[swift-evolution] [Proposal] Factory Initializers

Fri Mar 17 13:53:27 CDT 2017

> What's the motivation behind using `return` rather than self-assignment, like we currently have in inits for structs/enums and protocol extensions?

From the original discussion, it seemed the general consensus was that returning a value felt more natural than assigning to self. Here are some snippets from the original discussion:

Myself:
> I’m not opposed to assigning to self directly in convenience initializers (especially if there is already support for it in the ABI). My only concern would be that it feels less “natural” to do so than to simply return a value from the initializer. That being said, I think that’s a very negligible disadvantage (if even that), and if assigning to self is the easiest way to pull this off, I’m all for it.

Dave Abrahams:
> My instinct agrees with that.  Also, reassigning self raises the question of whether an object is allocated (and partly initialized?) before the reassignment.  Even if we can answer those questions in some clear way, I’d rather not have them come up at all.

Stephen Christopher:
> I agree. Were I to naively try this in Swift, I would expect to use a convenience initializer and return the instance I’d created. 

Ultimately I'm happy with either returning a value or assigning to self, depending on what the consensus is after a review.

> Is the "factory" keyword truly necessary, or could it be implied by "required" and using self-assignment in the init body?

Factory initializers would be closer to convenience initializers rather than required ones, and I think the keyword is semantically useful. Convenience/required initializers are expected to return a type matching that of the static type, while the "factory" keyword signifies that the return value may not match, just be upperbounded by the static type.

Additionally, when dealing with factory initializers in protocol extensions, I think it is much more clear in intent that the function will return a type that conforms to the protocol rather than a hypothetical "instance" of the protocol itself.

