Swift 4 "Cannot use mutating member on immutable value: 'self' is immutable"

JetForMe · September 14, 2017, 12:36am

Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
    ...
    static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
    static var pendingFetchers = [ModelFetcher]()
    static var currentFetcher: ModelFetcher?

    class
    func
    startNextFetcher()
    {
        self.managerDispatchQueue.async
        {
            guard
                self.currentFetcher == nil,
                let mf = self.pendingFetchers.popFirst()
                         ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
            else
            {
                return
            }

            self.currentFetcher = mf
            mf.start()
        }
    }
    ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.

···

--
Rick Mann
rmann@latencyzero.com

Zhao_Xin · September 14, 2017, 1:11am

Change `self` to `ModelFetcher`. You are calling a class static property,
not a class instance property.

Zhao Xin

···

On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users < swift-users@swift.org> wrote:

Moving to Swift 4, I'm running into an issue for which I can't seem to
find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
    ...
    static let managerDispatchQueue =
DispatchQueue(label: "Model Download Manager Queue")
    static var pendingFetchers =
[ModelFetcher]()
    static var currentFetcher: ModelFetcher?

    class
    func
    startNextFetcher()
    {
        self.managerDispatchQueue.async
        {
            guard
                self.currentFetcher == nil,
                let mf = self.pendingFetchers.popFirst()
                         ~~~~ ^ error: cannot
use mutating member on immutable value: 'self' is immutable
            else
            {
                return
            }

            self.currentFetcher = mf
            mf.start()
        }
    }
    ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a
monolithic app (the error shows up when this code is factored into a
framework). Other mutating references to self seem to compile okay (e.g.
"self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)").
Not sure what's special about this one.

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

JetForMe · September 14, 2017, 1:35am

Did this change in Swift 4?

I like being able to refer to the class in a class func as "self". If I ever change the name of the class, or refactor the code, or something, I don't have to change it in a bunch of places.

···

On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:

Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.

Zhao Xin

On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org> wrote:
Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
    ...
    static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
    static var pendingFetchers = [ModelFetcher]()
    static var currentFetcher: ModelFetcher?

    class
    func
    startNextFetcher()
    {
        self.managerDispatchQueue.async
        {
            guard
                self.currentFetcher == nil,
                let mf = self.pendingFetchers.popFirst()
                         ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
            else
            {
                return
            }

            self.currentFetcher = mf
            mf.start()
        }
    }
    ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

--
Rick Mann
rmann@latencyzero.com

JetForMe · September 14, 2017, 1:35am

Also, it works fine at a couple of other call sites. Why is this one special?

···

On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:

Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.

Zhao Xin

On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org> wrote:
Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
    ...
    static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
    static var pendingFetchers = [ModelFetcher]()
    static var currentFetcher: ModelFetcher?

    class
    func
    startNextFetcher()
    {
        self.managerDispatchQueue.async
        {
            guard
                self.currentFetcher == nil,
                let mf = self.pendingFetchers.popFirst()
                         ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
            else
            {
                return
            }

            self.currentFetcher = mf
            mf.start()
        }
    }
    ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

--
Rick Mann
rmann@latencyzero.com

JetForMe · September 14, 2017, 1:36am

Yeah, that's not it. I made the change you suggested, I get the same error.

···

On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:

Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.

Zhao Xin

On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org> wrote:
Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
    ...
    static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
    static var pendingFetchers = [ModelFetcher]()
    static var currentFetcher: ModelFetcher?

    class
    func
    startNextFetcher()
    {
        self.managerDispatchQueue.async
        {
            guard
                self.currentFetcher == nil,
                let mf = self.pendingFetchers.popFirst()
                         ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
            else
            {
                return
            }

            self.currentFetcher = mf
            mf.start()
        }
    }
    ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

--
Rick Mann
rmann@latencyzero.com

Zhao_Xin · September 14, 2017, 1:46am

I begin to think it is related to how you `popFirst()` implemented. Check
it or post it here.

Zhao Xin

···

On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann@latencyzero.com> wrote:

Yeah, that's not it. I made the change you suggested, I get the same error.

