I've come across the use of a sealed class in the ...

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[Marc]

I've come across the use of a sealed class in the Jetpack Compose Jetnews example code that I think has general applicability. I'm new to Kotlin and don't understand this construction of a generic class with 2 type parameters. I'd appreciate pointers to docs or articles that explain it. Thanks!

sealed class Result<out R> {
data class Success<out T>(val data: T) : Result<T>()
data class Error(val exception: Exception) : Result<Nothing>()
}

[dbaelz]

At first, I'm assume that the Result<out R> is a typo and it should be Result<out T>?

A good explanation of this construct and possible usage scenarios could be found in this article.
In my own simple words: Most prominent example might be the typical network response where you either have a success with a payload or an error with a reason. It avoids this (anti) pattern to have response classes with isSuccess boolean and a payload and error property and one of both is set depending on the success status.

[Marc]

Nope, Result<out R> is not a typo. This code snippet is lifted from the functioning Jetnews Compose example.

[diesieben07]

There is no class with two type parameters here. Result has one type parameter called R. Success has one type parameter called T. Success then extends Result and sets `Result`'s type parameter to whatever its own T is. So a Success<String> is a Result<String>.
Error is special, it has no type parameters, and instead every Error is a Result<Nothing>.

[Marc]

Thanks, for the explanation, Take! Your explanation makes things clear for me.

[Animesh Sahu]

Nothing is top-most type in the type system (subclass of every class) sort of imaginary, and hence no object can satisfy it.
For example:

var a = if(cond) 10 else 10.5
a = "Hello"  // error: a must be a Number

the type of a is inferred as Number , as Int and Double are subclasses of Number.

Similarly, if you do something like this:

var b = null

the inferred type would be closest intersection, in this case is Nothing? , a nullable Nothing, and hence you cannot assign anything to b.
This is not really a good example where it is used, but you can instead find better examples in 'contravariance' sections with in modifiers, to understand better.