Scala：为什么如果从GroupBy创建Map，则无法更改Map中的可变值

Question

我想创建一个键为整数且值为可变Set的Map对象。但是，当我从GroupBy函数创建Map对象时，可变集合中的值无法再更改。谁能告诉我为什么会这样？

import scala.collection.mutable

val groupedMap: Map[Int, mutable.Set[Int]] = 
    List((1,1),(1,2),(2,3))
        .groupBy(_._1)
        .mapValues(_.map(_._2).to[mutable.Set])

val originalMap: Map[Int, mutable.Set[Int]] =
    Map(1 -> mutable.Set(1, 2), 2 -> mutable.Set(3))

println(groupedMap) // Map(1 -> Set(1, 2), 2 -> Set(3))
println(originalMap) // Map(1 -> Set(1, 2), 2 -> Set(3))

groupedMap(1) += 99
originalMap(1) += 99

println(groupedMap) // Map(1 -> Set(1, 2), 2 -> Set(3))  <- HERE IS THE PROBLEM, THE VALUE 99 CAN NOT BE ADDED TO MY MUTABLE SET!
println(originalMap) // Map(1 -> Set(99, 1, 2), 2 -> Set(3))

Answer 1

.mapValues is lazy, meaning the function you give it is executed every time you access the value, so, when you do groupedMap(1) += 99, it runs your conversion, returns a Set to which you add 99, and discards it.

Then, when you print it, it runs the conversion again ... and prints the original contents.

If the above does not seem clear, try running this snippet as an illustration:

 val foo = Map("foo" -> "bar")
   .mapValues { _ => println("mapValues"); "baz" }

 println(foo("foo") + foo("foo"))

This is one of many problems you run into when using mutable data. Don't do it. In 99% of use cases in scala it is not needed. So, it is better to just pretend it does not exist at all, until you get enough grasp of the language to be able to determine definitively the remaining 1%.