map.updated（k，v）和map +（k，v）之间*有任何区别吗？ 我在一个而不是另一个上遇到编译错误

Question

I'm new (about a month) to Scala. I just came across a type-check error that I can't figure out

This snippet works as expected.

  def combine(m: Map[Char, Int], tup: (Char, Int)): Map[Char,Int] = tup match {
    case (key: Char, value: Int) => m updated (key, m.getOrElse(key, 0) + value)
  }

This snippet gives a type mismatch error on value

  def combine(m: Map[Char, Int], tup: (Char, Int)): Map[Char,Int] = tup match {
    case (key: Char, value: Int) => m + (key -> m.getOrElse(key, 0) + value)
  }

 found   : Int
 required: String
           case (key: Char, value: Int) => m + (key -> m.getOrElse(key, 0) + value)
                                                                             ^

Now, the docs on map say that ms updated (k,v) is equivalent to ms + (k->v) but that appears to not be the case.

Here's my setup:

SBT version 0.13.8
Scala version 2.11.8
Java SDK 1.8
IntlliJ IDEa

Answer 1

key -> m.getOrElse(key, 0) + value means, (key->m.getOrElse(key, 0)) + value . So, effectively, you are trying to add an Int to a tuple, no surprise, that does not work.

Try using parentheses: m + (key -> (m.getOrElse(key, 0) + value))