> On Feb 19, 2017, at 11:22 AM, Ole Begemann <[email protected]> wrote:
>
>
>> On 17 Feb 2017, at 01:26, Ben Cohen via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> 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>).
>> 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>).
>> Add a defaulting subscript get (e.g. counts[key, default: 0] += 1 or
>> grouped(key, default:[]].append(value)).
>> Add a group by-like init to create a Dictionary<K,[V]> from a sequence of V
>> and a closure (V)->K.
> Out of interest, how would you implement this? Does it require a generics
> feature that's slated for Swift 4? I tried two approaches that don't compile
> in a current Swift 3.1 snapshot (and I'm getting a segfault with both
> examples in a dev snapshot from 2017-02-14):
>
> 1)
>
> extension Dictionary {
> // error: same-type constraint 'Value' == '[S.Iterator.Element]' is
> recursive
> init<S: Sequence>(values: S, groupedBy: (S.Iterator.Element) -> Key)
> where Value == [S.Iterator.Element] {
> ...
> }
> }
> }
>
> 2)
>
> // error: reference to generic type 'Array' requires arguments in <...>
> extension Dictionary where Value == Array {
> init<S: Sequence>(values: S, groupedBy: (S.Iterator.Element) -> Key)
> where S.Iterator.Element == Value.Element {
> ...
> }
> }
> }
>
Oops, looks like a bug in the same-type constraint implementation. Ought to
work in 3.1. Minimal crasher repro:
extension Array {
func f<S: Sequence>()
where Element == S.Iterator.Element? {
}
}
I’ve raised https://bugs.swift.org/browse/SR-4008
It ought to be done with the first one. For the second, you can’t constrain
Value == Array because Array isn’t a type, needs to be Array<Something>.
As an (impractical) workaround, this compiles:
extension Dictionary {
subscript(k: Key, default default: Value) -> Value {
get { return self[k] ?? `default` }
set { self[k] = newValue }
}
}
extension Dictionary where Value: RangeReplaceableCollection {
init<S: Sequence>(grouping values: S, by: (S.Iterator.Element) -> Key)
where S.Iterator.Element == Value.Iterator.Element {
self = [:]
for x in values {
let k = by(x)
self[k, default: Value()].append(x)
}
}
}
let s = [10,20,22,30,31]
// have to explicitly type the Dictionary as a specific RRC
let d: [Int:[Int]] = Dictionary(grouping: s) { $0%10 }
print(d)
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