> On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:
>
> Change `self` to `ModelFetcher`. You are calling a class static
property, not a class instance property.
>
> Zhao Xin
>
> On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users < > swift-users@swift.org> wrote:
> Moving to Swift 4, I'm running into an issue for which I can't seem to
find an answer in google:
>
> "Cannot use mutating member on immutable value: 'self' is immutable"
>
> The code looks like:
>
> class
> ModelFetcher : NSObject, URLSessionDelegate
> {
> ...
> static let managerDispatchQueue =
DispatchQueue(label: "Model Download Manager Queue")
> static var pendingFetchers =
[ModelFetcher]()
> static var currentFetcher: ModelFetcher?
>
> class
> func
> startNextFetcher()
> {
> self.managerDispatchQueue.async
> {
> guard
> self.currentFetcher == nil,
> let mf = self.pendingFetchers.popFirst()
> ~~~~ ^ error:
cannot use mutating member on immutable value: 'self' is immutable
> else
> {
> return
> }
>
> self.currentFetcher = mf
> mf.start()
> }
> }
> ...
> }
>
> This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a
monolithic app (the error shows up when this code is factored into a
framework). Other mutating references to self seem to compile okay (e.g.
"self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)").
Not sure what's special about this one.
>
>
> --
> Rick Mann
> rmann@latencyzero.com
>
>
> _______________________________________________
> swift-users mailing list
> swift-users@swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>

--
Rick Mann
rmann@latencyzero.com

Jon_Shier · September 14, 2017, 1:59am

I think there’s something strange with popFirst. It doesn’t show up in the autocomplete in Xcode, but it compiles, and popLast doesn’t throw the same error. removeFirst doesn’t either, though it’s unsafe. Weird.

Jon

···

On Sep 13, 2017, at 9:47 PM, Zhao Xin via swift-users <swift-users@swift.org> wrote:

I begin to think it is related to how you `popFirst()` implemented. Check it or post it here.

Zhao Xin

On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann@latencyzero.com <mailto:rmann@latencyzero.com>> wrote:
Yeah, that's not it. I made the change you suggested, I get the same error.

> On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com <mailto:owenzx@gmail.com>> wrote:
>
> Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.
>
> Zhao Xin
>
> On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:
> Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:
>
> "Cannot use mutating member on immutable value: 'self' is immutable"
>
> The code looks like:
>
> class
> ModelFetcher : NSObject, URLSessionDelegate
> {
> ...
> static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
> static var pendingFetchers = [ModelFetcher]()
> static var currentFetcher: ModelFetcher?
>
> class
> func
> startNextFetcher()
> {
> self.managerDispatchQueue.async
> {
> guard
> self.currentFetcher == nil,
> let mf = self.pendingFetchers.popFirst()
> ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
> else
> {
> return
> }
>
> self.currentFetcher = mf
> mf.start()
> }
> }
> ...
> }
>
> This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.
>
>
> --
> Rick Mann
> rmann@latencyzero.com <mailto:rmann@latencyzero.com>
>
>
> _______________________________________________
> swift-users mailing list
> swift-users@swift.org <mailto:swift-users@swift.org>
> https://lists.swift.org/mailman/listinfo/swift-users
>

--
Rick Mann
rmann@latencyzero.com <mailto:rmann@latencyzero.com>

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

somu · September 14, 2017, 2:55am

Not sure why it is happening, but given below are my observations:

Simplified version:

var array = [1, 2, 3, 4, 5]
_ = array.popFirst() //error: cannot use mutating member on immutable value: 'array' is immutable
_ = array.popLast() //works ok

popFirst is not available in the Array documentation and “Jump to Definition” doesn’t seem to work.

Thanks and regards,
Muthu

···

On 14 Sep 2017, at 9:59 AM, Jon Shier via swift-users <swift-users@swift.org> wrote:

I think there’s something strange with popFirst. It doesn’t show up in the autocomplete in Xcode, but it compiles, and popLast doesn’t throw the same error. removeFirst doesn’t either, though it’s unsafe. Weird.

Jon

On Sep 13, 2017, at 9:47 PM, Zhao Xin via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

I begin to think it is related to how you `popFirst()` implemented. Check it or post it here.

Zhao Xin

On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann@latencyzero.com <mailto:rmann@latencyzero.com>> wrote:
Yeah, that's not it. I made the change you suggested, I get the same error.

