I believe Erica's point about RIGHT NAMES ONLY is the most clear. While a block-like syntax using $0 is nice, it fails to communicate whether it is a newvalue or an oldvalue.
Therefore instead of following a block-like pattern it should follow a delegate-function like pattern. This would remove the ambiguity that $0 or name changing would present. set (newValue: TheType) { let a = newValue } didSet (oldValue: TheType) { let b = oldValue } On Sat, Dec 3, 2016 at 10:06 PM Erica Sadun via swift-evolution < swift-evolution@swift.org> wrote: > [Original pitch: > https://gist.github.com/erica/f5c58c689a6f479606c6158077c1962b] > > GENERAL FEEDBACK > > 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. > > BUG REPORT > > 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 to address > this, regardless of whether the syntax changes or not. > > MENTIONING NAMES > > 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)) > 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. > > PREFERRED DESIGN > > 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 > > "NO CHANGE" > > 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.) > > REMOVING OVERRIDES > > 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. > > RIGHT NAMES ONLY > > 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. > > UPDATING PROPOSAL > > 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 <wh1pch...@gmail.com> wrote: > > I like this proposal! > > +1 > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution >
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