[swift-evolution] Dictionary Enhancements

[Brent Royal-Gordon]

> On Feb 16, 2017, at 4:26 PM, Ben Cohen via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 	• init from/merge in a Sequence of Key/Value pairs (already raised as SE-100: https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md).

Yes, I think this is a great idea.

> 	• make the Values view collection a MutableCollection (as in this PR: https://github.com/apple/swift-evolution/pull/555).

Sounds good to me.

> 	• Add a defaulting subscript get (e.g. counts[key, default: 0] += 1 or grouped(key, default:[]].append(value)).

I'd really like this to be a broader mechanism—either somehow permitting assignment through `??`:

	(counts[key] ?? 0) += 1

Or having some kind of `Dictionary` variant with a default value:

	var counts = DefaultedDictionary(elements, default: 0)
	counts[key] += 1

> 	• Add a group by-like init to create a Dictionary<K,[V]> from a sequence of V and a closure (V)->K.

Honestly, this is such a good idea that I might want it to be an extension method in `Sequence` or `Collection`:

	extension Sequence {
		func grouped<Key: Hashable>(by makeKey: (Element) throws -> Key) rethrows -> [Key: Element] {…}
	}

We don't expose `map` and `filter` as `Array` initializers; why would `grouped(by:)` be one?

> 	• Add Dictionary.filter to return a Dictionary.

Sure. If it took a `(Key, Value) -> Bool` predicate, that could even distinguish between the `Array`-returning version and the `Dictionary` one.

> 	• Add Dictionary.mapValues to return a Dictionary (can be more efficiently implemented than composition as the storage layout remains the same).

Yes, this is something I want.  So do 114 Stack Overflow users: <http://stackoverflow.com/questions/24116271/whats-the-cleanest-way-of-applying-map-to-a-dictionary-in-swift/24219069#24219069>

I'd actually prefer that this *not* take a `Key` parameter, because that's more likely to interfere with point-free styles of programming. Reasonable people can disagree on that, though.

> 	• Add capacity property and reserveCapacity() method.

Eh, maybe.

> 	• Have Dictionary.removeAtIndex return the Index of the next entry.

I'm not sure what the point of this is, and I'm not sure if you think this should also be done to equivalent APIs on e.g. `RangeReplaceableCollection`.

> 	• (once we have conditional conformance) Make dictionaries with Equatable values Equatable.

No-brainer—definitely want this one.

> Please reply here with any comments or questions on the above list, or any additions you believe are important that are missing from it.

I'd sort of like to see a protocol for key-value lookup types, sort of like `SetAlgebra` but for dictionaries. I've recently been building a custom dictionary (a `SortedDictionary` type which sorts its elements by key) and not having a protocol to describe dictionaries abstractly is a bit of an irritant.

-- 
Brent Royal-Gordon
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