[swift-evolution] Dictionary Enhancements
ben_cohen at apple.com
Sun Feb 19 17:08:46 CST 2017
> On Feb 17, 2017, at 11:20 PM, David Waite <david at alkaline-solutions.com> wrote:
>> On Feb 16, 2017, at 5:26 PM, Ben Cohen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at 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:
>> Add a defaulting subscript get (e.g. counts[key, default: 0] += 1 or grouped(key, default:].append(value)).
> Interesting, most of my scenarios aren’t one line subscript mutations like this, so I have been happy with counts[key] ?? 0, grouped(key) ?? , etc.
As well as a readability/discoverability benefit, there’s also a performance benefit: d[key] = (d[key] ?? ) + [value] is a linear-time operation, whereas d[key, default: ].append(value) should be constant time (there is some work needed on dictionary internals for this to work but the API ought to enable it once that’s done).
>> Add capacity property and reserveCapacity() method.
> would CoW copies also have the same reserved capacity?
Yes, it would mirror arrays, which behave like this. Note dictionaries already have this characteristic – they just don’t expose their capacity or allow you to preemptively expand it.
Any other behavior would be tricky. Until its mutated, a CoW copy is a literal copy, so must share the original’s capacity. Once the mutation forces a buffer copy, it would be pretty odd for that capacity to suddenly shrink.
> One way I could hypothetically copy around dictionaries that are using significantly more memory than I suspected,the other way algorithms would be quite confusing as an assignment to a variable might cause the capacity to revert.
> In terms of additional features, I have needed to group by value before, e.g. turn from K->V to V -> [K], which isn’t a simple reduce statement.
If there is a group-by operation from sequences, this ought to be composable from that given dictionaries are (Key,Value) sequences.
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