Variance/Covariance generics in Kotlin

Question

There is a sealed-class Result, which is parameterized with two types - success result (T) and error type (R).

It is inheritered by two classes:

a. Success - data class, accepts object T in the constructor

b. Error - data class, accepts object R in the constructor

I need to create a function, which returns Result object. The function has to be created in a way:

that the result of this function can be assigned to the variables of the following types:

    Result<Number, String>
    Result<Any, String>

that the result of this function can NOT be assigned to the variables of the following types:

    Result<Int, CharSequence>
    Result<Int, Any>

That is class Result must be covariant on T parameter and invariant on R parameter.

Animesh Sahu · Accepted Answer · 2020-06-06T06:42:08.617

2

You can use the declaration-site variance provided by Kotlin in the declaration of your Result class.

T -> By default T is invariant
out T -> makes T covariant
in T -> makes T contavariant

Example:

sealed class Result<out T, R>(val left: T? = null, val right: R? = null) {
    class Success<T, R>(data: T) : Result<T, R>(left = data)
    class Error<T, R>(data: R) : Result<T, R>(right = data)
}

fun main() {
    val res1 = Result.Success<String, Int>("Test")
    val res2: Result<Any, Int> = res1     // compiles successfully, T is covariant
    val res3: Result<String, Any> = res1  // doesn't compile, R is invariant (Type Mismatch)
}

A function can return Result as:

fun returnResult(): Result<String, Int> {
    val random = Random.nextBoolean() // random, for demonstration

    retrun if(random) {
        Result.Success("Success Example")
    } else {
        Result.Error(404)
    }
}

edited Jun 06 '20 at 06:42

answered Jun 06 '20 at 05:45

Animesh Sahu

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How about a separate function? – matua Jun 06 '20 at 06:17
@matua function? – Animesh Sahu Jun 06 '20 at 06:18
"I need to create a function, which returns Result object." - not simple assigning. I have difficulties with the syntax and if the function has to always parameterized. – matua Jun 06 '20 at 06:20
@matua function returns result object? a success or error? how would you wanna set the another generic? – Animesh Sahu Jun 06 '20 at 06:26
@matua, like the above? – Animesh Sahu Jun 06 '20 at 06:42
Is not returnResult supposed to be also parameterized? – matua Jun 06 '20 at 06:48
@matua what do you mean? If you want specific type of result from the funciton then you have specify it, how would you pass different type of results if you make function generic? – Animesh Sahu Jun 06 '20 at 06:50
I guess I understand now. Thank you very much! – matua Jun 06 '20 at 06:55

Variance/Covariance generics in Kotlin

1 Answers1