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Provide Custom Collections for Dictionary Keys and Values


This proposal addresses significant unexpected performance gaps when using dictionaries. It introduces type-specific collections for a Dictionary instance's keys and values properties.

New collection types provide efficient key lookup and mutable access to dictionary values, allowing in-place updates and copy-on-write optimization of stored values. The addition of these new types impacts the standard library ABI, since we won't be able to use types aliases from the existing types for keys and values.

Swift-evolution thread: [Proposal Draft] Provide Custom Collections for Dictionary Keys and Values


This proposal address two problems:

  • While a dictionary's keys collection is fine for iteration, its implementation is inefficient when looking up a specific key, because LazyMapCollection doesn't know how to forward lookups to the underlying dictionary storage.
  • Dictionaries do not offer value-mutating APIs. The mutating key-based subscript wraps values in an Optional. This prevents types with copy-on-write optimizations from recognizing they are singly referenced.

This proposal uses the following [String: [Int]] dictionary to demonstrate these problems:

var dict = ["one": [1], "two": [2, 2], "three": [3, 3, 3]]

Inefficient dict.keys Search

Swift coders normally test key membership using nil checks or underscored optional bindings:

if dict["one"] != nil {
    // ...
if let _ = dict["one"] {
    // ...

These approaches provide the expected performance of a dictionary lookup but they read neither well nor "Swifty". Checking the keys view reads much better but introduces a serious performance penalty: this approach requires a linear search through a dictionary's keys to find a match.

if dict.keys.contains("one") {
    // ...

A similar dynamic plays out when comparing dict.index(forKey:) and dict.keys.index(of:).

Inefficient Value Mutation

Dictionary values can be modified through the keyed subscript by direct reassignment or by using optional chaining. Both of these statements append 1 to the array stored by the key "one":

// Direct re-assignment
dict["one"] = (dict["one"] ?? []) + [1]

// Optional chaining

Both approaches present problems. The first is complex and hard to read. The second ignores the case where "one" is not a key in the dictionary, and is therefore less useful even if more streamlined. Furthermore, neither approach allows the array to grow in place—they introduce an unnecessary copy of the array's contents even though dict is the sole holder of its storage.

Adding mutation to a dictionary's index-based subscripting isn't possible. Changing a key stored at a particular index would almost certainly modify its hash value, rendering the index incorrect. This violates the requirements of the MutableCollection protocol.

Proposed Solution

This proposal adds custom collections for the keys and values dictionary properties. This follows the example set by String, which presents multiple views of its contents. A new Keys collection introduces efficient key lookup, while a new Values collection provides a mutable collection interface to dictionary values. Both new collections are nested in the Dictionary type.

These changes make the simple approach for testing whether a dictionary contains a key an efficient one:

// Fast, not slow
if dict.keys.contains("one") {
    // ...

As a mutable collection, values enables modification without copies or clumsy code:

if let i = dict.index(forKey: "one") {
    dict.values[i].append(1)  // no copy here
} else {
    dict["one"] = [1]

Both the keys and values collections share the same index type as Dictionary. This allows the above sample to be rewritten as:

// Using `dict.keys.index(of:)`
if let i = dict.keys.index(of: "one") {
} else {
    dict["one"] = [1]

Detailed design

  • The standard library introduces two new collection types: Dictionary.Keys and Dictionary.Values.
  • A Dictionary's keys and values properties change from LazyMapCollection to these new types.
  • The new collection types are not directly constructable. They are presented only as views into a dictionary.
struct Dictionary<Key: Hashable, Value>: ... {
    /// A collection view of a dictionary's keys.
    struct Keys: Collection {
        subscript(i: Index) -> Key { get }
        // Other `Collection` requirements

    /// A mutable collection view of a dictionary's values.
    struct Values: MutableCollection {
        subscript(i: Index) -> Value { get set }
        // Other `Collection` requirements
    var keys: Keys { get }
    var values: Values { get set }    
    // Remaining Dictionary declarations

Impact on existing code

The performance improvements of using the new Dictionary.Keys type and the mutability of the Dictionary.Values collection are both additive in nature.

Most uses of these properties are transitory in nature. Adopting this proposal should not produce a major impact on existing code. The only impact on existing code exists where a program explicitly specifies the type of a dictionary's keys or values property. In those cases, the fix would be to change the specified type.

Alternatives considered

  1. Add additional compiler features that manage mutation through existing key-based subscripting without the copy-on-write problems of the current implementation. This could potentially be handled by upcoming changes to copy-on-write semantics and/or inout access.

  2. Provide new APIs for updating dictionary values with a default value, eliminating the double-lookup for a missing key. The approach outlined in this proposal provides a way to remove one kind of double-lookup (mutating a value that exists) but doesn't eliminate all of them (in particular, checking for the existence of a key before adding).

    These could be written in a variety of ways:

    // Using a 'SearchState' type to remember key position
    // Using a two-argument subscript
    dict["one", withDefault: []].append(1)
    // Using a closure with an inout argument
    dict.withValue(forKey: "one") { (v: inout Value?) in
        if v != nil {
        } else {
            v = [1]
  3. Restructure Dictionary's collection interface such that the Element type of a dictionary is its Value type instead of a (Key, Value) tuple. That would allow the Dictionary type itself to be a mutable collection with an entries or keysAndValues view similar to the current collection interface. This interface might look a bit like this:

    let valuesOnly = Array(dict)
    // [[2, 2], [1], [3, 3, 3]]
    let keysAndValues = Array(dict.entries)
    // [("two", [2, 2]), ("one", [1]), ("three", [3, 3, 3])]
    let foo = dict["one"]
    // Optional([1])
    let i = dict.keys.index(of: "one")!