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

Andy Chou acchou2 at gmail.com
Sat Dec 17 19:40:57 CST 2016

I like that structs are value types in Swift, this encourages the use of immutable data. O'Caml has an operator "with" that allows for copying an existing struct with a change to one field. I looked at Lenses for this functionality and it seems like a lot to digest for something so simple. I also tried to implement this using a constructor, or a function, and it was not obvious how to do so without a lot of code duplication.

What's I'm looking for is something like this (not necessarily with this syntax):

struct Person {
    let name: String
    let address: String
    let phone: String

func f() {
    let andy = Person(name: "Andy", address: "1 Battery St., San Francisco, CA", phone: "1234567")
    let chris = andy.with(name: "Chris")
    let dave = andy.with(address: "50 Townsend St., San Francisco, CA")

I tried to implement a "with" function like this but default arguments cannot reference properties of self. Same problem trying to do this in a constructor.

Obviously it's possible to create an entirely new Person specifying the values from an existing Person, but this is very tedious with structures with many properties.

Anyone taken a look at this before? Any suggestions?


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