is there a way to assume from a generic that it's ...

# language-proposals

[Endre Deak]

is there a way to assume from a generic that it's a data class type therefore it has the copy method in place?

[jw]

No, but even if you could the signature of the function is unique to every type so you can't really abstract over it anyway

[kioba]

agree with the missing signature comment.

If you are interested in a way to allow generic copy you could use `receiver`/`context receiver` to provide copy mechanism to type arguments.

// data classes
data class A(val integer: Int)
data class B(val string: String)

// rule to be able to copy the type argument
fun interface GenericCopy<T>{
    fun T.copy(): T
}

// copy functions
val Acopy = GenericCopy<A> { copy(integer = 42) }
val Bcopy = GenericCopy<B> { copy(string = "42") }

// generic function with receiver 
fun <T> GenericCopy<T>.genericCopy(argument: T) : T = argument.copy()

// future generic function with context receiver
context(GenericCopy<T>)
fun <T> genericCopy(argument: T) : T = argument.copy()

// calling
fun main() {
    with(Acopy) {
        print(genericCopy(A(1)))  // A(integer=42)
    }

    with(Bcopy) {
        print(genericCopy(B("1"))) // B(string=42)
    }
}

in the above genericCopy function has a type argument T but the function can only be called in a scope of GenericCopy instance for T.

if your generic function has an argument, the specific cases could be implemented for each of the different data class types.