[swift-evolution] [Pitch] Adding a `mutate` clause to computed properties and subscripts

[Tim Vermeulen]

> On 11 Oct 2016, at 12:59, Jay Abbott <jay at abbott.me.uk> wrote:
> 
> This is interesting. I'm trying to evaluate your statement that "No setter would be needed if a mutation clause is provided" but I can't think exactly what the compiler would do in the case where there is a 'get' and 'mutate', but no 'set'...
> a) when you call a non-mutating function;

This would just call the getter and call the function on the result, exactly how it works right now.

> b) when you assign a new value.

A setter would be a special case of a “mutator”, where the starting value of the inout parameter is ignored. So the mutate clause would be invoked with `{ (x: inout T) in x = newValue }`. Does that make sense?

> c) when get and set aren't implemented with a matching hidden var (i.e. using a bit in an int var or storing in a dictionary).

I’m not sure what you mean by this. The getter and setter are implemented, but the mutator isn’t? Could you clarify?

> 
> 
> On Tue, 11 Oct 2016 at 11:26 Tim Vermeulen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Just having getters and setters doesn’t allow this (unless the optimiser is really smart about it). All the current API allows is grabbing whatever the `get` clause returns, mutating it, and then passing it into the `set` clause, whatever that does. The `set` clause might not do anything, or what it does could be seemingly unrelated to the `get` clause, so it’s not a trivial task to optimise this.
> 
>> On 11 Oct 2016, at 06:35, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> wrote:
>> 
>> 
>>> On Oct 10, 2016, at 9:53 PM, Tim Vermeulen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> There have been many instances where unexpected bad performance was caused by the interplay between getters, setters, mutations and the copy-on-write mechanism. For example:
>>> 
>>> struct Foo {
>>>     private var _array: [Int] = [1, 2, 3, 4, 5]
>>>     
>>>     var array: [Int] {
>>>         get { return _array }
>>>         set { _array = newValue }
>>>     }
>>> }
>>> 
>>> var foo = Foo()
>>> foo.array.append(6) // an O(n) operation
>>> 
>>> I propose a `mutate` clause which provides a `mutate` function with a single inout parameter (similar to how `set` provides `newValue`), which can be used instead of a setter:
>>> 
>>> var array: [Int] {
>>>     get { return _array }
>>>     mutate { mutate(&_array) }
>>> }
>>> 
>>> The compiler could then translate each mutation of `foo.array` to a closure with an inout parameter, which is then passed into the `mutate` clause (which in turn is executed with `_array` as its argument, as per the snippet above). For example, for `foo.array.append(6)`, the compiler would internally generate the closure `{ (arr: inout [Int]) in arr.append(6) }` and pass it into the `mutate` clause, `_array` is then passed as its parameter and the array is updated in constant time.
>>> 
>>> I apologise if that was too hard to follow.
>>> 
>>> No setter would be needed if a mutation clause is provided, but I see no good reason to do away with setters altogether, so this proposal would be purely additive.
>> 
>> If this is computationally better, why is it not the default behavior rather than an API change?
>> 
>> -- E
>> 
>> 
> 
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