在Scala中的andThen（）中键入推断

I am trying to understand how andThen works in Scala.

我试图了解如何和然后在Scala中工作。

Here's the code:

这是代码:

    def collectNames(fromList: List[Map[String,String]]) = {
      fromList.foldLeft(new StringBuffer)((x,y) => {
          x.append(y("Name")).append(",")
      }).toString
    }

    def getOriginalList = List(Map("Name" -> "NS","Age" -> "50"),Map ("Name" -> "SS", "Age" -> "45"))


    getOriginalList andThen collectNames

The compiler finds this disagreeable. It seems that collectNames is being given a Map[String,String] rather than a List[Map[String,String]].

编译器发现这令人不愉快。似乎collectNames被赋予Map [String,String]而不是List [Map [String,String]]。

What does it take me to be able to write that little snippet of 'flowing' code correctly?

能够正确编写那些“流动”代码的小片段我需要什么?

1 个解决方案

#1

The problem is that getOriginalList _ is a Function0 (i.e () => R) and has no compose or andThen.

问题是getOriginalList _是一个Function0(即()=> R)并且没有compose或andThen。

Therefore, you this won't compile: getOriginalList _ andThen collectNames _

因此,你不会编译:getOriginalList _然后collectNames _

Now if getOriginalList can be treated as a partially applied function1, it will work:

现在,如果可以将getOriginalList视为部分应用的function1,它将起作用:

def getOriginalList(a: Any) = List(Map("Name" -> "NS","Age" -> "50"))
val composed = getOriginalList _ andThen collectNames _

But in your case, composition-wise, you should probably consider getOriginalList as a val and use it as a function argument:

但在你的情况下,在组合方面,你应该将getOriginalList视为val并将其用作函数参数:

def collectNames(fromList: List[Map[String,String]]) = {
  fromList.foldLeft(new StringBuffer)((x,y) => {
      x.append(y("Name")).append(",")
  }).toString
}

def getOriginalList = List(Map("Name" -> "NS","Age" -> "50"),Map ("Name" -> "SS", "Age" -> "45"))

val comp = collectNames _ andThen ((s: String) => s.length())
val n = comp(getOriginalList)   // 6 = ("NS,SS,".length())

#1