Swift 4 Keypath for grouping sequences

Sequence+groupBy.swift

import Foundation

internal extension Sequence {
    func grouped<T>(by keyPath: KeyPath<Element, T>) -> [T: [Element]] {
        var groups = [T: [Element]]()
        for element in self {
            let key = element[keyPath: keyPath]
            if !groups.keys.contains(key) {
                groups[key] = [Element]()
            }
            groups[key]?.append(element)
        }
        return groups
    }
}