semantic/prototype/Doubt/Memo.swift

public struct Memo<A> {
	public var value: A {
		return _value.value.value()
	}

	private var _value: MutableBox<Thunk<A>>
}

private final class MutableBox<A> {
	init(_ value: A) {
		self.value = value
	}

	var value: A
}

private enum Thunk<A> {
	case Evaluated(A)
	case Unevaluated(() -> A)

	mutating func value() -> A {
		switch self {
		case let .Evaluated(a):
			return a
		case let .Unevaluated(f):
			let a = f()
			self = .Evaluated(a)
			return a
		}
	}
}