[swift-evolution] [Proposal Draft] partial initializers

[David Owens II]

> On Jan 10, 2016, at 7:44 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I have always considered the Flexible Memberwise Initialization proposal to be just a first step (as evidenced by the many future enhancements it discussed).  Its review has inspired new ideas and helped to shape my vision of the best long-term solution.    My final thoughts about the review can be found here: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160104/006176.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160104/006176.html>
> 
> Partial initializers is the second in a series of three proposals describing general features that can work together to form a complete solution.
> 
> The proposal drafts can be found at the following links:
> 
> * Parameter forwarding: https://github.com/anandabits/swift-evolution/blob/parameter-forwarding/proposals/NNNN-parameter-forwarding.md <https://github.com/anandabits/swift-evolution/blob/parameter-forwarding/proposals/NNNN-parameter-forwarding.md>
> * Partial initializers: https://github.com/anandabits/swift-evolution/blob/partial-initializers/proposals/NNNN-partial-initializers.md <https://github.com/anandabits/swift-evolution/blob/partial-initializers/proposals/NNNN-partial-initializers.md>
> * Property lists: https://github.com/anandabits/swift-evolution/blob/property-lists/proposals/NNNN-property-lists.md <https://github.com/anandabits/swift-evolution/blob/property-lists/proposals/NNNN-property-lists.md>
> 

The biggest drawback I see is that is doesn’t really address the ability to use normal functions to initialize the state of the type. For example, in order to implement a “reset” or “clear” mechanism, I still need to duplicate a bunch of init code.

class CFoo {
    private(set) var value: Int
    
    func reset() {
        value = 0
    }
    
    init() {
        // Really want to just call this...
        //reset()
        
        // But, instead, I need to duplicate the code from `reset()`
        value = 0
    }
    
    func inc() {
        ++value
    }
}

let c = CFoo()
c.inc()
c.value    // 1
c.reset()
c.value    // 0


Take the above code, the “init” functionality is really about putting the type in the correct state. I think I’d rather see a mechanism to make a non-initializer as adhering to the rules of an init() so that it could be used in multiple contexts. 

I think this modification makes your proposal much more interesting and applicable to more use cases.

-David
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