[Pitch] - Add `DictionaryProtocol` to the standard library

[AlexanderM (Alexander Momchilov)]

Hey Ben!

@Ben_Cohen

Creating a protocol purely to allow sugaring initializers as chained methods seems insufficient justification.

It's not only sugar for the initializers. I proposed a modification to @lorentey's idea:

@AlexanderM

I'd suggest taking an instance instead. You can still pass an empty instance, but this has the added utility of letting you accumulate into a pre-existing instance you might have. This would unlock the ability to use the convenience of map/filter/etc. in performance sensitive contexts, where you can skip the allocation (because you can reuse one "scratch" collection over and over).

The current initializer (e.g. Dictionary.init(grouping:by:)) strongly couples instance creation and grouping, so it's impossible to do just the grouping on its own into an existing instance (without making a new dictionary and merging it into the old).

The only way to take an instance as a parameter would be if there was some protocol that describes a mutable associative collection.