[swift-evolution] Reconsidering SE-0003 Removing var from Function Parameters and Pattern Matching

[Ben Langmuir]

> On Jan 22, 2016, at 9:45 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Jan 22, 2016, at 9:26 AM, David Farler via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hello everyone,
>> 
>> I'd like to reconsider SE-0003 for Swift 3 and propose cancelling the change in its entirety. After collecting feedback since Swift's open source launch, I no longer feel this is a good move and there are a few reasons why.
>> 
>> There are two main patterns that the removal penalizes:
>> 
>> - Get-Modify-Reassign
>> - Get-Modify-Return
>> 
>> I've found that many of the problems with this proposal stem from the uses before and after the "Modify" part, before returning or reassigning with the new value.
>> 
>> I've seen a few common responses to the var removal. Consider a `Rectangle` struct:
>> 
>> 
>> struct Rectangle {
>> var origin: (x: Double, y: Double)
>> var size: (width: Double, height: Double)
>> }
>> 
>> 
>> Even with mutable variables `origin` and `size`, this pattern would be impossible:
>> 
>> 
>> var selection = getRectangularSelection()
>> if var rect = selection?.rect {
>> // Mutate `rect` ...
>> selection.rect = rect
>> }
> 
> These examples don't make sense to me. None of them mutate the 'if var rect' binding at all. Are you sure this is what you meant?
> 
>> 
>> 
>> So, one might shadow the variable, which is not ideal:
>> 
>> 
>> var selection = getRectangularSelection()
>> if let rect = selection?.rect {
>> var rect = rect // Not so great
>> // Mutate `rect` ...
>> selection.rect = rect
>> }
>> 
>> 
>> Or, you might make a transformation function on `Rect`:
>> 
>> 
>> struct Rectangle {
>> var origin: (x: Double, y: Double)
>> var size: (width: Double, height: Double)
>> func withOrigin(x: Double, y: Double) -> Rect {
>>  var r = self
>>  r.origin = (x, y)
>>  return r
>> }
>> }
>> 
>> 
>> This is a much better solution than shadowing but you would need one of these for any property that you want to mutate and I think you'll agree that it doesn't scale with the language we have today. This response begs for a kind of initializer that takes all of the fields of the original struct except any that you want to override:
>> 
>> 
>> if let rect = selection?.rect.with(origin: newOrigin) {
>> // ...
>> }
> 
> You can approximate this today without any new language features:
> 
> protocol Updatable {}
> extension Updatable {
>  func with<T>(change: (inout T) -> ()) -> T {
>    var update = value
>    change(&update)
>    return update
>  }
> }
> 
> if let rect =  selection?.rect.with { $0.origin = newOrigin } {
> }
> 
> I think this approach generally leads to cleaner code, since it's not forcing you to bind names to otherwise uninteresting intermediate values.
> 
>> 
>> 
>> Straw syntax, but maybe you'll see something along these lines on swift-evolution in the future, which would provide a clear alternative to direct mutation patterns. Even then, I think having complementary patterns in the language isn't a bad thing.
>> 
>> These problems come up with the other variable bindings but the one that ended up bothering me the most was `guard var`:
>> 
>> 
>> func transform(selection: Rect?) {
>> guard let rect = selection else { return }
>> var _rect = rect
>> // Mutate `_rect` ...
>> }
>> 
>> 
>> One of the guard statement's main purposes is to conditionally bind a value as a peer in its own scope, not an inner scope like if statements. Not having var makes the guard statement much weaker.
>> 
>> There is certainly a bit of confusion about the nuances between value and reference semantics, who owns a value and when, how effects are propagated back to values, but I think we can attack the problem with more finesse.
>> 
>> Value types are one of the attractive features of Swift – because of their semantics, mutating algorithms are written in a familiar style but keeping effects limited to your unique reference. I don't think we should give that up now to address confusion about semantics, out of principle, or in anticipation of new language features. I propose cancelling this change for Swift 3 and continue to allow `var` in the grammar everywhere it occurs in Swift 2.2.
> 
> I disagree. We have a lot of evidence that 'if var' confuses people—a lot of users think that 'if var' and 'var' bindings in case patterns will write back to the original value when this isn't the case. Classes make this worse, since 'if var' *will* seem to work that way when projecting through optional class references, and the value vs reference semantics divide is confusing enough as it is.
> Those of us who do understand the semantics don't save anything either—I at least have to mentally audit any code I see using 'if var' to ensure that writeback wasn't intended by the original author.

Anecdata: I also find myself auditing every non-trivial use of ‘var’.

> Code is read and maintained more often than it's written, and it's written by more novices than experts, and 'if var' and its friends feel like very expert-writer-centric features to me.
> 
> -Joe
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