Why won't this compile?

class Foo(var values: MutableList<String>)

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

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.

That seems like something that should be fixed...

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

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]

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

🙏 Valhalla...