kotlin but it would be entirely new magic for Swift.

I want to agree unconditionally - especially because, as you note, the safety proof required for the first form is much higher as a result of it being a much broader and more powerful and thus potentially more edge-casey approach. Therefore it'll be more work to pitch. However, you then mention that Kotlin already does this - I know of Kotlin but am not that familiar with it; can you comment on how Kotlin does this and how well it's worked there?

The first approach implies a compiler mechanism with much broader implications, applications, and appeal (e.g. it probably ties in naturally if not as a prerequisite to constrained types and many other powerful language enhancements). If it does end up being implemented for those reasons, having a redundant if let optional { syntax - while not the end of the world - is, well, redundant.

Bikeshedding: I'm not sure I agree that the let number reads better than number != nil (though I'd get used to either form in practice, I'm sure). It's just that in plain English "if let number" is not a sentence, and it leaves the reader hanging with the natural question "if let number what?".