A Swift Playground for total distance functions on lists of CLLocation objects. https://medium.com/@kewindannerfjordremeczki/swift-4-2-corelocation-total-distance-traveled-ff881ba2a1ce

TotalDistance.swift

import CoreLocation

/// The locations used for the examples.
let locations = [CLLocation(latitude: 10.0, longitude: 12.0),
                 CLLocation(latitude: 10.1, longitude: 12.0),
                 CLLocation(latitude: 10.2, longitude: 12.0),
                 CLLocation(latitude: 10.1, longitude: 12.0)]

/* A BOILERPLATE SOLUTION */

func totalDistance(of locations: [CLLocation]) -> CLLocationDistance {
    var distance: CLLocationDistance = 0.0
    var previousLocation: CLLocation?
    
    locations.forEach { location in
        if let previousLocation = previousLocation {
            distance += location.distance(from: previousLocation)
        }
        previousLocation = location
    }
    
    return distance
}

print(totalDistance(of: locations)) // Prints 33182.563139193335


/* A RECURSIVE SOLUTION */

var mutableLocations = locations
func totalDistance(initialDistance: CLLocationDistance, initialLocation: CLLocation?, locations: inout [CLLocation]) -> CLLocationDistance {
    if locations.isEmpty {
        return initialDistance
    } else if let initialLocation = initialLocation {
        let firstLocation = locations.removeFirst()
        let distance = firstLocation.distance(from: initialLocation)
        return totalDistance(initialDistance: initialDistance + distance, initialLocation: firstLocation, locations: &locations)
    }
    return totalDistance(initialDistance: initialDistance, initialLocation: locations.removeFirst(), locations: &locations)
}

print(totalDistance(initialDistance: 0.0, initialLocation: nil, locations: &locations)) // Prints 33182.563139193335


/* REDUCE EXAMPLE */

print([2, 5, 3, 10].reduce(0, +)) // Prints 20

/* A REDUCED SOLUTION */

/// First closure example
let closure1: ((CLLocationDistance, CLLocation?), CLLocation) -> (CLLocationDistance, CLLocation?) = { tuple, location in
    guard let previousLocation = tuple.1 else {
        return (tuple.0, location)
    }
    return (tuple.0 + location.distance(from: previousLocation), location)
}

print(locations.reduce((0.0, nil), closure1).0) // Prints 33182.563139193335

/// Second closure example
let closure2: ((CLLocationDistance, CLLocation?), CLLocation) -> (CLLocationDistance, CLLocation) = { tuple, location in
    return (tuple.0 + location.distance(from: tuple.1 ?? location), location)
}

print(locations.reduce((0.0, nil), closure).0) // Prints 33182.563139193335

/// The one-liner solution
func totalDistanceReduce(of locations: [CLLocation]) -> CLLocationDistance {
    return locations.reduce((0.0, nil), { ($0.0 + $1.distance(from: $0.1 ?? $1), $1) }).0
}

print(totalDistanceReduce(of: locations)) // Prints 33182.563139193335


/* THE ICING ON THE CAKE */

extension Array where Element: CLLocation {
    
    /// Returns the total distance of the list of CLLocation objects.
    var totalDistance: CLLocationDistance {
        return reduce((0.0, nil), { ($0.0 + $1.distance(from: $0.1 ?? $1), $1) }).0
    }
}

print(locations.totalDistance) // Prints 33182.563139193335