Re: [swift-users] Question with calling a method which returns `Self` on super

[Joanna Carter via swift-users]

I really must do more research before making accusations ;-)

> Which still leaves my question of why I have to reimplement the default 
> copy(other:) method from the protocol's extension in the base class ???

Apparently, the answer is to remove the declaration of the copy(other:) method 
from the protocol itself and to leave the default implementation in the 
extension !

So I now end up with :

protocol Copyable
{
  init(other: Self)
}

extension Copyable
{
  func copy() -> Self
  {
return type(of: self).init(other: self)
  }
}


class Shape : Copyable
{
  var color: NSColor
  
  init(color: NSColor)
  {
self.color = color
  }
  
  required init(other: Shape)
  {
color = other.color
  }
}

… and all is well with the world :-)

Mind you, the compiler error messages were hardly helpful to diagnosing this 
particular problem :-(

Joanna

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Joanna Carter
Carter Consulting

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Re: [swift-users] Question with calling a method which returns `Self` on super

[Joanna Carter via swift-users]

> here is the scenario, I want to implement a Copying protocol which can be 
> used homogeneously
> 
> protocol Copying {
> func copy() -> Self
> }
> 
> Because this protocol contains neither associated-type-requirements or 
> self-requirements(requiring a method returning Self is different), it can 
> surely be used homogeneously.
> 
> let objects: [Copying] = 
> for copyable in objects {
> .
> }

But Copying does contain a Self requirement. It is used the result of the 
copy() method

Here is how I implement Copying :

protocol Copyable
{
  func copy() -> Self
  
  init(other: Self)
}

// This extension provides a default implementation
extension Copyable
{
  func copy() -> Self
  {
return type(of: self).init(other: self)
  }
}

implementing Copyable on a struct is simple :

struct Struct : Copyable
{
  var value: String
  
  init(value: String)
  {
self.value = value
  }
  
  init(other: Struct)
  {
value = other.value
  }
}

But implementing it on a class is slightly more troublesome.

class Shape : Copyable
{
  var color: NSColor
  
  init(color: NSColor)
  {
self.color = color
  }
}

At this stage, the compiler (obviously) screams that Copyable is not 
implemented but the first "error" it notices it marks as being on the default 
implementation in the extension:  Method 'copy()' in non-final class 'Shape' 
must return `Self` to conform to protocol 'Copyable'

Marking this error on the extension method, which obviously does exactly what 
the error says is missing ???

Implementing the copy() method in Shape kills this error message but leaves me 
wondering why I have to now repeat the exact same code in the implementing 
method that I wrote in the default method in the protocol extension ; something 
that the compiler is completely ignoring.

Implementing the protocol with a struct takes account of the default method but 
with a class ignores it. This smells remarkably like a bug !

Nonetheless, I now end up with the following code, complete with duplicate 
copy() method :

class Shape : Copyable
{
  var color: NSColor
  
  func copy() -> Self
  {
return type(of: self).init(other: self)
  }

  init(color: NSColor)
  {
self.color = color
  }
  
  required init(other: Shape)
  {
color = other.color
  }
}

Then I can go on to declare subclasses :

class Circle : Shape
{
  var radius: Double = 0.0
  
  init(color: NSColor, radius: Double)
  {
super.init(color: color)

self.radius = radius
  }

  required init(other: Shape)
  {
super.init(other: other)

if let other = other as? Circle
{
  radius = other.radius
}
  }
}


class Square : Shape
{
  var side: Double = 0.0
  
  init(color: NSColor, side: Double)
  {
super.init(color: color)

self.side = side
  }

  required init(other: Shape)
  {
super.init(other: other)

if let other = other as? Square
{
  side = other.side
}
  }
}

And, the, if I create an extension to Array :

extension Array where Element : Copyable
{
  func copy() -> Array
  {
var result = [Element]()

self.forEach { result.append($0.copy()) }

return result
  }
}

I can then use the following code to create an array of Shapes but, obviously 
not of Copyable, due to its restrictions on the Self requirements on the 
protocol.

{
  let shapes = [Circle(color: .black, radius: 5.0), Square(color: .black, side: 
5.0)]
  
  let copies = shapes.copy()
  
  …
}

Thus, another possible solution to your problem is to declare the required 
init(other: Self) in the protocol.

Which still leaves my question of why I have to reimplement the default 
copy(other:) method from the protocol's extension in the base class ???

Joanna

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Carter Consulting

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Re: [swift-users] Question with calling a method which returns `Self` on super

[吴君恺 via swift-users]

Thank you, Slava.

