meaning of scala underscore in list construct

Question

In the following code from "functional-programming-in-scala, what is _ mean here? I think it represent the result of sequence(t), but when I replaced it with sequence(t), it gives me an compile error. Why is that? What can I do to make this _ explicit?

Edit: I'm confused that whether this _ should be expand into result of sequence(t), list all use case for underscore here doesn't help here, I already reviewed it.

@ def sequence[A](a: List[Option[A]]): Option[List[A]] =
  a match {
      case Nil => Some(Nil)
      case h :: t => h flatMap (hh => sequence(t) map (hh :: _))
  }

defined function sequence

@

@ sequence(List(Some(1), Some(2))
  )
res1: Option[List[Int]] = Some(List(1, 2))

Replace _ with sequence(t)

def sequence[A](a: List[Option[A]]): Option[List[A]] =
a match {
    case Nil => Some(Nil)
    case h :: t => h flatMap (hh => sequence(t) map (hh :: sequence(t)))
}
cmd4.sc:4: value :: is not a member of Option[List[A]]
case h :: t => h flatMap (hh => sequence(t) map (hh :: sequence(t)))
                                                    ^
Compilation Failed

Possible duplicate of [What are all the uses of an underscore in Scala?](https://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala) — mkUltra, Sep 16 '18 at 17:39
I saw that already before ask this question. It doesn't help. And what I want to know is how it work specifically in this case. It shouldn't be a duplicate — xuanyue, Sep 16 '18 at 17:44

score 2 · Accepted Answer · answered Sep 14 '18 at 23:25

In every context, hh :: _ is simply a shortcut for _.::(hh), which in turn is a shortcut for x => x.::(h), or x => hh :: x. The type of the argument in this case is List[A] (because it's a list of As inside an Option). Therefore, your code does the same as the following code:

def sequence[A](a: List[Option[A]]): Option[List[A]] = 
  a match {
    case Nil => Some(Nil)
    case h :: t => h flatMap (hh => sequence(t) map ((xs: List[A]) => hh :: xs))
  }

Whether it's used inside flatMap or somewhere else, is completely irrelevant.

meaning of scala underscore in list construct

1 Answers1