我刚开始从eugene's blog post学习scalaz。我试图在我的代码中使用===但它返回一个编译错误
value === is not a member of object Red
这是我的代码
import scalaz._
sealed trait TrafficLight
case object Red extends TrafficLight
case object Yellow extends TrafficLight
case object Green extends TrafficLight
object Equality {
import Scalaz._
def eqInstance[A](f: (A,A) => Boolean): Equal[A] = new Equal[A]{
def equal(a1: A, a2: A): Boolean = f(a1,a2)
}
def trafficLightEqualFunction(a1: TrafficLight, a2: TrafficLight): Boolean = (a1, a2) match {
case (Red, Red) => true
case (Yellow, Yellow) => true
case (Green, Green) => true
case _ => false
}
implicit val trafficLightEqual: Equal[TrafficLight] = eqInstance(trafficLightEqualFunction)
def main(args: Array[String]){
println(Red === Red)
}
}
答案 0 :(得分:1)
这正是他的博客所抱怨的。 Red
的类型为Red.type
,此类型没有scalaz.Equal
的实例,因为Equal是不变的,所以尽管如此:
Red <: TrafficLight
没有关注
Equal[Red] <: Equal[TrafficLight]
。
您可以通过让Scala将Red视为TrafficLight来修改它:
println((Red:TrafficLight) === (Red:TrafficLight))