[swift-evolution] [Proposal Draft] Flexible memberwise initialization

Howard Lovatt howard.lovatt at gmail.com
Mon Jan 4 23:31:11 CST 2016

I was guessing that the current proposal does not change anything re.
default and current member wise initializers and so in addition to
suggesting Scala syntax I was also suggesting the transformation shown, or
its equivalent. The advantage of having a member wise init that has default
arguments and argument labels are considerable:

1. Allows lets as well as vars
2. Allows partial custom initialization
3. Eliminates need for other mechanisms, i.e. default and existing member
wise initialization

These facilities could be added to `memberwise init(...)` as well. In
particular, if a member wise init was present then an initialized property
could have a label, e.g.:

     class C {
         let example name: Type = initial
         memberwise init(...)

Would become the equivalent of:

     class C {
         let name: Type
         init(example name: Type = initial) {
             self.name = name

The Scala syntax is just a shorter alternative, ideally there be a
discussion of the pros and cons of the two syntax that included the
possibility of the wider set of objectives as outlined in the numbered
points above.

On Tuesday, 5 January 2016, Matthew Johnson <matthew at anandabits.com> wrote:

> On Jan 4, 2016, at 5:48 PM, Howard Lovatt <howard.lovatt at gmail.com
> <javascript:_e(%7B%7D,'cvml','howard.lovatt at gmail.com');>> wrote:
> Yes you can get close, but:
> 1. Its weird that you can only do this in an extension.
> This is the way the current implicit initializer works.  It is not
> synthesized if you define any initializers in the body of the type.  There
> are good reasons it works this way and the current proposal does not change
> those rules.
> 2. Its not quite the same as the proposal the current member-wise
> initialiser does not make the init arguments optional (the arguments
> themselves do not have defaults), i.e. with your example `let
> defaultOriginRect = Rect(size: Size(width: 5.0, height: 5.0))` fails
> whereas it would work for the proposal (this could also be true if the
> existing struct memberwise init and the new `memberwise init(..)` where
> changed to provide init argument defaults).
> The implicit memberwise initializer currently in the language does not
> provide defaults for parameters.  This proposal changes that behavior and
> provides defaults if the the member is a `var` and has an initial value.
> Unfortunately I was not able to find a solution to allow synthesized
> parameters for `let` members to have default values.  This is because the
> current semantics for `let` members do not allow the member to be
> initialized to anything other than the initial value if one is provided.  I
> am hoping a solution to this will be identified in the future and have
> suggested one possible mechanism `@default` in the future enhancements
> section.
> 3. Only ‘really' works for structs, the compiler doesn’t write a
> member-wise initialiser for classes (just a default initializer).
> That is true about the current behavior of the language but is not true
> with regards to the current proposal.
> 4. Still need the compiler to provide both default and member-wise
> initialisers, whereas this proposal would allow the existing default and
> member-wise initialisers to be deprecated and just the new member-wise
> initialiser would remain which would simplify the language and make it
> clear what was happening (this could also be true if a `memberwise
> init(..)` where added and existing compiler written inits removed).
> This proposal does not change anything with regard to the default
> initializer.
> On 5 Jan 2016, at 10:16 AM, Matthew Johnson <matthew at anandabits.com
> <javascript:_e(%7B%7D,'cvml','matthew at anandabits.com');>> wrote:
> struct Rect { var origin: Point = Point(), size: Size = Size() }
> extension Rect {
>        init(center: Point, size: Size) {
>            let originX = center.x - (size.width / 2)
>            let originY = center.y - (size.height / 2)
>            self.init(origin: Point(x: originX, y: originY), size: size)
>        }
> }

  -- Howard.
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