A riff on Erica Saudi's Swift-y implementation of a distance calculator between two coordinates (http://ericasadun.com/2016/01/11/make-this-swift-er-coordinate-distances/)

CoordinateDistanceCalculatable.swift

import CoreLocation
import Darwin

private enum EarthUnits: Double {
  case ImperialRadius = 3961.0
  case MetricRadius = 6373.0
}

postfix operator ° {}
postfix func °(degrees: Double) -> Double {return degrees * M_PI / 180.0}

protocol CoordinateDistanceCalculatable {
  var latitude: Double { get }
  var longitude: Double { get }
}

extension CoordinateDistanceCalculatable {
  func milesFrom(coordinate: CoordinateDistanceCalculatable) -> Double {
    let radius = EarthUnits.ImperialRadius
    
    let (dLat, dLon) = (latitude - coordinate.latitude, longitude - coordinate.longitude)
    
    let (sqSinLat, sqSinLon) = (pow(sin(dLat° / 2.0), 2.0), pow(sin(dLon° / 2.0), 2.0))
    
    let α = sqSinLat + sqSinLon * cos(latitude°) * cos(coordinate.latitude°)
    
    return 2.0 * radius.rawValue * atan2(sqrt(α), sqrt(1.0 - α))
  }
  
  func kilometersFrom(coordinate: CoordinateDistanceCalculatable) -> Double {
    let radius = EarthUnits.MetricRadius
    
    let (dLat, dLon) = (latitude - coordinate.latitude, longitude - coordinate.longitude)
    
    let (sqSinLat, sqSinLon) = (pow(sin(dLat° / 2.0), 2.0), pow(sin(dLon° / 2.0), 2.0))
    
    let α = sqSinLat + sqSinLon * cos(latitude°) * cos(coordinate.latitude°)
    
    return 2.0 * radius.rawValue * atan2(sqrt(α), sqrt(1.0 - α))
  }
}

extension CLLocationCoordinate2D: CoordinateDistanceCalculatable {}