Provide Custom Collections for Dictionary Keys and Values
This proposal address two problems:
- While a dictionary’s
keyscollection is fine for iteration, its implementation is inefficient when looking up a specific key, becauseLazyMapCollectiondoesn’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.[…]
Dictionary values can be modified through the keyed subscript by direct reassignment or by using optional chaining. Both of these statements append
1to the array stored by the key"one":// Direct re-assignment dict["one"] = (dict["one"] ?? []) + [1] // Optional chaining dict["one"]?.append(1)Both approaches present problems.
The proposed solution is an improvement but still seems a bit awkward.
Update (2017-02-20): Airspeed Velocity:
There are other changes likely for Swift 4 that would make this less awkward (e.g. to supply a default value when subscripting).
It’s just that this particular change was ABI impacting (changes the type of
Dictionary.Value) so was proposed during stage 1.
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