了解flatMap和Map(Understanding flatMap and Map)

 在Scala函数式编程 （ISBN：978-1617290657）的练习4.4中，我们需要遍历选项列表，并将这些选项中的值连接成一个包含列表的选项。 如果初始列表包含无选项，则最终选项也应为无。 
 
 功能签名 
 
def sequence[A](a: List[Option[A]]): Option[List[A]]
 
 样品1 
 
scala> sequence(List(Some(3), Some(5), Some(1)))
Option[List[Int]] = Some(List(3, 5, 1))
 
 样品2 
 
scala> sequence(List(Some(3), None, Some(1)))
Option[List[Int]] = None
 
 这是我在网上找到的解决方案： 
 
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 :: _))
}
 
 尽管我尽最大的努力来理解这个实现（通过在纸上写入函数跟踪），但我无法直观地理解这个函数。 我能够理解map和flatMap意味着什么，而不是在这个函数的上下文中。 
 
 是否有另一种方法来看问题，并从这个角度，直观地得出上述代码？ 

In exercise 4.4 of the book Functional Programming in Scala (ISBN: 978-1617290657), we need to run through a List of Options, and concatenate the values in those Options into a single Option containing a List. If the initial List contains a None Option, the final Option should be None as well.
 
Function Signature
 
def sequence[A](a: List[Option[A]]): Option[List[A]]
 
Sample 1
 
scala> sequence(List(Some(3), Some(5), Some(1)))
Option[List[Int]] = Some(List(3, 5, 1))
 
Sample 2
 
scala> sequence(List(Some(3), None, Some(1)))
Option[List[Int]] = None
 
This is a solution that I found online:
 
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 :: _))
}
 
Despite my best efforts at understanding this implementation (by writing the function trace on paper), I'm unable to get an intuitive grasp of this function. I am able to understand what map and flatMap means in isolation, but not in the context of this function.
 
Is there another way to look at the problem, and through that perspective, derive the code above intuitively?

原文：https://stackoverflow.com/questions/48866303