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

问题

Problem:

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

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

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

Does not compile, because scalac sees the _ type instead of a type constructor it expects. How to fix it, ideally without writing a novel?

Original question (before edits in reply to Dmytro's answer) had:

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] }] = ???

Is there a shorter way of writing the above two methods by using some wildcard expression? Simply using _ in High's type parameter position doesn't work as the kind doesn't match, and _[_] is not even a valid type expression.

回答1:

• If you make existential quantization outside High then it's just

  type T = High[F] forSome { type F[O] <: UpperBound[O] }
  
  def canEqual(that: Any) = that.isInstanceOf[T]
  
  def high(h: T) = ???
  

• If you make existential quantization inside High then since

  implicitly[(n forSome { type n <: Upper}) =:= Upper]
  implicitly[(m[O1] forSome { type m[O] <: UpperBound[O]}) =:= UpperBound[O1]]
  

  (and vice versa) it's just 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 𝑇 forSome { 𝑄 } where 𝑄 contains a clause type 𝑡[tps]>:𝐿<:𝑈 is equivalent to the type 𝑇′ forSome { 𝑄 } where 𝑇′ results from 𝑇 by replacing every covariant occurrence of 𝑡 in 𝑇 by 𝑈 and by replacing every contravariant occurrence of 𝑡 in 𝑇 by 𝐿.

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

来源：https://stackoverflow.com/questions/61041172/is-there-a-shorthand-for-type-variable-m-forsome-type-mo-upperboundo