问题
I'm a bit shaky on the rules as to when you need a _ after a method to use it as a function. For example, why is there a difference between Foo's and Nil's :: in the following?
def square(n: Int) = n * n
object Foo { def ::(f: Int => Int) = f(42) }
// ...
scala> Foo.::(square)
res2: Int = 1764
scala> Nil.::(square)
<console>:6: error: missing arguments for method square in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
Nil.::(square)
^
scala> Nil.::(square _)
res3: List[(Int) => Int] = List(<function1>)
回答1:
When you omit all parameters in a partially applied function expression, then you need to follow it with _ unless the compiler requires a function type in the place where you use it.
When you call the method :: on Foo, the compiler expects a type Int => Int for the parameter. So you can safely omit the underscore after square in that position.
However, the :: method on Nil can take a parameter of any type. So rather than assume that you meant to partially apply the function, it complains unless you make it absolutely explicit by adding _.
So those are the rules... I can't really enlighten you about why those are the rules; maybe somebody else who has better knowledge of the compiler, the type system, and the language design will be able to tell you why. But I assume that without these rules, there would be a danger of accidental ambiguity in many places.
来源：https://stackoverflow.com/questions/2064193/why-and-when-do-i-need-to-follow-a-method-name-with