There isn’t a way to mix data classes and Delegate...

# announcements

[Stephan Schroeder]

There isn’t a way to mix data classes and Delegates (while retaining the type we delegate to), is there??

fun main() {
    val r1 = Retailer(name = "test")
    val r2 = Retailer(name by lazy{"test"})
}

data class Retailer(
        val name: String
)

Playground link: https://pl.kotl.in/qfUC7Uzcj

[Michael de Kaste]

in order for kotlin to understand lazyness and delegation, it will propegate to a wrapper.

val name: String by lazy{"bla"}

just means that name is of type: Lazy<String>

what you could do is mask it

[Michael de Kaste]

data class Retailer(
        private var _name: Lazy<String>
) {
    val name: String
        get() = _name.value
    
    constructor(_name: String) : this(lazy{_name})
}

[Stephan Schroeder]

nice workaround, even though.
On a different note: I think you’re mistaken here:

val name: String by lazy{"bla"}
just means that name is of type: Lazy<String>

name is clearly of type String, otherwise this wouldn’t work:

fun main() {   
    val r3:String = Retailer2("test").name
}

class Retailer2(_name: String) {
    val name: String by lazy{_name}
}

There is some lower level engineering going on when using by not mere assignment.

[Burkhard]

name is of type String but it is backed by an internal property that is of type Lazy<String>.

val name by lazy { "someName" }

is the same as

private val nameDelegate = lazy{ "someName"}
val name: String
    get() = nameDelegate.value

In truth there is some more “magic” behind the value part of the delegate passing in some more information about the delegated property, but it isn’t used here.

[Stephan Schroeder]

good to know the internals 👍 but technically I was right then 😅. Saying "name is of type: Lazy<String>" is wrong, name is of type String (backed by a getter)! It's nameDelegate that is of type Lazy<String>!