[swift-evolution] "with" operator a la O'Caml?

[Jeremy Pereira]

> On 20 Dec 2016, at 17:32, Derrick Ho via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Jeremy,
> 
> The problem you present is not a mutability problem but rather a cache design problem. If your hash value truly is expensive then you only want to calculated it when you need to...
> 
> struct Person: Hashable
> {
>     var firstName: String {didSet { _hashValue = nil }}
>     var lastName: String {didSet { _hashValue = nil }}
>     private var _hashValue: Int?
>     var hashValue: Int { 
>         if _hashValue == nil {
>             _hashValue = firstName ^ lastName // the "expensive hash calculation"
>         }
>         return _hashValue!
>     }
> }
> 
> In the above implementation the hash value would only calculate the hash when firstName or lastName were changed.
> 
> However in your example your hash method would calculate a new one every time you copy a Person,

No it wouldn’t. Copying a struct does not run the initialiser again. 

So your solution is more complex and has more lines of code than mine. It is a good solution if your hash depends on properties that are truly meant to be mutable but if the properties are designed to be immutable, it is better to make them so and not have to introduce machinery in case they change unexpectedly.

> but mine would not. 
> 
> On Tue, Dec 20, 2016 at 6:44 AM Martin Waitz via swift-evolution <swift-evolution at swift.org> wrote:
> Am 2016-12-19 20:44, schrieb Erica Sadun via swift-evolution:
> 
> 
> > https://github.com/apple/swift-evolution/pull/346
> 
> 
> 
> 
> 
> -1
> 
> 
> I don't like where this is heading.
> 
> 
> 
> 
> 
> If you want to introduce method cascading, then have a look at Dart.
> 
> 
> 
> 
> 
> E.g. the example from the pull request could be something like this:
> 
> 
> 
> 
> 
>      let questionLabel = UILabel()
> 
> 
>          ..textAlignment = .Center
> 
> 
>          ..font = UIFont(name: "DnealianManuscript", size: 72)
> 
> 
>          ..text = questionText
> 
> 
> 
> 
> 
> The expression could still work on a mutable struct/class which later
> 
> 
> becomes
> 
> 
> immutable by using the `let` assignment.
> 
> 
> 
> 
> 
> The other example which silently creates a new instance is even worse.
> 
> 
> If you want to do something like this, then please do it more
> 
> 
> explicitly.
> 
> 
> E.g.:
> 
> 
> 
> 
> 
>      let fewerFoos = foos.clone()
> 
> 
>          ..remove(at: i)
> 
> 
> 
> 
> 
> Anyway, all of this is simply syntactic sugar and should wait...
> 
> 
> 
> 
> 
> --
> 
> 
> Martin
> 
> 
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