[英]Scala F-bounded polymorphism on object
I cannot write the following F-bounded polymorphism in Scala. 我不能在Scala中编写以下F-bounded多态。 Why? 为什么?
trait X[T <: X[T]]
object Y extends X[Y]
How can I express this and make it compile? 我怎样才能表达并编译?
It really seems like you ought to be able to write, 看起来你真的应该能够写作,
trait X[T <: X[T]]
object Y extends X[Y.type]
however, if you try that the compiler will give you an unhelpful (and I think spurious) error, 但是,如果你尝试编译器会给你一个无用的(我认为是虚假的)错误,
scala> object Y extends X[Y.type]
<console>:16: error: illegal cyclic reference involving object Y
object Y extends X[Y.type]
I say "spurious" because we can construct an equivalent object with a little bit of additional infrastructure, 我说“虚假”,因为我们可以构建一个具有一点额外基础设施的等效对象,
trait X[T <: X[T]]
trait Fix { type Ytype >: Y.type <: Y.type; object Y extends X[Ytype] }
object Fix extends Fix { type Ytype = Y.type }
import Fix.Y
If you wanted to experiment with this in real code, using a package object in place of object Fix
would make this idiom a little more usable. 如果你想在实际代码中试验这个,使用一个包对象来代替object Fix
会使这个成语更加有用。
Change it to : 将其更改为:
trait Y extends X[Y]
object
is not a type in Scala, but the so called companion object. object
不是Scala中的类型,而是所谓的伴随对象。 By defining object Y
, you can't express that it should extend trait T[Y]
, since that second Y
refers to a type Y
that hasn't been defined. 通过定义object Y
,您无法表达它应该扩展trait T[Y]
,因为第二个Y
指的是尚未定义的类型Y
You can however do the following: 但是,您可以执行以下操作:
trait Y extends X[Y] //If you try this on the REPL, do :paste before
object Y extends X[Y]
In this case the object Y
extends X[Y]
where the second Y is the trait you just define, make sure you keep this in mind. 在这种情况下,对象Y
扩展X[Y]
,其中第二个Y是您刚定义的特征,请确保记住这一点。
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