[swift-evolution] [Proposal] Uniform Initialization Syntax

[Gor Gyolchanyan]

The type of `self` could remain `inout Self` inside the failable initializer. The ability to assign nil would be a compiler magic (much like `return nil` is compiler magic) that is meant to introduce uniformity to the initialization logic.

The idea is to define all different ways initialization can take place and expand them to be used uniformly on both `self` and all its members, as well as remove the ways that do not make sense for their purpose.

Currently, there are 3 ways of initializing self as a whole:
	1. delegating initializer
	2. assigning to self
	3. returning nil

#1: The delegating initializer is pretty much perfect at this point, in my opinion, so no changes there.

#2: The only exception in assigning to self is the `nil` inside failable initializers.

#3:  The only thing that can be returned from an initializer is `nil`, which is compiler magic, so we can thing of it as a misnomer (because we aren't really **returning** anything).

If, for a second, we forget about potential factory initializers, returning anything from an initializer doesn't make much sense, because an initializer is conceptually meant to bring an existing object in memory to a type-specific valid state. This semantic was very explicitly in Objective-C with `[[MyType alloc] init]`. Especially since even syntactically, the initializer does not specify any return type, the idea of returning from an initializer is counter-intuitive both syntactically and semantically.

The actual *behavior* of `return nil` is very sensible, so the behavior, I imagine `self = nil`, would largely mean the same (except not needed to return immediately and allowing non-self-accessing code to be executed before return). Being able to assign `nil` to a non-optional (ExpressibleByNilLiteral doesn't count) may feel a bit wonky, but not as wonky as returning nil from something that is meant to initialize an object in-place and doesn't look like it should return anything.

# Factory Initializers

In case of factory initializers, the much discussed `factory init` syntax could completely flip this logic, but making the initializer essentially a static function that returns an object. In this case the initializer could be made to specify the return type (that is the supertype of all possible factory-created objects) and assigning to self would be forbidden because there is not self yet:

extension MyProtocol {

	public factory init(weCool: Bool) -> MyProtocol {
		self = MyImpl() // error: cannot assign to `self` in a factory initializer
		self.init(...) // error: cannot make a delegating initializer call in a factory initializer
		if weCool {
			return MyCoolImpl()
		} else {
			return MyUncoolImpl()
		}
	}

}

# In-place Member Initializers

In addition, member initialization currently is only possible with #2 (as in `self.member = value`), which could be extended in a non-factory initializer to be initializable in-place like this:

self.member.init(...)

This would compliment the delegating initialization syntax, while giving a more reliable performance guarantee that this member will not be copy-initialized.

> On Jun 9, 2017, at 1:32 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> If `self` is not of type `inout Self?`, then what is the type of `self` such that you may assign it a value of `nil`?
> 
> It certainly cannot be of type `inout Self`, unless `Self` conforms to `ExpressibleByNilLiteral`, in which case you are able to assign `self = nil` an unlimited number of times–but that has a totally different meaning.
> 
> Could `self` be of type `inout Self!`? Now that implicitly unwrapped optionals are no longer their own type, I’m not sure that’s possible. But even if it were, that seems unintuitive and potentially error-prone.
> 
> So I think Greg is quite right that, to enable this feature, `self` would have to be of type `inout Self?`–which is intriguing but potentially more boilerplatey than the status quo.
> On Fri, Jun 9, 2017 at 05:24 Gor Gyolchanyan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Good point, but not necessarily.
> Since you cannot access `self` before it being fully initialized and since `self` can only be initialized once, this would mean that after `self = nil`, you won't be allowed to access `self` in your initializer at all.You'll be able to do any potential, cleanup though.
> Also, since there can be only one `self = nil`, there's no reason to treat `self` as `inout Self?`, because the only place it can be `nil` is the place it cannot be accessed any more.
> 
> 
>> On Jun 9, 2017, at 7:45 AM, Greg Parker <gparker at apple.com <mailto:gparker at apple.com>> wrote:
>> 
>> 
>>> On Jun 8, 2017, at 5:09 AM, Gor Gyolchanyan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 1. Arbitrary `self` Assignments In Intializers
>>> 
>>> The first ideas is to allow `self = nil` inside failable initializers (essentially making `self` look like `inout Self?` instead of `inout Self` with magical `return nil`), so that all initializers uniformly can be written in `self = ...` form for clarity and convenience purposes. This should, theoretically, be nothing but a `defer { return nil }` type of rewrite, so I don't see any major difficulties implementing this. This is especially useful for failable-initializing enums where the main switch simply assigns to self in all cases and the rest of the initializer does some post-processing.
>> 
>> I don't see how to avoid source incompatibility and uglification of failable initializer implementations here. Allowing `self = nil` inside a failable initializer would require `self` to be an optional. That in turn would require every use of `self` in the initializer to be nil-checked or forced. I don't think that loss everywhere outweighs the gain of `self = nil` in some places.
>> 
>> 
>> -- 
>> Greg Parker     gparker at apple.com <mailto:gparker at apple.com>     Runtime Wrangler
>> 
>> 
> 
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