[swift-evolution] [Pitch] Add `mapValues` method to Dictionary

[Brent Royal-Gordon]

> I have a small remark though, wouldn’t it be better to let transform be of type (Key, Value) throws -> T instead of (Value) throws -> T? You can just ignore the key (with _) if you don’t need it, but I think it might come in handy in some cases.

The problem is, that closes the door to writing many simple maps in functional style. For instance, this:

	dictionaryOfNumbers.mapValues(abs)

Would have to become this:

	dictionaryOfNumbers.mapValues { _, v in abs(v) }

(It *might* be possible to do it with `$1`, but I'm not sure; there are some limitations around that.)

A value-value map is just simpler and cleaner, while almost always giving you what you need.

-- 
Brent Royal-Gordon
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