Semigroupy grouping magic in Swift

Semigroupy.swift

// What people often ask DefaultDict for, could be done with just one
// extension method for Optional.

extension Optional {
    mutating func mappend(z: Wrapped, _ f: (Wrapped, Wrapped) -> Wrapped) {
        self = map {x in f(x, z)} ?? z
    }
}

// --- Examples ------------------------------------------------------------

let strings = ["foo", "bar", "xyzzy", "oh", "good", "bad", "great"]

var groups = [Int: [String]]()

for s in strings {
    groups[s.characters.count].mappend([s], +)
}

print(groups) //=> [5: ["xyzzy", "great"], 2: ["oh"], 3: ["foo", "bar", "bad"], 4: ["good"]]

// ---

var counts = [Int: Int]()

for s in strings {
    counts[s.characters.count].mappend(1, +)
}

print(counts) //=> [5: 2, 2: 1, 3: 3, 4: 1]