[swift-users] Restricting associated values

[Travis Griggs]

> On Jun 18, 2017, at 10:33 PM, Howard Lovatt via swift-users <swift-users at swift.org> wrote:
> 
> To me Angle is a unit with two common representations: radians and degrees. It's not an enum because it doesn't have two values, it has one value that you can view in two ways.
> 
> Therefore I would make an Angle struct, something like:
> <snip>

Lots of different ways I think. I chose not between struct and enum, but used both:

struct Angle {
	enum Unit:CGFloat {
		case radians = 1.0
		case degrees = 57.29577951309314 // 360.0 / Tau
		case rotations = 0.1591549430918953 // 1.0 / Tau
	}
	
	// MARK: - Stored Properties
	var raw:CGFloat = 0.0
	var unit:Unit = .radians
        ….
        // MARK: - Left for the Student

I do a bit of UI programming with angles and have found rotations (I’ve drunk too much of the Tau manifesto koolaid probably) to be the most natural fit for a lot of things. Having an angle object allows me to layer over (via extensions) the various cocoa/uikit apis that take angles with type safe angle objects, and yet given me the ability to express said angles in whatever intermediate domain best fits.