> On 29 Jul 2016, at 18:42, Bram Beernink via swift-evolution
> <[email protected]> wrote:
>
> Hi all,
>
> Would it be possible to improve value and move semantics (performance) in
> Swift? Consider this possible Swift code in a future release of Swift:
>
> let array1 : [String] = ["Val1", "Val2"]
> let array2 = array1.appended(“Val3”) // Copy of array1 with “Val3” appended.
> array1 is left untouched. Nothing special yet.
> var array3 : [String] = [“Var1”]
> array3 = array3.appended(“Var2”) // array3 can just be mutated to add “Var2”,
> while maintaining value semantics. Swift can recognize that array3’s old
> state is not referenced anywhere in the future.
> let array4 = array2.appended("Val4").appended("Val5") // Copy of array2 with
> both "Val4" and "Val5" appended. In this case, “Val5” can also be appended by
> mutation.
>
> This example illustrates improved value semantics with a string array. But it
> would be good if this can work with any struct. Maybe via something similar
> to isUniquelyReferenced? Or maybe you can even have a “smart” self in a
> non-mutating func in a struct:
> struct Array<T> {
> func appended(e : T) -> Array<T> { // No mutating keyword!
> self.append(e) // self would either be mutated here if the current
> ref count of self is 1, and self is either a “rvalue” or self’s old state
> cannot possibly referenced anymore after this call. Otherwise, "self” would
> actually be a copy of self.
> return self
> }
> }
>
> I think that with such support it is encouraged to make more use of
> (immutable) value types while not sacrificing performance. Less mutations
> lead to more understandable code, which leads to less bugs.
>
> In any case, keep up the fast improvements to Swift.
>
> Best regards,
> Bram.
> _______________________________________________
> swift-evolution mailing list
> [email protected]
> https://lists.swift.org/mailman/listinfo/swift-evolution
It’s a known issue. See:
https://github.com/apple/swift/blob/master/docs/OptimizationTips.rst#advice-use-inplace-mutation-instead-of-object-reassignment
<https://github.com/apple/swift/blob/master/docs/OptimizationTips.rst#advice-use-inplace-mutation-instead-of-object-reassignment>
I’m not a compiler expert, but in theory I’d expect a smart-enough, fast-enough
optimiser to be able to replace those with mutating calls.
Karl
_______________________________________________
swift-evolution mailing list
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
