[swift-evolution] idea: immutable setters for structs and tuples?

Maximilian Hünenberger m.huenenberger at me.com
Wed Mar 23 04:08:08 CDT 2016

I cannot remember any use of such a feature but instead of a language feature there is also a reasonable library solution:

// inspired by the lens idea (of Joe Groff I think)
// there is probably a better name
func lens<T>(value: T, lensClosure: inout T -> ()) -> T {
        var value = value
        return value

// usage
let john = (firstName:"John", lastName:"Doe")
let alice = lens(john) { $0.firstName = "Alice" }

Kind regards
- Maximilian

> Am 23.03.2016 um 09:06 schrieb Zsolt Szatmári via swift-evolution <swift-evolution at swift.org>:
> Dear All,
>   The only thing I am really missing right now from Swift is described as following. This works in some other languages, e.g.  F#, Kotlin, and Haskell.
>   F# example (taken from https://fsharpforfunandprofit.com/posts/correctness-immutability/)
> let john = {firstName="John"; lastName="Doe"}
> let alice = {john with FirstName="Alice"}
>   Current way to do this in Swift is:
> let john = (firstName:"John", lastName:"Doe")
> var alice = john
> alice.firstName = "Alice"
>   This might seem to be a nuance, but it's more cumbersome (especially if one wants to do this frequently), and we are left with a var at the end.
>   Also, this idea rhymes with the current direction of removing var arguments from functions.
>   What do You think? Thank You.
> Zsolt
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