[swift-evolution] [Proposal Draft] Provide Custom Collections for Dictionary Keys and Values

[Nate Cook]

> On Oct 12, 2016, at 9:32 AM, plx via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The issue addressed is real; I’m not sure this is the best approach. 
> 
> In particular, unless I’m missing something obvious, the ownership strategy here would have to be:
> 
> - `DictionaryKeys` and `DictionaryValues` would each induce the expected +1 retain on the underlying storage
> - `DictionaryValues`’s mutations avoid triggering COW on the underlying storage by skipping the usual ownership check
> 
> …as otherwise it’s unclear how you’d do those in-place mutations (and this seems to be how the implementation works...is that correct?).

That's not quite right—when you access these views through the dictionary, they do not increment the storage retain count. This is the way slicing and views currently work on other mutable types. For example, when you reverse a slice of an array in-place, the slice doesn't get its own duplicate storage:

var a = Array(1...10)
a[0..<5].reverse()
a == [5, 4, 3, 2, 1, 6, 7, 8, 9, 10]

However, if you create a new variable out of the slice and reverse that, the slice does get its own storage:

var b = Array(1...10)
var bSlice = b[0..<5]
bSlice.reverse()
bSlice == [5, 4, 3, 2, 1]
b == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Strings and their views work the same way:

var s = "abcdefg"
s.characters.append("H")   // storage updated in place
s == "abcdefgH"

var sChars = s.characters  // no copy yet
sChars.removeLast()     // sChars gets its own copy before the mutation
s == "abcdefgH"
String(sChars) == "abcdefg"

var t = s   // no copy yet
t.characters.removeLast()  // t gets a new copy here
s == "abcdefgH"
t == "abcdefg"

I don't know the name of the compiler feature that enables this, but it's a critical part of the way views and slices work.

> With that design, it seems like you’d wind up allowing things like the below:
> 
>   // example A
>   let foo = [ “abc”: [1,2,3], “efg”: [4,5,6] ]
>   let bar = foo // shared storage, no COW
>   foo.values[foo.index(of: “abc”)!].append(789) // update shared storage, no COW
> 
>   // shared storage mutated,
>   // despite (a) both being `let` and (b) only foo.values getting touched
>   foo[“abc”] // [1, 2, 3, 789]
>   bar[“abc”] // [1, 2, 3, 789]

Example A isn't allowed—if foo and bar are both immutable, both of their `values` collections are also immutable, so there's no way to modify their shared storage.

>   // example B
>   var foo = [ “abc”: [1,2,3], “efg”: [4,5,6] ]
>   var bar = foo // shared storage, no COW
>   foo.values[foo.index(of: “abc”)!].append(789)
> 
>   // shared storage mutated only foo.values getting touched
>   foo[“abc”] // [1, 2, 3, 789]
>   bar[“abc”] // [1, 2, 3, 789]

Example B is incorrect—the mutation at `foo.values[...].append(789)` triggers a copy of the entire dictionary's underlying storage before allowing the mutation, since it knows that storage isn't uniquely referenced.

>   // example C
>   var foo = [ “abc”: [1,2,3], “efg”: [4,5,6] ]
>   var bar = foo 
>   bar[“abc”] = [1, 2, 3, 4] // COW triggered here, no shared storage
>   foo.values[foo.index(of: “abc”)!].append(789)
> 
>   // only `foo`’s storage mutated, b/c change to `bar` triggered COW
>   foo[“abc”] // [1, 2, 3, 789]
>   bar[“abc”] // [1, 2, 3, 4]

This is the current behavior and would remain the same after the proposed the changes.

> …where both A (by itself) and the B/C contrast seem very unwelcome.
> 
> Also, even if we assume we only ever make *responsible* use, having the stdlib include such directly-mutating views would seem likely to complicate any future concurrency plans.
> 
> To reiterate, I think the issue being addressed here is extremely important…I just don’t think I can get behind this type of solution (unless I’m grossly misunderstanding its mechanics).

Nate

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