[swift-evolution] [Proposal] Property behaviors

[Joe Groff]

> On Jan 19, 2016, at 3:10 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> The behavior list has to match exactly (or maybe as sets?).
> 
> Are you saying that there would be no ad-hoc composition of behaviors? This seems to imply that you'd need to implement every valid combination of behaviors by hand. That's a defensible position, given that it's easy to compose behaviors like "synchronized" in the wrong order, but significantly stifles behaviors like didSet/willSet that are more likely to be order-agnostic.

Perhaps we could use '...' to distinguish composable and non-composable behaviors:

// Implementation of exactly [lazy, synchronized]
behavior var [lazy, synchronized] _: Value where Self: Synchronizable { ... }

// Implementation of 'observed' composed with any other behavior
behavior var […, observed] _: Value { ... }

-Joe
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