Boom Boom Enum

To quote a fairly awful movie: “We were so, so wrong.” — me

Remember I said you couldn’t use Swift value types for a linked list? The real reason is because you can’t have references to other value instances, just copies (thus making bi-directional linked lists a recursion nightmare), but the compiler error was “a value type can’t refer to itself”.

Turns out, one Swift value type can refer to itself: enumerations. By adding the indirect keyword, you can use Swift enums to, for example, represent a binary tree structure:

indirect enum Tree {
	 case node(Int, left: Tree?, right: Tree?)
}

And it works! But is it a good idea? Hell, no!

Why not? Because accessing the “properties” of your data structure is a pain in the ass:

switch node {
case let .node (data, left, right):
	// Do something with data, left, and right
}

As far as I know, you need an otherwise extraneous switch statement, a case statement, and an extra set of local variables just to access the values. (Whereas for a class all you need is dot syntax.) And you need to do that everywhere you want to access them, every method.

And the compiler will complain if you don’t use all of the enumeration values, so you have to remember to use _ for those:

switch node {
case let .node (_, left, _):
	// Do something with left
}

I tried writing a full tree traversal implementation in Swift with enum-based trees and it was an unholy mess that I would not repeat.

Learn from me. Don’t use cool language features at the expense of maintainability.