Scala：映射字符串时错误的类型推断？

Question

I'm trying to compile simple helloworld in Scala, and get error "scala: value capitalize is not a member of Char" Why does compiler think that newW is Char?

val dict = Map(
    "hello" -> "olleh",
    "world" -> "dlrow"
  )

def translate(input: String): String = {
  input.split( """\s+""").map(w => dict.getOrElse(w.toLowerCase, w).map(newW => 
    (if (w(0).isUpper) newW.capitalize else newW))
  ).mkString(" ")
}

Answer 1

Here's what's happening:

input // is a string
.split( """\s+""") // is an Array[String]
.map(w => // w is a String, for each String in the Array[String]
  dict.getOrElse(w.toLowerCase, w) // is a String (returned by dict.getOrElse)
  .map(newW => // is a Char, for each Char in the String returned by dict.getOrElse

Answer 2

The second call to map in translate is mapping across the value returned from dict.getOrElse(...) , whose type is String , which can be implicitly treated as an Iterable[Char] . Thus, the compiler is correctly inferring that newW is of type Char and complaining when you try to call capitalize on it. You're probably looking for something along the lines of

def translate(input: String): String = {
  input.split( """\s+""").map(w => {
    val newW = dict.getOrElse(w.toLowerCase, w)
    (if (w(0).isUpper) newW.capitalize else newW)
  }).mkString(" ")
}

Update: By the way, that will fail at runtime if input is an empty string - it needs at least one more check for safety.