Just to clarify: It seems like the only ABI-affecting change here is the type
of keys/values. As you note at the end of your proposal, this should just be
Dictionary.Keys/Dictionary.Values regardless of whether we implement this
proposal or not, in which case this can be punted for Swift 4. It should be
fine to keep .Keys/.Values resilient so that we can change their implementation
details later if we want.
On the actual proposal: this is a pretty reasonable given Swift’s current
design and constraints. That said, I expect pushing forward on this kind of
thing right now is premature given the goals of Swift 4. A major aspect of
Swift 4 is reworking the way CoW semantics function internally, which could
drastically affect the way we approach this problem.
I’d really like if we could eliminate the “double search/hash” in the
no-existing-key case. There are ways to do this really cleanly, but they
probably involve more advanced CoW-safety propagation. In particular, you want
some way for the collection to return its search state to the caller so that
they can hand it back to insertion to just resume from there.
For instance:
map.entries[key] // An enum like Found(Value) | NotFound(SearchState)
.withDefault(value: []) // Unwrap the enum by completing the
NotFound(SearchState)
.append(1) // Now we have a value in both cases, we can append!
Or more complex:
map.entries[key]
.withDefault { /* logic that computes value */ }
.append(1)
I think this can be made to work in the current system if withDefault is
actually `[withDefault:]`, which is fine but a bit weird from a user’s
perspective.
In an ideal world the user could actually pattern match on the result of
`entries[key]`. In this way they could match on it and perform special logic in
both cases for really complex situations. This would make withDefault “just a
convenience”, so we aren’t pressured to add more methods like it every time
someone has a new Even More Complex use-case. e.g.:
switch map.entries[key] {
case .Found(entry):
if entry.value == 10 {
entry.remove()
print(“Found a value too many times! Moving key to fast-path auxiliary
structure…”)
} else {
entry.value += 1
}
case .NotFound(entry):
entry.insert(1)
print(“Found a value for the first time! Registering a bunch of extra
stuff…”)
}
But again, this is all dependent on a much more powerful SIL/ARC, and we just
don’t know what we’re going to get at this stage._______________________________________________
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