Re: [swift-users] Still can't derive from a generic class
Hi Joe
Thanks for your input on this.
> This is likely due to the runtime not handling cycles properly in type
> metadata initialization—a subclass T needs its base class metadata for
> BaseObject, which in turn needs the metadata for T to instantiate the
> generic type. This is something we plan to fix next year since it requires
> runtime ABI changes.
Hmmm. I'm finding this really frustrating, waiting for features that I have
been using for years, in another popular language (C#), to see the light of day.
Maybe application programmers are happy with Swift so far but, when it comes to
my speciality of designing frameworks, I am constantly finding myself blocked
by lack of metadata APIs :-(
I am talking about a couple of frameworks that underpin one of the largest
business management systems in Europe, for which the client is asking a
MacOS/iOS version.
> Would a protocol-based approach suffice? `Self` in a protocol extension would
> give you access to the concrete type in a similar way:
>
> protocol Base: AnyObject {
> var properties: [PartialKeyPath : AnyProperty] { get }
> }
>
> extension Base {
> func value(for keyPath: KeyPath) -> T?
> {
>guard let property = properties[keyPath] else
>{
> return nil
>}
>
>return property.getValue()
> }
>
> /*etc.*/
> }
>
> class Test: Base {
> let properties = [\Test.name: Property()]
>
> var name: String {
>get { return value(for: \.name) }
>set { set(value: newValue, for: \.name) }
> }
> }
At first glance, I thought, Yes! But then I realised that this then requires a
final class to cater for the use of Self in the protocol; something that would
be a blocker for the class hierarchies that most users of the framework would
want to create on top of the Base functionality.
I suppose I could look at using protocol-oriented composition but trying to
avoid associated types in protocols in frameworks that use generics extensively
is akin to hitting yourself repeatedly on the head with a hammer ;-)
And I really don't want to have to keep on repeating type-erasure boilerplate
code to achieve that.
I think the realisation that I may well lose the chance of a pretty massive
contract and that my client will lose an important business opportunity is
finally dawning :-(
Joanna
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Joanna Carter
Carter Consulting
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Re: [swift-users] Still can't derive from a generic class
> On Aug 30, 2017, at 11:17 AM, Joanna Carter via swift-users
> wrote:
>
> Hi Kenny
>
>> Just curious, and because I have a distinct lack of imagination: can you
>> share a concrete case of this pattern?
This is likely due to the runtime not handling cycles properly in type metadata
initialization—a subclass T needs its base class metadata for BaseObject,
which in turn needs the metadata for T to instantiate the generic type. This is
something we plan to fix next year since it requires runtime ABI changes. For
the particular pattern you've described:
> class BaseObject
> {
> private let properties: [PartialKeyPath : AnyProperty]
>
> init(properties: [PartialKeyPath : AnyProperty])
> {
>self.properties = properties
> }
>
> func value(for keyPath: KeyPath) ->
> valueType?
> {
>guard let property = properties[keyPath] else
>{
> return nil
>}
>
>return property.getValue()
> }
>
> func set(value: valueType?, for keyPath: KeyPath valueType>)
> {
>guard let property = properties[keyPath] else
>{
> return
>}
>
>property.set(value: value)
> }
> }
>
> class Test : BaseObject
> {
> var name: String?
> {
>get
>{
> return value(for: \.name)!
>}
>set
>{
> set(value: newValue, for: \.name)
>}
> }
>
> var number: Int?
> {
>get
>{
> return value(for: \.number)!
>}
>set
>{
> set(value: newValue, for: \.number)
>}
> }
>
> init()
> {
>super.init(properties: [\Test.name : Property(), \Test.number :
> Property()])
> }
> }
>
> Without the generic rootType parameter, all the getter and setter methods
> need an explicit mention of the derived class for the keypath :
>
> class Test : BaseObject
> {
> var name: String?
> {
>get
>{
> return value(for: \Test.name)!
>}
>set
>{
> set(value: newValue, for: \Test.name)
>}
> }
>
> …
> }
Would a protocol-based approach suffice? `Self` in a protocol extension would
give you access to the concrete type in a similar way:
protocol Base: AnyObject {
var properties: [PartialKeyPath : AnyProperty] { get }
}
extension Base {
func value(for keyPath: KeyPath) -> T?
{
guard let property = properties[keyPath] else
{
return nil
}
return property.getValue()
}
/*etc.*/
}
class Test: Base {
let properties = [\Test.name: Property()]
var name: String {
get { return value(for: \.name) }
set { set(value: newValue, for: \.name) }
}
}
-Joe
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Re: [swift-users] Still can't derive from a generic class
Hi Kenny
> Just curious, and because I have a distinct lack of imagination: can you
> share a concrete case of this pattern?
class BaseObject
{
private let properties: [PartialKeyPath : AnyProperty]
init(properties: [PartialKeyPath : AnyProperty])
{
self.properties = properties
}
func value(for keyPath: KeyPath) -> valueType?
