Swift: Append an element to an array in a dictionary value, creating the array/value if needed

Dictionary.Value+RangeReplaceableCollection.swift

import Foundation

extension Dictionary where Value: RangeReplaceableCollection {
    public mutating func append(element: Value.Iterator.Element, toValueOfKey key: Key) -> Value? {
        var value: Value = self[key] ?? Value()
        value.append(element)
        self[key] = value
        return value
    }
}

example.swift

var books: [String: [String]] = [
    "Phillip K Dick": [
        "Do Androids Dream of Electric Sheep?",
        "The Man in the High Castle",
    ],
    "Cory Doctorow": [
        "Down and Out in the Magic Kingdom",
        "Makers",
    ]
]

books.append(element: "Eastern Standard Tribe", toValueOfKey: "Cory Doctorow")
books
/*
[
    "Phillip K Dick": [
        "Do Androids Dream of Electric Sheep?",
        "The Man in the High Castle",
    ],
    "Cory Doctorow": [
        "Down and Out in the Magic Kingdom",
        "Makers",
        "Eastern Standard Tribe",
    ]
]
*/

books.append(element: "Parable of the Sower", toValueOfKey: "Octavia Butler")
books
/*
[
    "Phillip K Dick": [
        "Do Androids Dream of Electric Sheep?",
        "The Man in the High Castle",
    ],
    "Cory Doctorow": [
        "Down and Out in the Magic Kingdom",
        "Makers",
        "Eastern Standard Tribe",
    ],
    "Octavia Butler": [
      "Parable of the Sower",
    ]
]
*/