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

Honza Dvorsky jan.dvorsky at me.com
Sat May 21 14:15:38 CDT 2016

I see your point, I agree it would definitely be nicer to have both
variants, but is that a rule that *all* methods on Collection and
Dictionary etc also have lazy variants? I actually don't know (please tell
me if you do).

The question here is not really whether to add a lazy variant later (I
don't think anyone would object to that), but as that would require quite
substantial refactoring and abstraction change into a Map protocol (which
will definitely gather a lot of feedback and could drag on for months), I
don't see why we can't add the regular variant already and benefit from it
now. Another option is that there won't be demand for the lazy variant, in
which case it makes even less sense to block the regular variant right now
(and could even hurt this proposal).

Maybe I'm missing some information about the core team requiring always
adding both non-lazy and lazy variants, if so, please do tell me so that I
can re-evaluate my approach. If not, I'd like to keep the thread focused
strictly on the one method I'm proposing we add, for the benefits and
examples I provided.

The Dictionary initializers could slightly help things, but it'd require
nesting if you do more than one level, whereas `mapValues` would allow for
multiple transformations to be applied in sequence without additional
nesting (my example from the original pitch would turn into `var
descriptionTextLengths = Dictionary(Dictionary(repos.mapValues {
$0["description"].string }).mapValues { $0.characters.count })` which is
much uglier in my opinion (and kind of breaks the natural left-to-right

On Sat, May 21, 2016 at 8:01 PM Haravikk <swift-evolution at haravikk.me>

> On 21 May 2016, at 15:47, Honza Dvorsky <jan.dvorsky at me.com> wrote:
> While I agree that it'd be nice to add a Map abstraction into which we
> could move a lot of the Dictionary-ness, my original pitch is *just* about
> adding the specific implementation of `mapValues` in its regular, non-lazy
> form. My example was about only keeping a subset of the information in
> memory in a Dictionary to allow for quick and frequent access (lazy goes
> against that). I think it'd be better to get that in first, or at least
> evaluate that separately from a comprehensive refactoring of the
> Dictionary, which would just accumulate more opinions and slow this
> specific step down.
> Sorry, my point was that I think it’s better to wait until we can also do
> the lazy equivalent and have both done together, otherwise we end up with
> one map function that can work both lazily and one that never does. Sure
> that will require a refactoring into a protocol, but it seems to me that
> it’s better to do that as the first step, then add the feature after that.
> In the mean time extensions have you well covered for convenience.
> Another alternative to this feature might be to add a key/value pair
> constructor to Dictionary (it technically already has one, but it’s
> variadic only) so you could do something like this:
> let myTransformedDictionary = Dictionary(myIntegerDictionary.lazy.map {
> ($0, $1 + 5) })
> Since this would be a useful initialiser both now and in future. I dunno,
> it’s just my opinion, but I find it a bit weird to get half the
> implementation now, and could lead to misunderstandings with people trying
> to do myMap.lazy.mapValues (won’t be recognised) and wondering why there
> isn’t one.
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