Why. cant you have a custom setter on a constructor parameter? How can I make a property with a cust...

# language-evolution

[Sam Stone]

Why. cant you have a custom setter on a constructor parameter? How can I make a property with a custom setter be required when instantiating a class?

class Foo(
    var x: Int = 0
    var y: Int
        set(value) {
            field = value
            x += value
        }
)

[Loney Chou]

You can only have custom getter setter on properties within the block. It's just a design choice.

[Adam S]

you could move y to the class body, and pass in the initial value of y as a construction parameter

class Foo(
    var x: Int = 0,
    y: Int, // initial value of y
) {
  var y: Int = y // set the initial value of y
    set(value) {
      field = value
      x += value
    }
}

[Ayfri]

What about data classes then ?

[Ruckus]

@Ayfri That's not possible. The reasoning is "that's not what data classes are for".

[elizarov]

Exactly. Data classes are designed to be transparent wrappers over a group of related properties and must not do any custom processing on them.

[Ayfri]

Okay I see, thanks !

[solonovamax]

imo, you should try and keep your data classes to be immutable as much as possible

non-immutable data classes lead to code smell