[swift-users] Swift 4 "Cannot use mutating member on immutable value: 'self' is immutable"
Quinn "The Eskimo!"
eskimo1 at apple.com
Sun Sep 17 05:25:04 CDT 2017
On 15 Sep 2017, at 21:35, Vladimir.S via swift-users <swift-users at swift.org> wrote:
> … for me it is very strange decision to disallow a method because it is 'expensive'.
That’s pretty normal for Swift standard library protocols, which define not just the behaviour of the routine but expected performance. `popFirst()` is expected to be O(1) and that’s not possible with `Array`.
The rationale behind this decision is, I believe, related to generic algorithms. If I write generic code that uses `popFirst()`, I can only guarantee the complexity of my code if I can rely on `popFirst()` being O(1). If someone implements `popFirst()` as O(n), my generic algorithm might go from O(n^2) to O(n^3), which is quite a change.
On 16 Sep 2017, at 01:44, Rick Mann via swift-users <swift-users at swift.org> wrote:
> Is the compiler looking at the name "pop" and adding additional constraints (and then spewing a bogus error message)?
I’m not sure what’s going on here mechanically but, yes, the error message is bogus. This is exactly what SR-5515 is talking about.
If I were in your shoes I’d call this method something other than `popFirst()`. This falls under my standard “if you change the semantics, change the name” rule. Your implementation of `popFirst()` doesn’t conform to the semantics of `popFirst()` — it’s O(n) because `removeFirst()` is O(n) — and thus you want to avoid calling it `popFirst()`.
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Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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