scala - 类型推断错误

Question

给定以下代码:

implicit class RichString(s: String) {
  def !![T](createMessage: Long => T) = ()
}
case class SomeClass(i: Int, s: String, id: Long)

为什么会这样:

"some string" !! SomeClass(5, "test", _)

不编译，抛出以下错误:

Missing parameter type for expanded function ((x$1) => "some string".$bang$bang(SomeClass(5, "test", x$1)))
 "some string" !! SomeClass(5, "test", _)
                                       ^

Type mismatch;
 found   : cmd1.SomeClass
 required: Long => cmd1.SomeClass "some string" !! SomeClass(5, "test", _)

但是所有这些编译都很好:

val f = SomeClass(5, "test", _)
"some string" !! f

"some string" !! ( SomeClass(5, "test", _) )

"some string" !! { SomeClass(5, "test", _) }

PS:这个“一些字符串”!! SomeClass(5, "test", _: Long) 也会抛出类型不匹配。

Answer 1

正如消息所说，“一些字符串”!! SomeClass(5, "test", _) 扩展为 x => "some string"!! SomeClass(5, "test", x), 不是 "some string"!! { x => SomeClass(5, "test", x) }.所以 !! 没有收到一个函数，它收到一个 SomeClass，它不会编译。