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

[Brent Royal-Gordon]

> For this the key/value pair constructor for Dictionary would be the better option, since the .map() method essentially already lets you do this. Adding key transformation to .mapValues() would only make it less specialised (it’s called .mapValues after all ;), plus it would be incompatible with any simultaneous/future lazy implementation since you can’t just transform the keys lazily (you also need a way to transform them back if you intend to use them for lookups).

He was suggesting the transform be of type (Key, Value) -> NewValue; that is, it would be able to see what the key was, but it wouldn't be able to transform the key.

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Brent Royal-Gordon
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