Why won't this compile? ``` class Foo(var values: MutableList<String>) class Bar(var foo: Foo...

# getting-started

[benleggiero]

Why won't this compile?

class Foo(var values: MutableList<String>)

class Bar(var foo: Foo) {
    fun bar() {
        foo.values += "Baz"
    }
}

[karelpeeters]

There's an ambiguity between Collections.plus and MutableList.plusAssign. The former is only applicable when the field is a var, so a fix would be to make it a val. I don't think you can even have a var list: MutableList with`+=` working.

[benleggiero]

That seems like something that should be fixed...

[karelpeeters]

I don't really think there's even a fix.

[benleggiero]

Swift addresses this by having just one struct for both mutable and immutable versions, and then marking specific functions as mutating. At declaration time, var means "use the mutable version" and let means "use the immutable version. Copy-on-write ensures you don't mutate an immutable value passed to you.

So:

let array = [1, 2, 3]
array += 4 // compiler error
var array2 = array // copy
array2 += 4 // OK
print(array) // [1, 2, 3]
print(array2) // [1, 2, 3, 4]

[karelpeeters]

Well Kotlin doesn't have actual immutability nor any mutating guarantees about methods (yet), so I don't think that would work here.

[benleggiero]

🙏 Valhalla...