kotlin generics playground

generics.kt

// The ins and outs of Kotlin: https://kotlin.christmas/2019/22

// https://medium.com/@elye.project/in-and-out-type-variant-of-kotlin-587e4fa2944c
// https://kotlinlang.org/docs/reference/generics.html
// https://www.reddit.com/r/Kotlin/comments/dtfba1/covariance_vs_invariance/
// https://i.imgur.com/oKD9hPL.png
// https://docs.google.com/document/d/13eRz9uu8EuAUagcwEvLDUoWYB7TOyMgJWJyFLJnSB10/
// https://typealias.com/guides/ins-and-outs-of-generic-variance/


data class In<out T:Number>(val c:T)
data class Out<out E>(val c:E)

fun <R:Number?> foo(c:R): Out<R> {
    val o = Out(c=c)
    // return o - OK
    val c2 = 1.0
    val o2 = Out(c2)
   // return o2 // FAILS

    val c3 = 1.0  as R
    val o3 = Out(c3)
    //return o3 // OK

    //val c4 = 1.0  as? R?
    val o4 = Out(null as R)
    return o4 // OK
}


fun main() {
    val a:Double? = null
    val out = foo(a)

    println(out)
  //  In(1.0)
}