[swift-evolution] [Pitch] Removing Setter/Observer Name Overrides

Jean-Daniel dev at xenonium.com
Sun Dec 4 02:56:24 CST 2016

If you want to go to the side of consistency, why not just require the standard method syntax like any other place ?

set(foo newValue: valueType) {


So old/newValue will become a parameter label.

> Le 4 déc. 2016 à 04:06, Erica Sadun via swift-evolution <swift-evolution at swift.org> a écrit :
> [Original pitch: https://gist.github.com/erica/f5c58c689a6f479606c6158077c1962b <https://gist.github.com/erica/f5c58c689a6f479606c6158077c1962b>]
> I received a gratifying amount of feedback about my pitch here, on Twitter, 
> through email, on several Slack channels, and on IRC. I wanted to summarize 
> the feedback, to start a new round of discussion.
> * A majority of respondents believe the current feature is incorrectly designed 
>   and that this is our best opportunity to change it.
> * A majority of respondents disagree on *how* it should be changed.
> Before I commit to the (non-trivial) effort of pushing on this, I'd like to know if any 
> of the core team can chime in on the "preferred" design. Thank you.
> The notion that the compiler should check for `set(oldValue)`, `willSet(oldValue)`, 
> and `didSet(newValue)` and emit warnings or errors had pretty much  universal
> support. I have submitted https://bugs.swift.org/browse/SR-3310 <https://bugs.swift.org/browse/SR-3310> to address
> this, regardless of whether the syntax changes or not.
> A majority of respondents prefer that argument names always be mentioned, 
> whether or not they *can* be omitted. Consensus is that it's unSwifty
> to use pre-built `newValue` and `oldValue` arguments without mentioning
> them first.
> * The current system violates the principle of clarity. 
> * It adds too much magic (https://en.wikipedia.org/wiki/Magic_(programming)) <https://en.wikipedia.org/wiki/Magic_(programming))> 
>   at the point of use. 
> * It is inconsistent with the binding of variable names in closures.
> My original design, which I chose to provide the least impact on the compiler and 
> existing code, was the least popular option.
> The most popular design is that setters and property observers follow closures
> syntax,  namely that the old value and new value arguments be passed as $0, 
> and assignable using `name in`. Under this design, a setter looks like:
> ```
> set { newValue in ... } // or
> set { somethingElse in ... } // or
> set { use $0 here }
> ```
> Swift loses the "magic" newValue and oldValue, but any developer who
> normally prefers to mention the name before use has a simple, visible
> and easy way to retain that clarity. 
> * Mirrors closure syntax
> * Easy to use
> * Loses magic names
> * Encourages documenting names in context
> The second most popular design is "leave things as they are" (but implement the bug
> report.) Developers with good style habits will use mandatory `newValue` and `oldValue`
> names in their setter and observer declarations. No proposal is needed, and the bug
> report guards against potential errors.
> I would appreciate knowing whether the core team feels that the support for "no change",
> even from a smaller group of developers, disqualifies this issue from the high bar of Phase 1.
> (This group also included the most developers who self-reported that they did not
>  use the override feature.)
> A third design entirely loses the ability to override variables or mention their names. 
> This was in fact my *original* original design that I did not submit after sufficient 
> devs told me they wanted to always spell out magic argument names. 
> Finally, the least popular design is my original pitch. (Only allow the "right" names,
> and allow them to be omitted.) This design has the least impact on the language, 
> causes the least breaking for most use-cases, and allows most pro coders to continue
> using the "mention all names" approach.
> I am happy to update the proposal for the "closure-like" design. I believe there *was*
> reasonable consensus that the current system is out of step with Swift's design goals
> to push forward. However, I want this to go through another round of feedback.
> Thank you in advance for your comments. If this does move forward to a proposal, it
> must be discussed and decided in the first phase of Swift 4 as the change *is* breaking.
> -- Erica
>> On Dec 1, 2016, at 10:22 PM, Derrick Ho <wh1pch81n at gmail.com <mailto:wh1pch81n at gmail.com>> wrote:
>> I like this proposal!
>> +1
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