> On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com <mailto:owenzx@gmail.com>> wrote:
>
> Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.
>
> Zhao Xin
>
> On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:
> Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:
>
> "Cannot use mutating member on immutable value: 'self' is immutable"
>
> The code looks like:
>
> class
> ModelFetcher : NSObject, URLSessionDelegate
> {
> ...
> static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
> static var pendingFetchers = [ModelFetcher]()
> static var currentFetcher: ModelFetcher?
>
> class
> func
> startNextFetcher()
> {
> self.managerDispatchQueue.async
> {
> guard
> self.currentFetcher == nil,
> let mf = self.pendingFetchers.popFirst()
> ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
> else
> {
> return
> }
>
> self.currentFetcher = mf
> mf.start()
> }
> }
> ...
> }
>
> This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.
>
>
> --
> Rick Mann
> rmann@latencyzero.com <mailto:rmann@latencyzero.com>
>
>
> _______________________________________________
> swift-users mailing list
> swift-users@swift.org <mailto:swift-users@swift.org>
> https://lists.swift.org/mailman/listinfo/swift-users
>

--
Rick Mann
rmann@latencyzero.com <mailto:rmann@latencyzero.com>

_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

JetForMe · September 14, 2017, 11:36pm

I think there’s something strange with popFirst. It doesn’t show up in the autocomplete in Xcode, but it compiles, and popLast doesn’t throw the same error. removeFirst doesn’t either, though it’s unsafe. Weird.

extension
Array
{
  public
  mutating
  func
  popFirst()
    -> Array.Element?
  {
    if self.count > 0
    {
      return self.removeFirst()
    }

    return nil
  }
}

···

On Sep 13, 2017, at 18:59 , Jon Shier <jon@jonshier.com> wrote:

Jon

On Sep 13, 2017, at 9:47 PM, Zhao Xin via swift-users <swift-users@swift.org> wrote:

I begin to think it is related to how you `popFirst()` implemented. Check it or post it here.

Zhao Xin

On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann@latencyzero.com> wrote:
Yeah, that's not it. I made the change you suggested, I get the same error.

> On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:
>
> Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.
>
> Zhao Xin
>
> On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org> wrote:
> Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:
>
> "Cannot use mutating member on immutable value: 'self' is immutable"
>
> The code looks like:
>
> class
> ModelFetcher : NSObject, URLSessionDelegate
> {
> ...
> static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
> static var pendingFetchers = [ModelFetcher]()
> static var currentFetcher: ModelFetcher?
>
> class
> func
> startNextFetcher()
> {
> self.managerDispatchQueue.async
> {
> guard
> self.currentFetcher == nil,
> let mf = self.pendingFetchers.popFirst()
> ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
> else
> {
> return
> }
>
> self.currentFetcher = mf
> mf.start()
> }
> }
> ...
> }
>
> This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.
>
>
> --
> Rick Mann
> rmann@latencyzero.com
>
>
> _______________________________________________
> swift-users mailing list
> swift-users@swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

--
Rick Mann
rmann@latencyzero.com

eskimo · September 14, 2017, 8:13am

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.

If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.

var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.

let mf = self.pendingFetchers.popFirst()
~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’

* * *

@Rick, there’s two things in play here:

* The diagnostic is clearly bogus and you should definitely file a bug about that.

<Issues · apple/swift · GitHub;

Please post your bug number, just for the record.

* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?

Share and Enjoy

···

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org> wrote:
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

VladimirS · September 14, 2017, 12:14pm

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.

If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.

var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

···

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org> wrote:

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.

let mf = self.pendingFetchers.popFirst()
~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’

* * *

@Rick, there’s two things in play here:

* The diagnostic is clearly bogus and you should definitely file a bug about that.

<Issues · apple/swift · GitHub;

Please post your bug number, just for the record.

* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?

Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

_______________________________________________
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JetForMe · September 15, 2017, 8:20pm

I meant to add: I implemented it based on the declaration of popLast().

···

On Sep 15, 2017, at 13:05 , Roderick Mann via swift-users <swift-users@swift.org> wrote:

On Sep 13, 2017, at 18:59 , Jon Shier <jon@jonshier.com> wrote:

  I think there’s something strange with popFirst. It doesn’t show up in the autocomplete in Xcode, but it compiles, and popLast doesn’t throw the same error. removeFirst doesn’t either, though it’s unsafe. Weird.

extension
Array
{
  public
  mutating
  func
  popFirst()
    -> Array.Element?
  {
    if self.count > 0
    {
      return self.removeFirst()
    }

    return nil
  }
}

Jon

On Sep 13, 2017, at 9:47 PM, Zhao Xin via swift-users <swift-users@swift.org> wrote:

I begin to think it is related to how you `popFirst()` implemented. Check it or post it here.

