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[英]How to workaround using a covariant type parameter in a contravariant position
[英]Is there any alternatives to using covariant type in contravariant position?
我正在寻找一种从现有实例创建新实例的模式,这样我就可以通过遍历其上方的层次结构来计算Tile的UltimateBase。 我尝试了以下操作,但是在buildTile方法的参数库中获得了“协变类型A在协变位置出现”的信息 。 是否有替代模式来构建依赖于先前实例的实例?
trait Color
trait Blue extends Color
trait Green extends Color
trait Tile[+A] {
def declaredBase: Option[(Double, Tile[A])]
final val ultimateBase: Option[(Double, Tile[A])] = ??? // Some implementation, not important
// Can't be protected[this]
def buildTile(name: String, multiple: Double, base: Tile[A]): Tile[A]
}
case class BlueTile(name: String, declaredBase: Option[(Double, Tile[Blue])]) extends Tile[Blue] {
override def buildTile(name: String, multiple: Double, base: Tile[Blue]): Tile[Blue] = {
// Something more complicated here
this
}
}
谢谢。
1)您可以分别声明协变位置的类型,并在子类中实现它:
trait Tile[+A] {
type T <: A
def declaredBase: Option[(Double, Tile[A])]
final val ultimateBase: Option[(Double, Tile[A])] = None // Some implementation, not important
// Can't be protected[this]
def buildTile(name: String, multiple: Double, base: Tile[T]): Tile[A]
}
case class BlueTile(name: String, declaredBase: Option[(Double, Tile[Blue])]) extends Tile[Blue] {
type T = Blue
override def buildTile(name: String, multiple: Double, base: Tile[T]): Tile[Blue] =
{
println(base.declaredBase)// Something more complicated here
this
}
}
scala> val x = new BlueTile("", None)
x: BlueTile = BlueTile(,None)
scala> val a: Tile[Color] = new BlueTile("", None).buildTile("", 1.0, x) //covariance works
None
a: Tile[Color] = BlueTile(,None)
它在这种特殊情况下有效,因为您实际上可以最终指定类型T。
2)另一种方法是隐式添加def buildTile
:
scala> :paste
// Entering paste mode (ctrl-D to finish)
trait Tile[+A] {
def declaredBase: Option[(Double, Tile[A])]
final val ultimateBase: Option[(Double, Tile[A])] = None // Some implementation, not important
}
case class BlueTile(name: String, declaredBase: Option[(Double, Tile[Blue])]) extends Tile[Blue]
implicit class BuildFromBlue(t: Tile[Blue]) {
def buildTile(name: String, multiple: Double, base: Tile[Blue]): Tile[Blue] = {
println(base.declaredBase)// Something more complicated here
t
}
}
// Exiting paste mode, now interpreting.
defined trait Tile
defined class BlueTile
defined class BuildFromBlue
scala> val x = new BlueTile("", None)
x: BlueTile = BlueTile(,None)
scala> val a: Tile[Color] = new BlueTile("", None).buildTile("", 1.0, x) //covariance works
None
a: Tile[Color] = BlueTile(,None)
3A)最后一种选择是通过存在类型的协方差。 您可以将Tile[A]
声明为不变式,但在需要时需要Tile[_ <: A]
:
trait Tile[A] {
def declaredBase: Option[(Double, Tile[A])]
def buildTile(name: String, multiple: Double, base: Tile[A]): Tile[A]
}
case class BlueTile(name: String, declaredBase: Option[(Double, Tile[Blue])]) extends Tile[Blue] {
override def buildTile(name: String, multiple: Double, base: Tile[Blue]): Tile[Blue] = this
}
scala> val x = new BlueTile("", None)
x: BlueTile = BlueTile(,None)
scala> val a: Tile[_ <: Color] = new BlueTile("", None).buildTile("", 1.0, x)
a: Tile[_ <: Color] = BlueTile(,None)
1,2,3A)但是在转换为更大的Tile[Color]
类型后,不能在相反的位置使用它:
scala> a.buildTile("", 1.0, a)
<console>:18: error: type mismatch;
found : Tile[_$1(in value res20)] where type _$1(in value res20) <: Color
required: Tile[_$1(in value a)]
a.buildTile("", 1.0, a)
^
3B)您只需将base
与变量存在类型绑定即可实现:
trait Tile[+A] {
def declaredBase: Option[(Double, Tile[A])]
def buildTile(name: String, multiple: Double, base: Tile[_ >: A]): Tile[A]
}
case class BlueTile(name: String, declaredBase: Option[(Double, Tile[Blue])]) extends Tile[Blue] {
override def buildTile(name: String, multiple: Double, base: Tile[_ >: Blue]): Tile[Blue] = {println(base.declaredBase); this}
}
scala> val x = new BlueTile("", None)
x: BlueTile = BlueTile(,None)
scala> val a: Tile[Color] = new BlueTile("", None).buildTile("", 1.0, x: Tile[Blue])
None
a: Tile[Color] = BlueTile(,None)
scala> a.buildTile("", 1.0, a) //you can do it now
None
res23: Tile[Color] = BlueTile(,None)
它的工作原理是[_ >: A]
实际上是说您不需要liskov替代来作为base
。 它将允许从Tile[Color]
使用buildTile
。 但是,这种方法将未结合的base
到Tile[_ >: Color]
满足里氏取代为Tile[+A]
本身:
scala> a.buildTile("", 1.0, a: Tile[Any]) //Tile[Any] also works
None
res27: Tile[Color] = BlueTile(,None)
因此,我建议将A
限制为Color
(或可能在Tile
某处):
trait Tile[+A <: Color]{...}
...
scala> a.buildTile("", 1.0, a)
None
res33: Tile[Color] = BlueTile(,None)
scala> a.buildTile("", 1.0, a: Tile[Any])
<console>:18: error: type arguments [Any] do not conform to trait Tile's type parameter bounds [+A <: Color]
a.buildTile("", 1.0, a: Tile[Any])
^
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