Multimap.md

Multimap.swift

Being able to insert into a Dictionary<Key, Array<Values>> without having to explicitly handle the fact that the value array is nil on the very first insert for a given key.

The code isn't difficult, but it is repetitive boilerplate. And it is also a decent example of extending a generic type in Swift.

Multimap.swift

import Foundation


extension Dictionary where Value: RangeReplaceableCollection {
	/// Append a new value to a Dictionary being treated as a multimap. The dictionary can map to any `RangeReplaceableCollection` (of values).
	mutating func append(_ value: Value.Element, for key: Dictionary.Key) {
		var array = self[key]
		if array == nil {
			array = Value() // RangeReplaceableCollection includes an init() requirement: see https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20161219/029658.html
			self[key] = array
		}

		array!.append(value)
	}
}