Superclass constraint cannot depend on type parameter


(Jon Shier) #1

Swifters:
  I just encountered the error “Superclass constraint ‘RepeatableNetworkOperation<U>’ cannot depend on a type parameter” on this definition:

class RepeatedNetworkOperation<T, U where U: Decodable, U == U.DecodedType, U: Repeatable, T: RepeatableNetworkOperation<U>>: RepeatedOperation<T> { … }

where the relevant other classes are defined as:

class InterruptibleNetworkOperation<T: Decodable where T == T.DecodedType>: GroupOperation { … }
class RepeatedOperation<T where T: NSOperation>: GroupOperation { … }
class RepeatableNetworkOperation<T where T: Decodable, T == T.DecodedType, T: Repeatable>: InterruptibleNetworkOperation<T>, Repeatable { … }

I understand the error, but am I correct in thinking this is just a limitation of Swift’s current type and/or generics systems? I’m not a language theorist, but it seems perfectly reasonable that something like this should work. If it is just a limitation and not something I’m doing wrong, are there any suggestions for a workaround? The class I’m trying to write would really clean up my code.

Jon


(Slava Pestov) #2

Hi Jon,

You are right that this is a limitation of the current implementation in Swift 2.2. I may have fixed this recently in master:

https://github.com/apple/swift/commit/3aacf5d805768527b59d24e6da2a03911b3dd5b0

It would be great if you could try the latest compiler with your code.

Slava

···

On Mar 9, 2016, at 12:48 PM, Jon Shier via swift-users <swift-users@swift.org> wrote:

Swifters:
  I just encountered the error “Superclass constraint ‘RepeatableNetworkOperation<U>’ cannot depend on a type parameter” on this definition:

class RepeatedNetworkOperation<T, U where U: Decodable, U == U.DecodedType, U: Repeatable, T: RepeatableNetworkOperation<U>>: RepeatedOperation<T> { … }

where the relevant other classes are defined as:

class InterruptibleNetworkOperation<T: Decodable where T == T.DecodedType>: GroupOperation { … }
class RepeatedOperation<T where T: NSOperation>: GroupOperation { … }
class RepeatableNetworkOperation<T where T: Decodable, T == T.DecodedType, T: Repeatable>: InterruptibleNetworkOperation<T>, Repeatable { … }

I understand the error, but am I correct in thinking this is just a limitation of Swift’s current type and/or generics systems? I’m not a language theorist, but it seems perfectly reasonable that something like this should work. If it is just a limitation and not something I’m doing wrong, are there any suggestions for a workaround? The class I’m trying to write would really clean up my code.

Jon

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(Jon Shier) #3

Slava:
  I won’t be able to test my actual code, as the Operations and Alamofire libraries haven’t been updated for Swift 3 (which I wouldn’t expect), but I’ll try to put together a test cast that is similar and see if it works? When will this change be part of the package I can download for Xcode 7.3?

Jon

···

On Mar 17, 2016, at 1:01 PM, Slava Pestov <spestov@apple.com> wrote:

Hi Jon,

You are right that this is a limitation of the current implementation in Swift 2.2. I may have fixed this recently in master:

https://github.com/apple/swift/commit/3aacf5d805768527b59d24e6da2a03911b3dd5b0

It would be great if you could try the latest compiler with your code.

Slava

On Mar 9, 2016, at 12:48 PM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Swifters:
  I just encountered the error “Superclass constraint ‘RepeatableNetworkOperation<U>’ cannot depend on a type parameter” on this definition:

class RepeatedNetworkOperation<T, U where U: Decodable, U == U.DecodedType, U: Repeatable, T: RepeatableNetworkOperation<U>>: RepeatedOperation<T> { … }

where the relevant other classes are defined as:

class InterruptibleNetworkOperation<T: Decodable where T == T.DecodedType>: GroupOperation { … }
class RepeatedOperation<T where T: NSOperation>: GroupOperation { … }
class RepeatableNetworkOperation<T where T: Decodable, T == T.DecodedType, T: Repeatable>: InterruptibleNetworkOperation<T>, Repeatable { … }

I understand the error, but am I correct in thinking this is just a limitation of Swift’s current type and/or generics systems? I’m not a language theorist, but it seems perfectly reasonable that something like this should work. If it is just a limitation and not something I’m doing wrong, are there any suggestions for a workaround? The class I’m trying to write would really clean up my code.

Jon

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