I will use the trick you provided.
And knowing that this bug has already been under tracking is a great comfort to 
me, really :)

Lincoln

> 在 2017年8月11日，上午11:29，Slava Pestov  写道：
> 
> 
>> On Aug 10, 2017, at 8:23 PM, Slava Pestov via swift-users 
>> > wrote:
>> 
>> Hi Lincoln,
>> 
>> This is a known issue with ‘super’ method calls: 
>> https://bugs.swift.org/browse/SR-1736 
>> 
>> A workaround is to cast the result of the supermethod call to Self 
>> explicitly:
>> 
>> let copied = super.copy() as! Self
> 
> I’m sorry, but this is nonsense. We don’t allow ‘Self’ to refer to the 
> dynamic Self type yet.
> 
> You could do something like this I guess,
> 
> func cast(from: T, to: U.Type) -> U {
>   return from as! U
> }
> 
> let copied = cast(from: super.copy(), to: type(of: self))
> 
> Slava
> 
>> 
>> Slava
>> 
>>> On Aug 10, 2017, at 8:16 PM, 吴君恺 via swift-users >> > wrote:
>>> 
>>> Hi all,
>>> 
>>> here is the scenario, I want to implement a Copying protocol which can be 
>>> used homogeneously
>>> 
>>> protocol Copying {
>>> func copy() -> Self
>>> }
>>> 
>>> Because this protocol contains neither associated-type-requirements or 
>>> self-requirements(requiring a method returning Self is different), it can 
>>> surely be used homogeneously.
>>> 
>>> let objects: [Copying] = 
>>> for copyable in objects {
>>> .
>>> }
>>> 
>>> It will work if I make one of my base class conform to Copying
>>> 
>>> class Shape: Copying {
>>> var color: UIColor?
>>> 
>>> required init() {}
>>> 
>>> func copy() -> Self {
>>> let copied = type(of: self).init()
>>> copied.color = color
>>> return copied
>>> }
>>> }
>>> 
>>> The implementation of `copy` above is forced by compiler to avoid any 
>>> explicit specification of type `Shape`, so that the returning value will 
>>> have a dynamic type, which is just what I want. Here, the type of `copied` 
>>> is `Self`
>>> 
>>> However, if I try to make a subclass of Shape, I can't find a elegant way 
>>> to implement this `copy` method in that subclass, the following code will 
>>> not compile.
>>> 
>>> class Circle: Shape {
>>> var radius: Float = 5
>>> 
>>> func copy() -> Self {
>>> let copied = super.copy() 
>>> copied.radius = radius // compilation error
>>> return copied
>>> }
>>> }
>>> 
>>> The compiler will complain that `copied` has no property `radius`. It turns 
>>> out that calling copy() on super will yield a value of type Shape, rather 
>>> than Self. 
>>> 
>>> Swift now forbids explicit conversion to `Self` (I totally agree with this 
>>> rule), and will automatically allow `Self` to be treated as a specific type 
>>> in some circumstances. But for this case, I wonder whether this is the 
>>> correct behavior or a bug? Why calling `super.copy()` not be able to get a 
>>> `Self`?
>>> 
>>> I did find a work-around for this problem afterwards, but this question 
>>> really haunts me...
>>> 
>>> Cheers,
>>> Lincoln.
>>> ___
>>> swift-users mailing list
>>> swift-users@swift.org 
>>> https://lists.swift.org/mailman/listinfo/swift-users 
>>> 
>> 
>> ___
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>> https://lists.swift.org/mailman/listinfo/swift-users
> 

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Re: [swift-users] Question with calling a method which returns `Self` on super

[Slava Pestov via swift-users]

> On Aug 10, 2017, at 8:23 PM, Slava Pestov via swift-users 
>  wrote:
> 
> Hi Lincoln,
> 
> This is a known issue with ‘super’ method calls: 
> https://bugs.swift.org/browse/SR-1736 
> 
> A workaround is to cast the result of the supermethod call to Self explicitly:
> 
> let copied = super.copy() as! Self

I’m sorry, but this is nonsense. We don’t allow ‘Self’ to refer to the dynamic 
Self type yet.

