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.
If one of you have specific ideas about the potential Map protocol, I encourage you to start a separate thread for that, to focus the conversation on the parameters of what it would look like. I guess I'm now asking - would you support a proposal for adding the basic mapValues function as the first step, with the potential extendability to a Map protocol allowing for a lazy version? Because I'd like to keep the proposal as focused as possible to increase the chance of an on-point discussion. Thanks, Honza On Sat, May 21, 2016 at 3:27 PM Matthew Johnson <[email protected]> wrote: > > > Sent from my iPad > > > On May 21, 2016, at 8:45 AM, Haravikk via swift-evolution < > [email protected]> wrote: > > > > I think that before this can be done there needs to be an abstraction of > what a Dictionary is, for example a Map<Key, Value> protocol. This would > allow us to also implement the important lazy variations of what you > suggest, which would likely be more important for very large dictionaries > as dictionaries are rarely consumed in their entirety; in other words, > calculating and storing the transformed value for every key/value pair is > quite a performance overhead if only a fraction of those keys may actually > be accessed. Even if you are consuming the whole transformed dictionary the > lazy version is better since it doesn’t store any intermediate values, you > only really want a fully transformed dictionary if you know the > transformation is either very costly, or transformed values will be > accessed frequently. > > > > Anyway, long way of saying that while the specific implementation is > definitely wanted, the complete solution requires a few extra steps which > should be done too, as lazy computation can have big performance benefits. > > > > That and it’d be nice to have a Map protocol in stdlib for defining > other map types, such as trees, since these don’t require Hashable keys, > but dictionaries do. > > +1 to defining map abstractions in the standard library (separating read > only from read write). The value associatedtype should not take a position > on optionality, allowing for maps which have a valid value for all possible > keys. I have done similar things in other languages and found it extremely > useful. It is not uncommon to have code that just needs to read and / or > write to / from a map without having concern for the implementation of the > map. > > One issue I think we should sort out along side this is some kind of > abstraction which allows code to use functions or user-defined types > without regard for which it is accessing. The map abstraction would build > on this abstraction, allowing single argument functions to be viewed as a > read only map. > > One option is to allow functions to conform to protocols that only have > subscript { get } requirements (we would probably only allow them to be > subscripted through the protocol interface). I think this feels like the > most Swifty direction. > > Another option is to take the path I have seen in several languages which > is to allow overloading of the function call "operator". I originally > wanted this in Swift but now wonder if the first option might be a better > way to accomplish the same goals. > > -Matthew > > > > >> On 21 May 2016, at 11:27, Honza Dvorsky via swift-evolution < > [email protected]> wrote: > >> > >> Hello everyone, > >> > >> I have added a very simple, but powerful method into a Dictionary > extension on multiple projects in the last weeks, so I'd like to bring up > the idea of adding it into the standard library, in case other people can > see its benefits as well. > >> > >> Currently, Dictionary conforms to Collection with its Element being the > tuple of Key and Value. Thus transforming the Dictionary with regular map > results in [T], whereas I'd find it more useful to also have a method which > results in [Key:T]. > >> > >> Let me present an example of where this makes sense. > >> > >> I recently used the GitHub API to crawl some information about > repositories. I started with just names (e.g. "/apple/swift", > "/apple/llvm") and fetched a JSON response for each of the repos, each > returning a dictionary, which got saved into one large dictionary as the > end of the full operation, keyed by its name, so the structure was > something like > >> > >> { > >> "/apple/swift": { "url":..., "size":...., "homepage":... }, > >> "/apple/llvm": { "url":..., "size":...., "homepage":... }, > >> ... > >> } > >> > >> To perform analysis, I just needed a dictionary mapping the name of the > repository to its size, freeing me to discard the rest of the results. > >> This is where things get interesting, because you can't keep this > action nicely functional anymore. I had to do the following: > >> > >> let repos: [String: JSON] = ... > >> var sizes: [String: Int] = [:] > >> for (key, value) in repos { > >> sizes[key] = value["size"].int > >> } > >> // use sizes... > >> > >> Which isn't a huge amount of work, but it creates unnecessary mutable > state in your transformation pipeline (and your current scope). And I had > to write it enough times to justify bringing it up on this list. > >> > >> I suggest we add the following method to Dictionary: > >> > >> extension Dictionary { > >> public func mapValues<T>(_ transform: @noescape (Value) throws -> > T) rethrows -> [Key: T] { > >> var transformed: [Key: T] = [:] > >> for (key, value) in self { > >> transformed[key] = try transform(value) > >> } > >> return transformed > >> } > >> } > >> > >> It is modeled after Collection's `map` function, with the difference > that > >> a) only values are transformed, instead of the Key,Value tuple and > >> b) the returned structure is a transformed Dictionary [Key:T], instead > of [T] > >> > >> This now allows a much nicer workflow: > >> > >> let repos: [String: JSON] = ... > >> var sizes = repos.mapValues { $0["size"].int } > >> // use sizes... > >> > >> and even multi-step transformations on Dictionaries, previously only > possible on Arrays, e.g. > >> var descriptionTextLengths = repos.mapValues { $0["description"].string > }.mapValues { $0.characters.count } > >> > >> You get the idea. > >> > >> What do you think? I welcome all feedback, I'd like to see if people > would support it before I write a proper proposal. > >> > >> Thanks! :) > >> Honza Dvorsky > >> > >> _______________________________________________ > >> swift-evolution mailing list > >> [email protected] > >> https://lists.swift.org/mailman/listinfo/swift-evolution > > > > _______________________________________________ > > swift-evolution mailing list > > [email protected] > > https://lists.swift.org/mailman/listinfo/swift-evolution > >
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