在 Scala 中是否有一个 shorthand for type variable 'm forSome { type m[O] <: UpperBound[O] }`？

Question

问题：

trait UpperBound[O]
trait High[F[O] <: UpperBound[O]]

def canEqual(that :Any) = that.isInstanceOf[High[_]]

def high(h :High[_]) = ???

不编译，因为 scalac 看到 _ 类型而不是它期望的类型构造函数。如何修复它，最好不写小说？

原始问题（在对 Dmytro 的回答进行编辑之前）有：

def canEqual(that :Any) = that.isInstanceOf[High[m forSome { type m[O] <: UpperBound[O] }]]

def high(h :High[m forSome { type m[O] <: UpperBound[O] }] = ???

有没有更短的方法可以使用通配符表达式来编写上述两个方法？简单地在 High 的类型参数位置使用 _ 是行不通的，因为种类不匹配，并且 _[_] 甚至不是一个有效的类型表达式。

Answer 1

如果你在 High 之外进行存在量化，那么它只是

type T = High[F] forSome { type F[O] <: UpperBound[O] }

def canEqual(that: Any) = that.isInstanceOf[T]

def high(h: T) = ???

如果在 High 内部进行存在量化，那么自
```
implicitly[(n forSome { type n <: Upper}) =:= Upper]
implicitly[(m[O1] forSome { type m[O] <: UpperBound[O]}) =:= UpperBound[O1]]
```
（反之亦然）只是 High[UpperBound]
```
implicitly[High[m forSome { type m[O] <: UpperBound[O] }] =:= High[UpperBound]]

def canEqual(that: Any) = that.isInstanceOf[High[UpperBound]]

def high(h: High[UpperBound]) = ???
```
An existential type forSome { } where </code> contains a clause type <code>[tps]>:<: is equivalent to the type ′ forSome { } where ′ results from </code> by replacing every covariant occurrence of <code> in </code> by <code> and by replacing every contravariant occurrence of in </code> by <code>.

https://scala-lang.org/files/archive/spec/2.13/03-types.html#simplification-rules

在 Scala 中是否有一个 shorthand for type variable 'm forSome { type m[O] <: UpperBound[O] }`？

Is there a shorthand for type variable 'm forSome { type m[O] <: UpperBound[O] }` in Scala?

scala

types

existential-type

higher-kinded-types