CODEWITHSUNDEEP
scalalambdasyntactic-sugar
xuanyue
xuanyue

Reputation: 1428

meaning of scala underscore in list construct

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

Upvotes: 0

Views: 97

Answers (1)

Andrey Tyukin
Andrey Tyukin

Reputation: 44918

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.

Upvotes: 2

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