[swift-evolution] [Pitch] consistent public access modifiers

[Adrian Zubarev]

A short example where I personally wanted a public-but-not-open protocol:

public protocol SubscriptParameterType {
     
    // This property was needed to prevent the client from breaking
    // the library by conforming to the protocol, but I'd like to  
    // keep it invisible for the client, or even better prevent the
    // client from conforming to the protocol.
    var parameter: Document.SubscriptParameter { get }
}

extension Document {
     
    public enum SubscriptParameter {
             
        case string(String)
        case integer(Int)
    }
}

extension String : SubscriptParameterType {
     
    public var parameter: Document.SubscriptParameter {
         
        return .string(self)
    }
}

extension Int : SubscriptParameterType {
     
    public var parameter: Document.SubscriptParameter {
         
        return .integer(self)
    }
}

// Somewhere inside the `Document` type
public subscript(firstKey: String, parameters: SubscriptParameterType...) -> Value? { … }
That implementation enables more safe queries of my Document type like document["key1", intIndexInstance, stringKeyInstance, 10, "key"] rather than document["key1/\(intIndexInstance)/\(stringKeyInstance)/10/key"].



-- 
Adrian Zubarev
Sent with Airmail

Am 15. Februar 2017 um 17:03:32, Matthew Johnson via swift-evolution (swift-evolution at swift.org) schrieb:


On Feb 15, 2017, at 9:59 AM, Rien <Rien at Balancingrock.nl> wrote:


On 15 Feb 2017, at 16:45, Matthew Johnson <matthew at anandabits.com> wrote:


On Feb 15, 2017, at 9:35 AM, Rien <Rien at Balancingrock.nl> wrote:


On 15 Feb 2017, at 16:11, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:


On Feb 15, 2017, at 5:59 AM, Jeremy Pereira via swift-evolution <swift-evolution at swift.org> wrote:


On 15 Feb 2017, at 11:11, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:


Our philosophy in general, however, is to default to the behavior which preserves the most flexibility for the library designer.

Actually, I thought the philosophy was to preserver type safety. When did that change?

Also, when was the library designer prioritised ahead of the application developer?


Both open and non-open classes are common, but we chose to give non-open classes the `public` keyword because that's the flexibility-preserving option.

No it isn’t, it’s the flexibility restricting option. The consumer of an open class can subclass it. The consumer of a public class cannot subclass it. How is the second more flexible than the first?

It reduces complexity for the library author by allowing them to opt-out of the complexity involved in supporting unknown, user-defined subclasses.  It is important to allow libraries to have this flexibility. They are free to declare a class `open` if they want to allow subclassing. It’s even possibly for a library to declare all classes `open` if it wishes to do so.  But *requiring* that would reduce the design space libraries are allowed to explore and / or introduce fragility by moving the subclass restriction to a comment.


Why would a library author want to prohibit subclasses?
A library user can always wrap the class and subclass the wrapper.

This is composition, not inheritance.  The most important difference is that a wrapper cannot override methods, it can only wrap and / or forward them.  This means that when the superclass calls a method on `self` that method *always* invokes its version of that method rather than a subclass override.  This is a very important difference.


Agreed, however that does not answer the question why would a library developer want to disallow subclassing?
I do not see a use case for that. I.e. a feature that cannot be implemented without it. (without “open”)

The feature it enables is more robust libraries and the ability for library authors to better reason about their code.  You may not find this benefit enough to be worth a language feature, but many of us do.


Rien.


There are cases where subclassing does not make sense. And thus preventing subclasses adds information for those users that don’t RTFM. But that imo is not worth the impact extra complexity places on all other users.

Rien.



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