Zhao Xin

On Thu, Sep 14, 2017 at 9:36 AM, Roderick Mann <rmann@latencyzero.com> wrote:
Yeah, that's not it. I made the change you suggested, I get the same error.

On Sep 13, 2017, at 18:11 , Zhao Xin <owenzx@gmail.com> wrote:

Change `self` to `ModelFetcher`. You are calling a class static property, not a class instance property.

Zhao Xin

On Thu, Sep 14, 2017 at 8:37 AM, Rick Mann via swift-users <swift-users@swift.org> wrote:
Moving to Swift 4, I'm running into an issue for which I can't seem to find an answer in google:

"Cannot use mutating member on immutable value: 'self' is immutable"

The code looks like:

class
ModelFetcher : NSObject, URLSessionDelegate
{
   ...
   static let managerDispatchQueue = DispatchQueue(label: "Model Download Manager Queue")
   static var pendingFetchers = [ModelFetcher]()
   static var currentFetcher: ModelFetcher?

   class
   func
   startNextFetcher()
   {
       self.managerDispatchQueue.async
       {
           guard
               self.currentFetcher == nil,
               let mf = self.pendingFetchers.popFirst()
                        ~~~~ ^ error: cannot use mutating member on immutable value: 'self' is immutable
           else
           {
               return
           }

           self.currentFetcher = mf
           mf.start()
       }
   }
   ...
}

This code compiled fine in Xcode 8, or in Xcode 9/Swift 3.2 as a monolithic app (the error shows up when this code is factored into a framework). Other mutating references to self seem to compile okay (e.g. "self.currentFetcher = nil" or "self.pendingFetchers.remove(at: idx)"). Not sure what's special about this one.

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Jon_Shier · September 14, 2017, 2:48pm

It also doesn’t explain why removeFirst would work but popFirst doesn’t. The mutation to the underlying array should be the same, the only different is that one returns the element optionally. It also doesn’t explain why popFirst doesn’t show up I the autocomplete but compiles anyway.

Jon

···

On Sep 14, 2017, at 8:16 AM, Vladimir.S via swift-users <swift-users@swift.org> wrote:

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
    ~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
                   * * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
<Issues · apple/swift · GitHub;
Please post your bug number, just for the record.
* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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Jon_Shier · September 14, 2017, 3:05pm

This seems oddly similar to [SR-5659] String.UnicodeScalarView has breaking changes in latest beta · Issue #48229 · apple/swift · GitHub, but [Thing] should meet the appropriate criteria for popFirst to work properly. Yet the error we’re seeing here is the same that was originally seen when trying to use popFirst on a String. Definitely feels like some sort of Swift bug here.

Jon

···

On Sep 14, 2017, at 10:48 AM, Jon Shier via swift-users <swift-users@swift.org> wrote:

  It also doesn’t explain why removeFirst would work but popFirst doesn’t. The mutation to the underlying array should be the same, the only different is that one returns the element optionally. It also doesn’t explain why popFirst doesn’t show up I the autocomplete but compiles anyway.

Jon

On Sep 14, 2017, at 8:16 AM, Vladimir.S via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
    ~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
                   * * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
<Issues · apple/swift · GitHub;
Please post your bug number, just for the record.
* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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Jon_Shier · September 14, 2017, 7:01pm

Looks like this is sort of tracked in SR-5515popFirst error message is silly <https://bugs.swift.org/browse/SR-5515>\. I’m not sure what a good answer is here, since it seems like it isn’t supposed to be available if it can’t guarantee O(1) behavior. Still seems like Array should have it, but in any case, this looks like a bad error message at least.

Jon

···

On Sep 14, 2017, at 11:05 AM, Jon Shier via swift-users <swift-users@swift.org> wrote:

  This seems oddly similar to [SR-5659] String.UnicodeScalarView has breaking changes in latest beta · Issue #48229 · apple/swift · GitHub, but [Thing] should meet the appropriate criteria for popFirst to work properly. Yet the error we’re seeing here is the same that was originally seen when trying to use popFirst on a String. Definitely feels like some sort of Swift bug here.

