Geometric types with units

type-with-unit.swift

public protocol MeasurementUnit {
  // If we have one of this unit, how many millimeters is it?
  class var asMillimeters: Double { get }
}

public class Millimeter : MeasurementUnit {
  public class var asMillimeters: Double {
    get {
      return 1.0
    }
  }
}
public class Inch : MeasurementUnit {
  public class var asMillimeters: Double {
    get {
      return 25.4
    }
  }
}

public struct Point<T:MeasurementUnit> {
  let x: Double = 0.0;
  let y: Double = 0.0;
		
  func convert<U:MeasurementUnit>() -> Point<U> {
    let scale = U.asMillimeters / T.asMillimeters
    let xc = x * scale
    let yc = y * scale
    return Point<U>(x:xc, y:yc)
  }
}

public func toIn<T:MeasurementUnit>(p:Point<T>) -> Point<Inch> {
  return p.convert()
}

let r_mm = Point<Millimeter>(x:1.0, y:2.0)
let r_in = toIn(r_mm)

println("r_in:\(r_in.x), \(r_in.y)\n")