[英]Type mismatch with generics sum type
I have a sum trait, that looks like as follow: 我有一个总和特征,如下所示:
sealed trait Sum[+A, +B]
final case class Failure[A](value: A) extends Sum[A, Nothing]
final case class Success[B](value: B) extends Sum[Nothing, B]
When I try to create a new variable as: 当我尝试创建一个新变量时:
val s1: Sum[Int, Nothing] = Success(4)
I've got following error: 我遇到以下错误:
Error:(5, 41) type mismatch;
found : Int(4)
required: Nothing
val s1: Sum[Int, Nothing] = Success(4)
Why? 为什么?
And why this is working: 以及为什么这样做:
val s1: Sum[Int, Int] = Success(4)
因为B
是第二个类型参数,而不是第一个:
val s1: Sum[Nothing, Int] = Success(4)
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