[swift-evolution] [Proposal] Uniform Initialization Syntax

[Gor Gyolchanyan]

Disclaimer: I do realize that any of the following ideas may have been discussed before and/or there might be a good reason for their lack of implementation, so please go easy in me. 🙂

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.

2. Arbitrary `return` Statements In Intializers

The second idea is to allow `return ...` inside all initializers, which should also, theoretically, be a simple rewrite to `self = ...; return`. This one is to complement the existing `return nil` and allow some super-short initializers with a switch that returns in all cases or a more complex initializer that has a lot of guard statements.

2.1. Future Factory Initializers

In addition, the `return ...` syntax has the benefit for potential factory initializers. So far, the proposals for factory initializers involved a keyword attached to the initializer, which just complicates the lexical structure of the language and adds unnecessary complication to the interface of types. Currently, factory initializers imported from Objective-C or from C using the `__attribute__((swift_name("MyType.init(self:...)")))` look like normal initializers (an in case of C, the return value doesn't even have to be related to the enclosing type in any way), but behave as you'd expect: you call the initializer and the result is a value that *should* be a subtype of the type you've called the initializer for. So, if in the future Swift gets native factory initializers (including, most importantly, in protocols), it won't require special syntax, because simply returning an instance of a subtype (with a compile-time check, of course) would look and behave very intuitively. This would also be very useful for singletons, which would use a private initializer for creating the instance and a public factory initializer for returning it.

3. Failable Member Initialization

The third idea is to allow writing `self.member = MemberType?(...)` or `self.member = .init?(...)` (the exact syntax is up to debate) inside failable initializers, which would be simply rewritten as:

guard let _self_member = MemberType(...)  else {
	return nil
}
self.member = _self_member

This will dramatically reduce the boilerplate and visual clutter form complex failable initializers that call other failable initializers. A good use case would be complex `LosslessStringConvertible` types with many `LosslessStringConvertible` members.

So, what do you guys think?