小编All*_*hou的帖子

在Scala中,存在类型有以下两种形式:

 // placeholder syntax
 List[_]
 // forSome
 List[T forSome {type T}]

但是,似乎第二种形式不能出现在方法类型参数位置(至少在我写下面的方式).

  // placeholder syntax is Okay
  scala> def foo[List[_]](x: List[_]) = x
  foo: [List[_]](x: List[_])List[_]

  scala> def foo[List[t forSome {type t}]](x: List[_]) = x
  <console>:1: error: ']' expected but 'forSome' found.
  def foo[List[T forSome {type T}]](x: List[_]) = x
                  ^

  // being as upper bound is also Okay
  scala> def foo[A <: List[T forSome { type T }]](x: A) = x
  foo: [A <: List[T forSome { type T …