Combining map with reduce

The following lines show an imperative code version of a for in loop that calculates the sum of all the highestScore values for the games:

var sum = 0
for game in gameRepository.getAll() {
    sum += game.highestScore
}
print(sum)

The code is very easy to understand. The sum variable has a starting value of 0, and each iteration of the for in loop retrieves a Game instance from the Array<Game> returned by the gameRepository.getAll method and increases the value of the sum variable with the value of the highestScore property.

We can combine the map and reduce operations to create a functional version of the previous imperative code to calculate the sum of all the highestScore values for the games. The next lines chain a call to map to a call to reduce to achieve this goal. Take a look at the following code:

let highestScoreSum = gameRepository.getAll().map({ $0.highestScore }).reduce(0, combine: {
    sum, highestScore in
    return sum + highestScore
})
print...