You could do something like this I guess,

func cast(from: T, to: U.Type) -> U {
  return from as! U
}

let copied = cast(from: super.copy(), to: type(of: self))

Slava

> 
> Slava
> 
>> On Aug 10, 2017, at 8:16 PM, 吴君恺 via swift-users > > wrote:
>> 
>> Hi all,
>> 
>> here is the scenario, I want to implement a Copying protocol which can be 
>> used homogeneously
>> 
>> protocol Copying {
>> func copy() -> Self
>> }
>> 
>> Because this protocol contains neither associated-type-requirements or 
>> self-requirements(requiring a method returning Self is different), it can 
>> surely be used homogeneously.
>> 
>> let objects: [Copying] = 
>> for copyable in objects {
>> .
>> }
>> 
>> It will work if I make one of my base class conform to Copying
>> 
>> class Shape: Copying {
>> var color: UIColor?
>> 
>> required init() {}
>> 
>> func copy() -> Self {
>> let copied = type(of: self).init()
>> copied.color = color
>> return copied
>> }
>> }
>> 
>> The implementation of `copy` above is forced by compiler to avoid any 
>> explicit specification of type `Shape`, so that the returning value will 
>> have a dynamic type, which is just what I want. Here, the type of `copied` 
>> is `Self`
>> 
>> However, if I try to make a subclass of Shape, I can't find a elegant way to 
>> implement this `copy` method in that subclass, the following code will not 
>> compile.
>> 
>> class Circle: Shape {
>> var radius: Float = 5
>> 
>> func copy() -> Self {
>> let copied = super.copy() 
>> copied.radius = radius // compilation error
>> return copied
>> }
>> }
>> 
>> The compiler will complain that `copied` has no property `radius`. It turns 
>> out that calling copy() on super will yield a value of type Shape, rather 
>> than Self. 
>> 
>> Swift now forbids explicit conversion to `Self` (I totally agree with this 
>> rule), and will automatically allow `Self` to be treated as a specific type 
>> in some circumstances. But for this case, I wonder whether this is the 
>> correct behavior or a bug? Why calling `super.copy()` not be able to get a 
>> `Self`?
>> 
>> I did find a work-around for this problem afterwards, but this question 
>> really haunts me...
>> 
>> Cheers,
>> Lincoln.
>> ___
>> swift-users mailing list
>> swift-users@swift.org 
>> https://lists.swift.org/mailman/listinfo/swift-users
> 
> ___
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Re: [swift-users] Question with calling a method which returns `Self` on super

[Slava Pestov via swift-users]

Hi Lincoln,

This is a known issue with ‘super’ method calls: 
https://bugs.swift.org/browse/SR-1736 

A workaround is to cast the result of the supermethod call to Self explicitly:

let copied = super.copy() as! Self

Slava

> On Aug 10, 2017, at 8:16 PM, 吴君恺 via swift-users  
> wrote:
> 
> Hi all,
> 
> here is the scenario, I want to implement a Copying protocol which can be 
> used homogeneously
> 
> protocol Copying {
> func copy() -> Self
> }
> 
> Because this protocol contains neither associated-type-requirements or 
> self-requirements(requiring a method returning Self is different), it can 
> surely be used homogeneously.
> 
> let objects: [Copying] = 
> for copyable in objects {
> .
> }
> 
> It will work if I make one of my base class conform to Copying
> 
> class Shape: Copying {
> var color: UIColor?
> 
> required init() {}
> 
> func copy() -> Self {
> let copied = type(of: self).init()
> copied.color = color
> return copied
> }
> }
> 
> The implementation of `copy` above is forced by compiler to avoid any 
> explicit specification of type `Shape`, so that the returning value will have 
> a dynamic type, which is just what I want. Here, the type of `copied` is 
> `Self`
> 
> However, if I try to make a subclass of Shape, I can't find a elegant way to 
> implement this `copy` method in that subclass, the following code will not 
> compile.
> 
> class Circle: Shape {
> var radius: Float = 5
> 
> func copy() -> Self {
> let copied = super.copy() 
> copied.radius = radius // compilation error
> return copied
> }
> }
> 
> The compiler will complain that `copied` has no property `radius`. It turns 
> out that calling copy() on super will yield a value of type Shape, rather 
> than Self. 
> 
> Swift now forbids explicit conversion to `Self` (I totally agree with this 
> rule), and will automatically allow `Self` to be treated as a specific type 
> in some circumstances. But for this case, I wonder whether this is the 
> correct behavior or a bug? Why calling `super.copy()` not be able to get a 
> `Self`?
> 
> I did find a work-around for this problem afterwards, but this question 
> really haunts me...
> 
> Cheers,
> Lincoln.
> ___
> swift-users mailing list
> swift-users@swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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