Why does the type of the result of the

left()

function here must have

R?

, although

R

is already by definition nullable?

class Either<L, R> private constructor(
    private val left: L,
    private val right: R,
) {

    companion object {

        fun <L, R> left(left: L): Either<L, R?> {
            return Either(left, null)
        }
    }
}

What makes you say that

R

is by definition nullable?

R

is only nullable if the caller of this function specifies it as such. This signature forces

Either.left<Int, String>(42)

to return an

Either<Int, String?>

. Were it not for the so called "definitely nullable"

R?

, it would be expected to return

Either<Int, String>

which is not possible given the body of the function (so defining the function without

?

would not compile)

You're right, I was thinking, because

R : Any?

that it's guaranteed to be nullable. But indeed it depends on the concrete instantiation of the class. It can be nullable, but doesn't have to.

You're welcome 🙂