{
guard let property = properties[keyPath] else
{
return nil
}
return property.getValue()
}
func set(value: valueType?, for keyPath: KeyPath)
{
guard let property = properties[keyPath] else
{
return
}
property.set(value: value)
}
}
class Test : BaseObject
{
var name: String?
{
get
{
return value(for: \.name)!
}
set
{
set(value: newValue, for: \.name)
}
}
var number: Int?
{
get
{
return value(for: \.number)!
}
set
{
set(value: newValue, for: \.number)
}
}
init()
{
super.init(properties: [\Test.name : Property(), \Test.number :
Property()])
}
}
Without the generic rootType parameter, all the getter and setter methods need
an explicit mention of the derived class for the keypath :
class Test : BaseObject
{
var name: String?
{
get
{
return value(for: \Test.name)!
}
set
{
set(value: newValue, for: \Test.name)
}
}
…
}
It was all so simple in C# :(
Joanna
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Carter Consulting
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Re: [swift-users] Still can't derive from a generic class
Just curious, and because I have a distinct lack of imagination: can you share
a concrete case of this pattern?
Thanks!
-Kenny
> On Aug 29, 2017, at 10:04 AM, Joanna Carter via swift-users
> wrote:
>
> Hi
>
> I would have hoped by now that it should be possible to do :
>
> class BaseObject
> {
> }
>
> class Test : BaseObject
> {
> }
>
> All compiles well but, at runtime, when calling let test = Test(), I get a
> "EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" error.
>
> This is something I have been able to do in C# for many a year. Why oh why
> can I still not do it in Swift ?
>
> Joanna
>
> --
> Joanna Carter
> Carter Consulting
>
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Re: [swift-users] Still can't derive from a generic class
Just curious, and because I have a distinct lack of imagination: can you share
a concrete case of this pattern?
Thanks!
-Kenny
> On Aug 29, 2017, at 10:04 AM, Joanna Carter via swift-users
> wrote:
>
> Hi
>
> I would have hoped by now that it should be possible to do :
>
> class BaseObject
> {
> }
>
> class Test : BaseObject
> {
> }
>
> All compiles well but, at runtime, when calling let test = Test(), I get a
> "EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" error.
>
> This is something I have been able to do in C# for many a year. Why oh why
> can I still not do it in Swift ?
>
> Joanna
>
> --
> Joanna Carter
> Carter Consulting
>
> ___
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> swift-users@swift.org
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Re: [swift-users] Still can't derive from a generic class
Ah, I see. Making a subclass that inherits from a superclass generic to the
subclass doesn’t seem like it should be a thing, but I’m not a type theorist.
In any case, this is a bug (unexpected runtime crash and all) and you should
file it.
Jon
> On Aug 29, 2017, at 1:25 PM, Joanna Carter
> wrote:
>
> Hi Jon
>
>> Le 29 août 2017 à 19:20, Jon Shier a écrit :
>>
>> This works fine for me in a playground in the latest Xcode 9 beta:
>>
>> class Test { }
>>
>> class Base { }
>>
>> class Sub: Base { }
>>
>> let sub = Sub()
>
> That may well work but what I want is to be able to do is simpler than that :
>
> class Base { }
>
> class Test : Base { }
>
> let test = Test()
>
>
> Joanna
>
> --
> Joanna Carter
> Carter Consulting
>
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Re: [swift-users] Still can't derive from a generic class
Hi Jon
> Le 29 août 2017 à 19:20, Jon Shier a écrit :
>
> This works fine for me in a playground in the latest Xcode 9 beta:
>
> class Test { }
>
> class Base { }
>
> class Sub: Base { }
>
> let sub = Sub()
That may well work but what I want is to be able to do is simpler than that :
class Base { }
class Test : Base { }
let test = Test()
Joanna
--
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Carter Consulting
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Re: [swift-users] Still can't derive from a generic class
This works fine for me in a playground in the latest Xcode 9 beta:
class Test { }
class Base { }
class Sub: Base { }
let sub = Sub()
Jon
> On Aug 29, 2017, at 1:04 PM, Joanna Carter via swift-users
> wrote:
>
> Hi
>
> I would have hoped by now that it should be possible to do :
>
> class BaseObject
> {
> }
>
> class Test : BaseObject
> {
> }
>
> All compiles well but, at runtime, when calling let test = Test(), I get a
> "EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" error.
>
> This is something I have been able to do in C# for many a year. Why oh why
> can I still not do it in Swift ?
>
> Joanna
>
> --
> Joanna Carter
> Carter Consulting
>
> ___
> swift-users mailing list
> swift-users@swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
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[swift-users] Still can't derive from a generic class
Hi
I would have hoped by now that it should be possible to do :
class BaseObject
{
}
class Test : BaseObject
{
}
All compiles well but, at runtime, when calling let test = Test(), I get a
"EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" error.
This is something I have been able to do in C# for many a year. Why oh why can
I still not do it in Swift ?
Joanna
--
Joanna Carter
Carter Consulting
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