Thoughts inline. > On Feb 16, 2017, at 4:26 PM, Ben Cohen via swift-evolution > <swift-evolution@swift.org> wrote: > > Hi swift-evolution, > > Following up on Ted’s post regarding the opening up of stage 2, I’m starting > a thread to discuss improvements to the Dictionary type. > > Here is a list of commonly requested changes/enhancements to Dictionary, all > of which would probably be appropriate to put together into a single > evolution proposal: > > init from/merge in a Sequence of Key/Value pairs (already raised as SE-100: > https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md > > <https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md>). +1. I have wanted this since Swift 1.
> make the Values view collection a MutableCollection (as in this PR: > https://github.com/apple/swift-evolution/pull/555 > <https://github.com/apple/swift-evolution/pull/555>). I think Nate’s proposal covers this case well. > Add a defaulting subscript get (e.g. counts[key, default: 0] += 1 or > grouped(key, default:[]].append(value)). I am indifferent to this. I am happy using ??. I guess it could be slightly more efficient because it avoids wrapping and unwrapping the optional. > Add a group by-like init to create a Dictionary<K,[V]> from a sequence of V > and a closure (V)->K. +1. I would use this. > Add Dictionary.filter to return a Dictionary. +1. > Add Dictionary.mapValues to return a Dictionary (can be more efficiently > implemented than composition as the storage layout remains the same). +1000. I have also been asking for this since the beginning. I built my own version (and use it frequently), but as you say, the standard library can do it much more efficiently. I would also like to see an in-place version as well. One design detail. Even though it is only mapping the values, I would like it to pass the key to the closure as well. It occasionally figures in to the mapping logic. > Add capacity property and reserveCapacity() method. +0.5. I could see it being useful occasionally. > Have Dictionary.removeAtIndex return the Index of the next entry. No opinion on this. > (once we have conditional conformance) Make dictionaries with Equatable > values Equatable. +1 > Please reply here with any comments or questions on the above list, or any > additions you believe are important that are missing from it. > I would also like to see a version of map which returns a dictionary and handles key collisions: let newDict = myDict.map(collision: {k,v1,v2 in v2}) { (k,v) in ... } The collision parameter would take a throwing closure and handle the case of a key conflict (by returning the value to use, throwing, or trapping). It would have a default value so that it would only have to be specified if a different behavior was desired. In advanced cases, the collision could be used to accumulate values together. Because of this, I would actually like to see this on *collection* (not just dictionary). The map closure is handed each element of the sequence (which in the case of dictionary is a key/value tuple), and expects a return value of a key/value tuple. The collision block is called when a key is returned which has already been used to figure out what value to use. This might choose a winner, or it could act like reduce, building a value from the components. As a concrete example of what this allows, I could take in an array of words/strings [“apple”, “aardvark”, …] and then do the following to get a count of how many words start with each letter: let letterFrequency = words.map(collision:{$1+$2}) { (String($0.characters.first ?? “”) ,1)} print(letterFrequency[“a”]) //number of words starting with “a" I am ok using a term other than ‘map' if that is easier on the compiler, but I would like this functionality. At the simple end, it allows map functionality for both keys and values. At the advanced end, it acts as a categorizing reduce over a sequence/collection. You can even trivially implement the proposed groupBy with it (this is on Collection, but you could do an init on dict the same way): func grouped<K>(by categorizer: (Element)->K ) -> [K:Element] { return self.map(collision:{$1+$2}) {(categorizer($0), [$0])} } Thanks, Jon > All methods added to the standard library increase complexity, so need a > strong justification to reduce the risk of API sprawl. When requesting > additions/modifications, please keep the following questions in mind: > > Is the suggested addition a common operation that many would find useful? Can > it be flexible enough to cover different needs? > Will it encourage good practice? Might it be misused or encourage > anti-patterns? > Can the operation be composed simply from existing std lib features? Is that > composition intuitive/readable? > Is writing the equivalent by hand hard to get right? Are there common > correctness traps that this addition would help avoid? > Is writing the equivalent by hand hard to make efficient? Are there common > performance traps that this addition would help avoid? > Might a native implementation be able to execute more efficiently, by > accessing internals, than the equivalent implementation using public APIs? > As he has already written up SE-100 and another Dictionary proposal, Nate > Cook has kindly offered to collate a new omnibus proposal for Dictionary, > which will then get pitched here. > > I will send another email about enhancements to Sequence/Collection > algorithms shortly. > > Thanks! > > Ben > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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