Jon

On Sep 14, 2017, at 10:48 AM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

  It also doesn’t explain why removeFirst would work but popFirst doesn’t. The mutation to the underlying array should be the same, the only different is that one returns the element optionally. It also doesn’t explain why popFirst doesn’t show up I the autocomplete but compiles anyway.

Jon

On Sep 14, 2017, at 8:16 AM, Vladimir.S via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
    ~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
                   * * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
<Issues · apple/swift · GitHub;
Please post your bug number, just for the record.
* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

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VladimirS · September 15, 2017, 11:08am

Looks like this is sort of tracked in SR-5515popFirst error message is silly <https://bugs.swift.org/browse/SR-5515>\. I’m not sure what a good answer is here, since it seems like it isn’t supposed to be available if it can’t guarantee O(1) behavior. Still seems like Array should have it, but in any case, this looks like a bad error message at least.

But can .removeFirst() guarantee this? I don't believe these two methods a very different in implementation. And, as I said, for me it is very strange decision to disallow a method because it is 'expensive'. I hope this is just a bug, that will be fixed.

Vladimir.

···

On 14.09.2017 22:02, Jon Shier via swift-users wrote:

Jon

On Sep 14, 2017, at 11:05 AM, Jon Shier via swift-users <swift-users@swift.org >> <mailto:swift-users@swift.org>> wrote:

This seems oddly similar to [SR-5659] String.UnicodeScalarView has breaking changes in latest beta · Issue #48229 · apple/swift · GitHub, but [Thing] should meet the appropriate criteria for popFirst to work properly. Yet the error we’re seeing here is the same that was originally seen when trying to use popFirst on a String. Definitely feels like some sort of Swift bug here.

Jon

On Sep 14, 2017, at 10:48 AM, Jon Shier via swift-users <swift-users@swift.org >>> <mailto:swift-users@swift.org>> wrote:

It also doesn’t explain why removeFirst would work but popFirst doesn’t. The mutation to the underlying array should be the same, the only different is that one returns the element optionally. It also doesn’t explain why popFirst doesn’t show up I the autocomplete but compiles anyway.

Jon

On Sep 14, 2017, at 8:16 AM, Vladimir.S via swift-users <swift-users@swift.org >>>> <mailto:swift-users@swift.org>> wrote:

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org >>>>> <mailto:swift-users@swift.org>> wrote:

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
    ~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
                   * * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
<Issues · apple/swift · GitHub;
Please post your bug number, just for the record.
* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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JetForMe · September 16, 2017, 12:44am

In my case, it's my own implementation of popFirst(), so I'm not sure why it's complaining at all. Who cares if it's slow? The declaration is identical to popLast(), with the exception that I'm defining mine in an extension that's not Generic (I don't know if you can do it any other way). But otherwise, my declaration is identical to popLast().

Is the compiler looking at the name "pop" and adding additional constraints (and then spewing a bogus error message)?

···

On Sep 15, 2017, at 13:35 , Vladimir.S via swift-users <swift-users@swift.org> wrote:

On 14.09.2017 22:02, Jon Shier via swift-users wrote:

Looks like this is sort of tracked in SR-5515popFirst error message is silly <https://bugs.swift.org/browse/SR-5515>\. I’m not sure what a good answer is here, since it seems like it isn’t supposed to be available if it can’t guarantee O(1) behavior. Still seems like Array should have it, but in any case, this looks like a bad error message at least.

But can .removeFirst() guarantee this? I don't believe these two methods a very different in implementation. And, as I said, for me it is very strange decision to disallow a method because it is 'expensive'. I hope this is just a bug, that will be fixed.

Vladimir.

Jon

On Sep 14, 2017, at 11:05 AM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

This seems oddly similar to [SR-5659] String.UnicodeScalarView has breaking changes in latest beta · Issue #48229 · apple/swift · GitHub, but [Thing] should meet the appropriate criteria for popFirst to work properly. Yet the error we’re seeing here is the same that was originally seen when trying to use popFirst on a String. Definitely feels like some sort of Swift bug here.

