Scala map with function call results in references to the function instead of results-CodePudding

I have a list of keys for which I want to fetch data. The data is fetched via a function call for each key. I want to end up with a Map of key -> data. Here's what I've tried:

  case class MyDataClass(val1: Int, val2: Boolean)

  def getData(key: String): MyDataClass = {
    // Dummy implementation
    MyDataClass(1, true)
  }

  def getDataMapForKeys(keys: Seq[String]): Map[String, MyDataClass] = {
    val dataMap: Map[String, MyDataClass] =  keys.map((_, getData(_))).toMap
    dataMap
  }

This results in a type mismatch error:

type mismatch;
 found   : scala.collection.immutable.Map[String,String => MyDataClass]
 required: Map[String,MyDataClass]
    val dataMap: Map[String, MyDataClass] =  keys.map((_, getData(_))).toMap

Why is it setting the values in the resulting Map to instances of the getData() function, rather than its result? How do I make it actually CALL the getData() function for each key and put the results as the values in the Map?

CodePudding user response：

The code you wrote is the same as the following statements:

keys.map((_, getData(_)))

keys.map(x => (x, getData(_)))

keys.map(x => (x, y => getData(y)))

This should clarify why you obtain the error.

As suggested in the comments, stay away from _ unless in simple cases with only one occurrences.

CodePudding user response：

The gist of the issue is (_, getData(_))) is creating a Tuple instead of a map entry for each key that is being mapped over. Using -> creates a Map which is what you want.

...
val dataMap: Map[String, MyDataClass] =  keys.map(key =>  (key -> getData(key))).toMap
...