Open Radar

 
Summary:
Properties have some great functionality: they can be lazy, computed, stored, and constant; the willSet/didSet hooks are extremely handy. Providing an abstraction for interacting with these properties would enable a lot of very convenient proxies/decorators, like observable properties, copy-on-write properties, etc.

Currently you can implement these manually:

class Container<T> {
	var contents: T {
		get {
			return observableContents.value
		}
		set {
			observableContents.value = newValue
		}
	}
	private var observableContents: ObservableProperty<T> // assume this exists
}

However, this adds a lot of noise and boilerplate. If ObservableProperty<T>, CopyOnWriteProperty<T>, etc. could instead adopt a protocol in the language, these implementation details could be made much more convenient, up to the point of being transparent (i.e. drop-in) to adopt and use, while still being visible in the types as documentation:

class Container<T> {
	var contents: ObservableProperty<T> // seeing that this is an ObservableProperty<T>, which conforms to Swift.Property, the language can now ensure that get/set/willSet/didSet are all performed using ObservableProperty<T>’s implementations
}

A list of things this would enable:

- observable properties

- copy on write

- lazy evaluators in the vein the delay/force functions in Scheme:

protocol RecursiveExpression { … }

class RecursiveBinaryExpression : RecursiveExpression {
	let left: Lazy<RecursiveBinaryExpression>
	let right: Lazy<RecursiveBinaryExpression>
}

- automatically invoked reference type boxes for e.g. recursive structs

struct List<T> {
	var value: T
	var next: Box<List<T>?>
}

- memoizing (in the vein of the language’s extant lazy, and that sweet memoize {} combinator I keep going on about)

- similarly, *fixpoints* (ooooh, ahhhh)

- debug logging

typealias Infrastructure = CriticalInfrastructure<LogLevel.Error> // set to Error in development, SilentImplosion in production
class CriticalInfrastructure<LogLevel> {
	var crucialDetails: Log<LogLevel, Int>
}


Steps to Reproduce:
N/A

Expected Results:
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Actual Results:
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Regression:
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Notes:
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