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

Derrick Ho wh1pch81n at gmail.com
Tue Dec 20 11:32:01 CST 2016


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
        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, 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


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


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



     let fewerFoos = foos.clone()

         ..remove(at: i)

Anyway, all of this is simply syntactic sugar and should wait...




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