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

Kevin Ballard kevin at sb.org
Fri Jan 8 13:05:53 CST 2016

I had another thought last night about this proposal. Since it appears to be working as syntactic sugar, if I write something like

var x: Int
memberwise init(...) {
    // what does x refer to here?

Inside of the memberwise init, does the bare identifier `x` refer to the property, or to the implicit parameter? I think it should refer to the property, because making it refer to the implicit parameter means you'd be referring to a variable that was never visibly declared anywhere. AFAIK the only precedent for that is the implicit argument `newValue` to setters, `oldValue` to didSet observers, and `error` to catch blocks, but those are all constant identifiers that mean the same thing in every such context, as opposed to memberwise init where the implicit parameters are different for each init. More generally, we should avoid exposing such non-declared variables without a compelling reason.

The other argument in favor of making `x` refer to the property is because that's just generally more useful. All the properties match the values of the parameters on the first line of the user-supplied body to the memberwise init, so there's no benefit to be had from leaving the parameters accessible to user code.

As for how to actually accomplish this, assuming this feature is implemented as an AST transformation, ideally we'd have some way to synthesize identifiers in the AST that cannot possibly be referenced by user code (e.g. some form of identifier namespacing if it exists, or some form of gensym-like behavior). Alternatively, if there's no particularly good way to do this, we could just use something like `__name` as the internal parameter name.

-Kevin Ballard

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