[swift-evolution] [Review] SE-0018 Flexible Memberwise Initialization

[Wallacy]

I like the idea of the proposal (not 100%) but i really dislike the "Future
enhancements" part:

*@default* is unnecessary in my opinion, i think just write the "future"
variable declaration ("memberwised") and put the default value is enough:

struct S {
    let s: String
    let i: Int

    // user declares:
    memberwise init(s: String = "hello",...) {}

    // compiler synthesizes:
    init(s: String = "hello", i: Int) { // because s: String matches
the sintaxe, so the compiler will not (re)synthesize.
        /* synthesized */ self.s = s
        /* synthesized */ self.i = i
    }
}



*memberwise properties* and *@nomemberwise* are too much trouble for little
gain.  Hand write the init by itself will be more easy and clear.

The fist part when the proposal focuses only on the automation of (maybe
only undeclared) variables on init using "..." placeholder as insertion
point, I believe that can be a nice feature.


Em sex, 8 de jan de 2016 às 18:13, Thorsten Seitz via swift-evolution <
swift-evolution at swift.org> escreveu:

> Am 08.01.2016 um 19:58 schrieb Kevin Ballard via swift-evolution <
> swift-evolution at swift.org>:
>
> On Fri, Jan 8, 2016, at 12:56 AM, Thorsten Seitz wrote:
>
>
>
> Am 08.01.2016 um 00:41 schrieb Kevin Ballard via swift-evolution <
> swift-evolution at swift.org>:
>
> On Thu, Jan 7, 2016, at 03:11 PM, Matthew Johnson wrote:
>
>
>
> On Jan 7, 2016, at 3:31 PM, Kevin Ballard <kevin at sb.org> wrote:
>
> On Thu, Jan 7, 2016, at 07:12 AM, Matthew Johnson wrote:
>
> Do you have an example of where you would want a caller to initialize a
> property, but then overwrite the value they provide *during
> initialization*?
>
>
> Sure, how about something like a Rect type that always guarantees it's in
> "standard" form (e.g. no negative sizes):
>
> struct StandardRect {
>     var origin: CGPoint
>     var size: CGSize {
>         didSet {
>             // ensure standardized form here
>         }
>     }
>
>     memberwise init(...) {
>         if size.width < 0 {
>             origin.x += size.width
>             size.width = -size.width
>         }
>         if size.height < 0 {
>             origin.y += size.height
>             size.height = -size.height
>         }
>     }
> }
>
>
> This is a good example.  Thanks!
>
>
> Actually I do not like this example for several reasons: (1) I would make
> the rectangle an immutable type with let properties, (2) the didSet already
> seems to do what is encoded in the memberwise init, so this seems to be
> redundant, (3) the memberwise init is so complex that having the automatic
> initialization feature is not really worth it for this example, especially
> as it seems to require using var properties instead of let properties to do
> the overwriting.
>
>
> 1) Why would you make it immutable? That helps nothing and only serves to
> make the type harder to use. Structs should _rarely_ be immutable, you
> should always default to mutable and only make things immutable if there's
> a good reason for it. If the struct itself is in an immutable position then
> it inherits the immutability, which handles all of the reasons why you
> might otherwise default to immutable.
>
>
> Hmm, food for thought… guess I still haven’t completely understood Swift’s
> handling of immutability… thanks for pointing that out!
>
> 2) didSet isn't triggered in init. There's no redundancy.
>
>
> You are right, of course. I forgot that when I wrote the mail.
>
> 3) You really really want var properties anyway, it's pointless to use let
> properties.
>
> I think cases like this will be rare so I still think a warning is a good
> idea.  Something like -Wno-overwrite-memberwise-init would allow it to be
> suppressed in cases where you actually do intend to do this.  Would that
> satisfy you?
>
>
> No. It's not appropriate to have the only way to suppress a warning on
> perfectly legal code to be passing a flag to the swiftc invocation.
> Especially because we have no precedent yet for even having flags like that.
>
> What's wrong with the suggestion to make the warning behave the same way
> as dead store warnings (e.g. warn if the property is overwritten without
> any prior reads)? We already have logic for doing this kind of analysis.
>
>
>
> I think this would not be sufficient, because this would not allow
> overwriting a property based on the value of another property which might
> be necessary as well.
>
>
> That seems much less likely to be necessary, because if you're doing that,
> then you're completely ignoring one of your parameters.
>
>
>
> Actually isn’t this what happens in your example? The property origin is
> overwritten without being read, so this would generate the warning, or did
> I understand something wrong?
>
>
> Origin is being modified. Modification reads it first. `x += 2` reads `x`
> before writing to it.
>
>
> I stand corrected.
>
> -Thorsten
>
>
>
> -Kevin Ballard
>
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