Unit types

unit-types.swift

import Foundation

@objc protocol MeasurementUnit {
  // If we have one of this unit, how many millimeters is it?
  class var asMillimeters: Double { get }
  var value: Double { get }
    
  init(_:Double)
}

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

  let value: Double
    
  required init(_ value:Double) {
    self.value = value
  }

}

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

  let value: Double
  required init(_ value:Double) {
    self.value = value
  }
}

func convert<T:MeasurementUnit, U:MeasurementUnit>(m: T) -> U {
  let factor = U.asMillimeters / T.asMillimeters
  return U(m.value * factor)
}

let in_measurement: MeasurementUnit = Inch(5)
let mm_measurement: Millimeter = convert(in_measurement)

// xcrun -sdk macosx swiftc unit-type.swift
// ... which then crashes.