[swift-evolution] [Proposal] Property behaviors

[Chris Lattner]

> On Dec 17, 2015, at 4:36 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
>>  
>> It looks to me that the only benefit this has versus IOUs is you can use a `let` instead of a `var`. It's worth pointing out that this actually doesn't even replace IOUs for @IBOutlets because it's commonly useful to use optional-chaining on outlets for code that might run before the view is loaded (and while optional chaining is possible with behavior access, it's a lot more awkward).
> 
> There are several advantages over IUO that I see. The behavior can ensure that a mutable delayed var never gets reset to nil, and can dynamically enforce that an immutable delayed var/let isn't reset after it's been initialized. It also communicates intent; as I mentioned to Matthew in another subthread, 

Another benefit is that this allows elimination of an entire use-case for IOUs.  This makes it more appetizing and possible to push IOUs further off into a corner.  I’m personally interested in the idea of IOUs evolving into something that isn’t a first class type - instead they could only exist in argument and return types.  Eliminating a common reason that people use them for properties would help with that.

-Chris
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