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

Slava Pestov spestov at apple.com
Sun Dec 25 09:23:00 CST 2016

> On Dec 17, 2016, at 8:40 PM, Andy Chou via swift-evolution <swift-evolution at swift.org> wrote:
> 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?
> Andy

What’s the advantage of this over declaring the stored properties of the struct ‘var’ and then doing this?

var chris = andy
chris.name = “Chris"


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