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

Pierre Monod-Broca pierremonodbroca at gmail.com
Tue Dec 20 01:54:10 CST 2016

But for a struct to be immutable, you don't need all its properties to be let. (I guess that's Derrick's point)

struct Person { /* . . . var properties . . . */ }
let john = Person() // john is completely immutable

If a struct has var properties, you can do a mutable copy, change it, assign it to a new let, and you take advantage of immutability again.

let jim = { var copy = john
  copy.firstname = "Jim"
  return copy

func with<T>(_ original: T, transform: (inout T) -> ()) -> T {
  var copy = original
  return copy

let jane = with(john) { $0.firstname = "Jane" }
// jane is also immutable
(I didn't check this compiles)


> Le 19 déc. 2016 à 23:08, Andy Chou via swift-evolution <swift-evolution at swift.org> a écrit :
> Value semantics help reduce the issues around mutability, but they don't go away completely. I would like to create structs that are completely immutable after construction. Turning the properties into vars unfortunately loses this idea.
> The proposed 'with' function doesn't construct new instances, which means the let constants are already set. Nick's solution works, as it's basically a copy constructor that allows for changes while the new object is constructed. But it needs to be created for each struct. Which I'm fine with :)
> Andy

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