LazyStore.swift

struct LazyStore<Key: Hashable, Value> {

    private var store: [Key : Value] = [:]

    let builderClosure: Key -> Value?

    init(builderClosure: Key -> Value?) {
        self.builderClosure = builderClosure
    }

    private mutating func insertIfNeeded(key: Key) {
        guard store[key] == nil else {
            return
        }
        store[key] = builderClosure(key)
    }

    mutating func addObjectForKey(key: Key) {
        insertIfNeeded(key)
    }

    mutating func valueForKey(key: Key) -> Value? {
        insertIfNeeded(key)
        return store[key]
    }

}

var myStore = LazyStore<String, Int> { $0.hashValue }

myStore.valueForKey("Hello")