映射与FlatMap on String [英] Map versus FlatMap on String

问题描述

从 Scala中的功能规划原则收听收藏讲座，我看到了这个例子： / p>

  scala> val s =Hello World
 
 scala> s.flatMap（c =>（。+ c））//前缀每个元素带有一个句点
 res5：String = .H.e.l.l.o。 .World 
  

然后，我很好奇为什么Odersky先生没有使用地图这里。但是，当我尝试地图时，我得到的结果不同于我的预期。

  scala> s.map（c =>（。+ c））
 res8：scala.collection.immutable.IndexedSeq [String] = Vector（.H，.e，.l，.l， 
。，.W，.o，.r，.l，
  

预期以上调用返回一个String，因为我是映射 -ing，即应用一个函数到序列中的每个项目，然后返回一个新的序列。

但是，我可以执行映射而不是 flatmap sList：List [Char] = List（H，e，l，l，o，，W，o，r，l，d）

scala> sList .map（c =>。+ c）
res9：List [String] = List（.H，.e，.l，.l，.o，。，.W， ，.r，.l，.d）

为什么是$ code IndexedSeq [ String] String上的调用 map 的返回类型

解决方案

这个行为的原因是为了将map应用于String，Scala将字符串视为一系列字符（ IndexedSeq [String] ）。这是由于地图调用所得到的，对于所述序列的每个元素，应用该操作。由于Scala将字符串作为序列应用 map ，这就是 map 返回。

flatMap 然后简单地调用该顺序的 flatten ，然后转换它返回一个字符串

Listening to the Collections lecture from Functional Programming Principles in Scala, I saw this example:

scala> val s = "Hello World"

scala> s.flatMap(c => ("." + c)) // prepend each element with a period
res5: String = .H.e.l.l.o. .W.o.r.l.d

Then, I was curious why Mr. Odersky didn't use a map here. But, when I tried map, I got a different result than I expected.

scala> s.map(c => ("." + c))
res8: scala.collection.immutable.IndexedSeq[String] = Vector(.H, .e, .l, .l, .o, 
                                                          ". ", .W, .o, .r, .l, 

I expected that above call to return a String, since I'm map-ing, i.e. applying a function to each item in the "sequence," and then returning a new "sequence."

However, I could perform a map rather than flatmap for a List[String]:

scala> val sList = s.toList
sList: List[Char] = List(H, e, l, l, o,  , W, o, r, l, d)

scala> sList.map(c => "." + c)
res9: List[String] = List(.H, .e, .l, .l, .o, ". ", .W, .o, .r, .l, .d)

Why was a IndexedSeq[String] the return type of calling map on the String?

解决方案

The reason for this behavior is that, in order to apply "map" to a String, Scala treats the string as a sequence of chars (IndexedSeq[String]). This is what you get as a result of the map invocation, where for each element of said sequence, the operation is applied. Since Scala treated the string as a sequence to apply map, that is what mapreturns.

flatMap then simply invokes flatten on that sequence afterwards, which then "converts" it back to a String

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