[swift-evolution] Proposal: 'selfless' keyword for refactoring duplicate code from initialisers

[Colin Cornaby]

+1 on not being sure selfless is the right keyword, but liking the direction of this proposal. I haven’t hit this issue yet in my Swift projects, but I see this pattern all the time in Obj-C.

I’m trying to play devils advocate with myself and figure out if there is a way around without adding a new concept to the language… Not coming up with much so far.

> On Dec 15, 2015, at 4:30 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Dec 15, 2015, at 5:59 PM, Ross O'Brien via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Hi all,
>> 
>> I'm a new member of the list, so apologies if this is a duplicate of an existing idea or if there's already a way to do this in Swift 2.1 that I've missed.
>> 
>> In Objective C, and C-like languages, an initialiser function represents a stage after allocation of memory where properties are given values. In Swift, init appears to precede (or overlap with) allocation. The benefit of this is that for type-safety reasons, all properties of a type (or new properties of a derived type) can be verified as having values. The disadvantage, and one of the stumbling blocks for those who learned Objective-C, is that until all the properties have values, the instance does not exist and instance functions cannot be called.
>> 
>> There's an invisible threshold in Swift init() functions marking this transition. In derived classes it's the point where super.init() is called - after the derived type has provided initial values, but before any type functions can be called.
>> 
>> Some types have multiple initialisers, and may be duplicating a lot of code in those distinct inits before they cross the threshold. This code can't be refactored into an instance function because the instance doesn't exist yet. The instance function may not even require the use of any properties of the type.
>> 
>> If the compiler can read an init function and its varied control flow and determine a threshold where all properties have values, presumably it can read the code of any function called before that threshold, determine which properties they read and which they assign to, and provide a warning if a path assigns to a constant a second time, etc.. But this isn't currently happening.
>> 
>> I'm guessing there are multiple contributing factors for this: the combinatorial explosion of possible control flow paths with functions (particularly if they're recursive); the possibility that the function calls are used by the compiler to mark the end of a control flow path, by which point it can determine whether everything has a value; the function genuinely can't exist without allocation. I don't know the reasons but I'd be interested to learn them.
>> 
>> I'm proposing the keyword 'selfless' for a function which could be called before the threshold. It either only uses local properties or properties belonging to the type - never to the 'super' type (in the case of a derived class). It can't call any instance functions which aren't themselves selfless.
>> 
>> Example of use:
>> class FooView : UIView
>> {
>>     var property : Int
>> 
>>     init()
>>     {
>>         initialiseProperty()
>>         super.init()
>>     }
>> 
>>     init(frame:CGRect)
>>     {
>>         initialiseProperty()
>>         super.init(frame)
>>     }
>> 
>>     selfless func initialiseProperty()
>>     {
>>         property = 4
>>     }
>> }
>> 
>> Is this something of interest?
>> 
>> Regards,
>> Ross O'Brien
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> 
> +1. This is something that I was planning to propose. It comes up particularly often in Cocoa objects that implement NSCoding, where you have to implement both init(coder:) and the designated initializer. Currently, if you have a bunch of complicated code involved in setting defaults for your properties, in a manner that’s too complex to solve with simple default values, you end up with a lot of copy-paste code in the two initializers, which can easily get out of sync if one is edited without being diligent about editing the other one in the same way. The exception, of course, if if you make init(coder:) a convenience initializer, but then subclasses cannot call super’s implementation of init(coder:), which makes this unworkable in a lot of circumstances.
> 
> I’m not sure “selfless” is the right keyword for this, but some sort of feature along these lines would be incredibly helpful.
> 
> Charles
> 
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