Jon

On Sep 14, 2017, at 10:48 AM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

It also doesn’t explain why removeFirst would work but popFirst doesn’t. The mutation to the underlying array should be the same, the only different is that one returns the element optionally. It also doesn’t explain why popFirst doesn’t show up I the autocomplete but compiles anyway.

Jon

On Sep 14, 2017, at 8:16 AM, Vladimir.S via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

On 14.09.2017 11:14, Quinn "The Eskimo!" via swift-users wrote:

On 14 Sep 2017, at 03:56, somu subscribe via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

popFirst is not available in the Array …

Right. This makes sense when you consider the standard setup for an array, namely, a variable length buffer of items. Removing the first element is expensive, whereas removing the last element (`popLast()`) is cheap.
If you run your simplified example in Xcode 8 you get an error that hints at what’s going on.
var array = [1, 2, 3, 4, 5]
_ = array.popFirst()
   ~~~~~~^~~~~~~~
error: 'ArraySlice<Int>' is not convertible to '[Int]'

Sorry, could you clarify, I can't understand.
What should be the error message here and why at all we should have an error here?

Are we forced to use this code to pop first element in array? :
var array = [1, 2, 3, 4, 5]
let x = array[0...].popFirst()
print(x!) // 1
print(array) // [2,3,4,5]

And why we shouldn't be allowed to have popFirst() for array? Yes, it is expensive, but is it a right reason to disallow it totally? In many situations we have a small arrays and don't care so much how fast the first element will be pop'ed.
I hope I'm just missing something.

Vladimir.

Notably, I put Rick’s code into Xcode 8 (Xcode 8.3.3 on macOS 10.12.6 with a new command line tool project) and I get the same error there.
let mf = self.pendingFetchers.popFirst()
        ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
error: 'ArraySlice<ModelFetcher>' is not convertible to '[ModelFetcher]’
                  * * *
@Rick, there’s two things in play here:
* The diagnostic is clearly bogus and you should definitely file a bug about that.
<Issues · apple/swift · GitHub;
Please post your bug number, just for the record.
* You wrote:

This code compiled fine in Xcode 8 …

My tests indicate that it doesn’t. I suspect that you changed something else during the Swift 4 migration and that’s why you’re seeing it fail. Can you take another look at the original Xcode 8 code to see what’s else got changed?
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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eskimo · September 17, 2017, 10:25am

… 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 15 Sep 2017, at 21:35, Vladimir.S via swift-users <swift-users@swift.org> wrote:

On 16 Sep 2017, at 01:44, Rick Mann via swift-users <swift-users@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()`.

Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

JetForMe · September 18, 2017, 1:45am

All right, I'm happy to change the name to "safeRemoveFirst()", but I'm a bit irritated that there's an implicit performance constraint based on the name alone, without any obvious decorator syntax.

···

On Sep 17, 2017, at 03:25 , Quinn The Eskimo! via swift-users <swift-users@swift.org> wrote:

On 15 Sep 2017, at 21:35, Vladimir.S via swift-users <swift-users@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@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()`.

--
Rick Mann
rmann@latencyzero.com

VladimirS · September 18, 2017, 12:52pm

… 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.

Do I understand this correctly?: To protect *me* from using popFirst() with possible O(n) complexity in my *generic code*, there is no better solution in Swift than just *hide* the method from me even in non-generic code. Even if I probably don't care about the complexity for my 5 elements array.
And so, I should use hacky code like this(which doesn't produce warnings/errors) to fight with compiler:
let x = array[0...].popFirst()

Can't understand/accept this, sorry. If you are saying "this is a current limitation we have" - ok, but when you are saying "That’s pretty normal" - I don't understand what "normal" means here. I even can understand if compiler can provide us with some 'popFirstNComplexity()'(or warning with a way to silence it) when 'popFirst' shouldn't be accessible because of reasons mentioned by you. But just hide the method.

Also, could you clarify, why .removeFirst() is different? We can use it without problems:

var array = [1, 2, 3, 4, 5]
array.removeFirst() // no warnings
print(array) // [2,3,4,5]

Vladimir.

···

On 17.09.2017 13:25, Quinn "The Eskimo!" via swift-users wrote:

On 15 Sep 2017, at 21:35, Vladimir.S via swift-users <swift-users@swift.org> wrote:

On 16 Sep 2017, at 01:44, Rick Mann via swift-users <swift-users@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()`.

Share and Enjoy
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Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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