[swift-evolution] [Proposal] Property behaviors

[Matthew Johnson]

I have been thinking further about the compiler diagnostics for `delayed`.  It might be interesting to consider making various compiler diagnostics available to any behavior rather than having a special case in the compiler for `delayed`.  

Here are some examples:

* By default properties with a behavior must be initialized in phase one just like normal properties.
* Behaviors can opt-in to a relaxed requirement that the property must be initialized *somewhere* in the initializer, but not necessarily phase one.  Delayed would opt-in to this.
* Behaviors can opt-in to a requirement that the property *cannot* be set outside of an initializer.  Delayed would opt-in to this.
* Behaviors can opt-in to a requirement that the property *cannot* be set anywhere.  A variation of lazy might opt-in to this.  (clear would still work as it is part of the implementation of lazy)

Allowing behaviors to specify diagnostic behavior like this would probably be *possible* in the ad-hoc proposal.  However, it would probably be a lot more clear and elegant if we adopt the “behavior declaration” idea.

Matthew


> On Dec 21, 2015, at 11:23 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I played around a bit with the idea of a special behavior declaration. I think it feels a lot nicer, though it also feels like a much bigger language change if we go this route. Inside the declaration, you need to specify:
> - what accessors the behavior supports, to be implemented by properties using the behavior,
> - if the behavior controls storage, what that storage is, and what initialization logic it requires,
> - if the behavior requires an initializer, and whether that initializer is used eagerly at property initialization or deferred to later, and
> - what operations the behavior offers, if any.
> 
> Here's a quick sketch of how a behavior declaration could look. As a strawman, I'll use 'var behavior' as the introducer for a property behavior (leaving the door open to 'func behavior', 'struct behavior', etc. in the possible future). If you were going to reinvent computed properties from whole cloth, that might look like this:
> 
> var behavior computed<T> {
>   // A computed property requires a `get` and `set` accessor.
>   accessor get() -> T
>   accessor set(newValue: T)
> 
>   // Accessors for the property
>   get { return get() }
>   set { set(newValue) }
> }
> 
> lazy might look something like this:
> 
> var behavior lazy<T> {
>   // lazy requires an initializer expression, but it isn't
>   // used until after object initialization.
>   deferred initializer: T
> 
>   // The optional storage for the property.
>   var value: T?
> 
>   // Initialize the storage to nil.
>   init() {
>     value = nil
>   }
> 
>   // Accessors for the property.
>   mutating get {
>     if let value = value {
>       return value
>     }
>     // `initializer` is implicitly bound to the initializer expr as a
>     // `@noescape () -> T` within the behavior's members.
>     let initialValue = initializer()
>     value = initialValue
>     return initialValue
>   }
> 
>   set {
>     value = newValue
>   }
> 
>   // clear() operation for the behavior.
>   mutating func clear() {
>     value = nil
>   }
> }
> 
> Some behaviors like `lazy` and `resettable` want to take control of the storage to manage their semantics, but many behaviors are adapters independent of how the underlying behavior behaves. These kinds of behavior are easy to compose with other behaviors and to override base class properties with. You could use inheritance-like syntax to indicate a "wrapping" behavior like this, and commandeer `super` to refer to the underlying property. For instance, `synchronized`:
> 
> var behavior synchronized<T>: T {
>   get {
>     return sync { return super }
>   }
>   set {
>     return sync { return super }
>   }
> }
> 
> or `observing` didSet/willSet:
> 
> var behavior observing<T>: T {
>   accessor willSet(oldValue: T, newValue: T) { }
>   accessor didSet(oldValue: T, newValue: T) { }
> 
>   get { return super }
>   set {
>     let oldValue = super
>     willSet(oldValue, newValue)
>     super = newValue
>     didSet(oldValue, newValue)
>   }
> }
> 
> If you want to refer back to the containing `self`, we could support that too, and by treating behavior functions specially we should be able to maintain coherent semantics for backreferencing value types as well. Implementing `synchronized` with a per-object lock could look like this:
> 
> var behavior synchronizedByObject<T>: T where Self: Synchronizable {
>   get {
>     return self.withLock { return super }
>   }
>   set {
>     return self.withLock { return super }
>   }
> }
> 
> (though the juxtaposed meanings of `super` and `self` here are weird together…we'd probably want a better implicit binding name for the underlying property.)
> 
> -Joe
> 
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