> On Mar 17, 2017, at 1:15 PM, Zach Waldowski via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Big +1.
> 
> Two small nits/questions:
> 
> - What's the motivation behind using `return` rather than self-assignment, like we currently have in inits for structs/enums and protocol extensions? I didn't follow the original discussion in depth, so excuse me if this has been hashed out before.
> - Is the "factory" keyword truly necessary, or could it be implied by "required" and using self-assignment in the init body?
> 
> Best,
>   Zachary Waldowski
>   zach at waldowski.me
> 
> 
>> On Fri, Mar 17, 2017, at 12:26 PM, Riley Testut via swift-evolution wrote:
>> 
>> 
>> Hi again everyone!
>> 
>> Now that Swift 4 Stage 2 proposals are being considered, I thought it might be time to revisit this proposal and see if it might align with the goals set forth for Swift 4.
>> 
>> As a quick tl;dr, this proposal describes a new "factory initializer" that would allow you to return a value from an initializer. This would have several benefits, as mentioned in the proposal itself as well as throughout this mailing list. For convenience, here's a link to the proposal on GitHub: https://github.com/rileytestut/swift-evolution/blob/master/proposals/NNNN-factory-initializers.md
>> 
>> Would love to hear any more comments on this proposal, and if we feel this is appropriate for considering for Swift 4 I'll happily re-open the pull request!
>> 
>> Riley Testut
>> 
>>> On Nov 19, 2016, at 7:45 AM, arkadi daniyelian <arkdan at icloud.com> wrote:
>>> i would appreciate this feature.
>>> 
>>> For unexperienced developers, its often hard to recognize *when* factory is a good fit to do the job, and how exactly approach the implementation. I imagine having this feature built into the language may help to choose and implement factory when its the right thing to do.
>>> 
>>>> On Nov 18, 2016, at 12:23 AM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Is there any chance of reviving this? It seems to me that since this would require Swift initializers to be implemented internally in such a way that they can return a value (as Objective-C init methods do), it may affect ABI stability and thus may be germane to the current stage of Swift 4 development.
>>>> 
>>>> Charles
>>>> 
>>>>> On Dec 17, 2015, at 3:41 PM, Riley Testut via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> Recently, I proposed the idea of adding the ability to implement the "class cluster" pattern from Cocoa (Touch) in Swift. However, as we discussed it and came up with different approaches, it evolved into a functionality that I believe is far more beneficial to Swift, and subsequently should be the focus of its own proposal. So here is the improved (pre-)proposal:
>>>>> 
>>>>> # Factory Initializers
>>>>> 
>>>>> The "factory" pattern is common in many languages, including Objective-C. Essentially, instead of initializing a type directly, a method is called that returns an instance of the appropriate type determined by the input parameters. Functionally this works well, but ultimately it forces the client of the API to remember to call the factory method instead, rather than the type's initializer. This might seem like a minor gripe, but given that we want Swift to be as approachable as possible to new developers, I think we can do better in this regard.
>>>>> 
>>>>> Rather than have a separate factory method, I propose we build the factory pattern right into Swift, by way of specialized “factory initializers”. The exact syntax was proposed by Philippe Hausler from the previous thread, and I think it is an excellent solution:
>>>>> 
>>>>> class AbstractBase {
>>>>>  public factory init(type: InformationToSwitchOn) {
>>>>>      return ConcreteImplementation(type)
>>>>>  }
>>>>> }
>>>>> 
>>>>> class ConcreteImplementation : AbstractBase {
>>>>> 
>>>>> }
>>>>> 
>>>>> Why exactly would this be useful in practice? In my own development, I’ve come across a few places where this would especially be relevant:
>>>>> 
>>>>> ## Class Cluster/Abstract Classes
>>>>> This was the reasoning behind the original proposal, and I still think it would be a very valid use case. The public superclass would declare all the public methods, and could delegate off the specific implementations to the private subclasses. Alternatively, this method could be used as an easy way to handle backwards-compatibility: rather than litter the code with branches depending on the OS version, simply return the OS-appropriate subclass from the factory initializer. Very useful.
>>>>> 
>>>>> ## Protocol Initializers
>>>>> Proposed by Brent Royal-Gordon, we could use factory initializers with protocol extensions to return the appropriate instance conforming to a protocol for the given needs. Similar to the class cluster/abstract class method, but can work with structs too. This would be closer to the factory method pattern, since you don’t need to know exactly what type is returned, just the protocol it conforms to.
>>>>> 
>>>>> ## Initializing Storyboard-backed View Controller
>>>>> This is more specific to Apple Frameworks, but having factory initializers could definitely help here. Currently, view controllers associated with a storyboard must be initialized from the client through a factory method on the storyboard instance (storyboard. instantiateViewControllerWithIdentifier()). This works when the entire flow of the app is storyboard based, but when a single storyboard is used to configure a one-off view controller, having to initialize through the storyboard is essentially use of private implementation details; it shouldn’t matter whether the VC was designed in code or storyboards, ultimately a single initializer should “do the right thing” (just as it does when using XIBs directly). A factory initializer for a View Controller subclass could handle the loading of the storyboard and returning the appropriate view controller.
>>>>> 
>>>>> Here are some comments from the previous thread that I believe are still relevant:
>>>>> 
>>>>> 
>>>>>> On Dec 9, 2015, at 1:06 PM, Philippe Hausler <phausler at apple.com> wrote:
>>>>>> 
>>>>>> I can definitely attest that in implementing Foundation we could have much more idiomatic swift and much more similar behavior to the way Foundation on Darwin actually works if we had factory initializers.
>>>>> 
>>>>> 
>>>>>> On Dec 7, 2015, at 5:24 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>>>>>> 
>>>>>> A `protocol init` in a protocol extension creates an initializer which is *not* applied to types conforming to the protocol. Instead, it is actually an initializer on the protocol itself. `self` is the protocol metatype, not an instance of anything. The provided implementation should `return` an instance conforming to (and implicitly casted to) the protocol. Just like any other initializer, a `protocol init` can be failable or throwing.
>>>>>> 
>>>>>> Unlike other initializers, Swift usually won’t be able to tell at compile time which concrete type will be returned by a protocol init(), reducing opportunities to statically bind methods and perform other optimization tricks. Frankly, though, that’s just the cost of doing business. If you want to select a type dynamically, you’re going to lose the ability to aggressively optimize calls to the resulting instance.
>>>>> 
>>>>> 
>>>>> I’d love to hear everyone’s thoughts on this!
>>>>> 
>>>>> Best,
>>>>> Riley Testut
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