[swift-evolution] [Suggestion] Case-based dispatch for enum methods

[Brent Royal-Gordon]

> Is the lack of comments due to general disinterest in such a thing or did my mail go amiss somehow? ;)

I suspect it has something to do with the very similar thread I started about a week before you. :^) <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160321/013153.html> Your use of `default` is new and very interesting, though.

I've actually been thinking about this a little more, and it occurred to me that we might be able to borrow Joe Groff's accessor idea from the property behaviors proposal. As you may recall, accessors are a way to specify a method (or, perhaps, a computed property) that can be implemented by the property to customize the behavior; accessors are implemented in a curly-bracket-enclosed block attached to the property. In property behaviors, they look something like this:

	var behavior getObserver<Value>: Value {
		accessor func willGet() {}
		accessor func didGet() {}
		
		var value: Value
		
		get {
			willGet()
			let v = value
			didGet()
			return v
		}
		set {
			value = newValue
		}
		...
	}
	
	@getObserver var x: Int {
		willGet { print("Getting x") }
	} 

Using the same mechanism in enums might remove the redundant boilerplate caused by redeclaring the same methods in every case, and also reduce the appearance that cases are types. Your example might look like:

	enum MyTree {
		mutating accessor func insert(value: T)
		
		case Leaf1(Float, Float) {
			insert {
				…
			}
		}
		case Parent1(Int, Float) {
			insert {
				…
			}
		}
		default {
			insert {
				…
			}
		}
	}

Or, to use my example:

	enum Suit: Int {
		accessor var isRed: Bool { get }
		accessor var description: String { get }
		
		case hearts {
			description { return "♥️" }
			isRed { return true }
		}
		case spades {
			description { return  "♠️" }
		}
		case diamonds {
			description { return  "♦️" }
			isRed { return true }
		}
		case clubs {
			description { return  "♣️" }
		}
		default {
			isRed { return false }
		}
	}

-- 
Brent Royal-Gordon
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