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

[Ross O'Brien]

Lazy properties can't make use of parameters of the init() without storing
them, and can't calculate parameters to be used in a super.init() (or
decide which super.init() is called, if there are several to choose from).
Part of the idea of the selfless function is to keep local lets/vars at
local scope rather than type scope, and allow functions which perform
calculations in an initialisation to be reusable.
Static functions can perform these calculations but can't set instance
properties.
At the moment, for an init() to use initial parameters without storing
them, all the code for setting properties has to be in the init; I don't
think there's a way to break an init up into a sequence of smaller function
calls, each dealing with a lower level of abstraction of the init. i.e.
something like this:

init(<initial arguments>)
{
    initialiseColor(<subset of arguments>)
    initialiseSize(<subset of arguments>)
    initialiseFont(<subset of arguments>)
}


On Wed, Dec 16, 2015 at 1:06 AM, Marc Knaup <marc at knaup.koeln> wrote:

> In its current state with the initial example -1 from me for the proposal.
>
> The example could easily be written like this:
>
> class FooView: UIView {
>
>     var property = 4
>
>     init() {
>         super.init()
>     }
>
>     init(frame: CGRect) {
>         super.init(frame)
>     }
> }
>
>
> In cases where the initially value is computed in a complex way a closure
> can be used:
>
> class FooView: UIView {
>
>     var property: Int = {
>         // some complicated computation
>         return value
>     }()
>
>     init() {
>         super.init()
>     }
>
>     init(frame: CGRect) {
>         super.init(frame)
>     }
> }
>
> And that value could even delegate computation to static methods of the
> class.
>
> In cases where the computation is even more complex and refers to other
> properties a lazy var can be used, which even allows the value to refer
> to self:
>
> class FooView: UIView {
>
>     private(set) lazy var property: Int = {
>         // some complicated computation which can use self
>         return value
>     }()
>
>     init() {
>         super.init()
>     }
>
>     init(frame: CGRect) {
>         super.init(frame)
>     }
> }
>
>
>
> On Wed, Dec 16, 2015 at 1:30 AM, 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> 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
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>> +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
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>
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