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

[Dan Appel]

I don't understand - why do we need a map protocol for a lazy mapvalues?
On Sat, May 21, 2016 at 11:12 PM Brent Royal-Gordon <brent at architechies.com>
wrote:

> > Obviously if it's required to have a lazy variant for all collection
> methods, I'd probably pull back this proposal, as it would automatically be
> rejected by the core team.
>
> I'm not a member of the core team, but I doubt it's necessary.
>
> I don't think it makes much sense to wait for a lazy implementation. If
> there *is* going to be a Map protocol, then the best way to implement it
> will require an advanced generics feature that's still in the proposal
> stages:
>
>         // Note: I'm simplifying signatures here.
>
>         protocol Map: Collection {
>                 associatedtype Key
>                 associatedtype Value
>                 associatedtype Iterator: IteratorProtocol where
> Iterator.Element == (Key, Value)
>
>                 subscript (key: Key) -> Value? { get }
>                 func mapValues<T>(transform: Value -> T) -> [Key: T]
>         }
>
> And implementing the laziness will similarly work best with not only that
> feature, but also conditional conformances:
>
>         protocol LazyMap: Map, LazyCollectionProtocol {
>                 subscript (key: Base.Key) -> Base.Value? { get }
>                 func mapValues<T>(transform: Value -> T)(…) ->
> LazyMappedValuesMap<Base, T>
>         }
>
>         extension LazyCollection: LazyMap where Base: Map {
>                 …
>         }
>
>         struct LazyMapValuesMap<Base: Map, NewValue>: Map {
>                 …
>         }
>
> If we *don't* wait, on the other hand, we're going to end up in
> manual-specification and GYB hell. (And even then, I believe conditional
> conformances are not enough to force all LazyCollectionProtocol conformers
> to support Map if they have a Map base. You'd need conditional *conformance
> requirements*, which are *not* planned because it's not safe to add a
> requirement to a protocol.)
>
> I just don't think laziness is so crucial that we should stop the presses
> until we have it. `mapValues` carries water all by itself, even without a
> matching `lazy.mapValues`.
>
> --
> Brent Royal-Gordon
> Architechies
>
> --
Dan Appel
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