What does dollar sign do in scala

Question

When I was reading spark source code here, I saw code like $(a_variable). What does it mean?

I copy the code here:

  final val blockSize: IntParam = new IntParam(this, "blockSize",
    "Block size for stacking input data in matrices. Data is stacked within partitions." +
      " If block size is more than remaining data in a partition then " +
      "it is adjusted to the size of this data. Recommended size is between 10 and 1000",
    ParamValidators.gt(0))

  /** @group getParam */
  final def getBlockSize: Int = $(blockSize)

Answer 1

That isn't special Scala syntax, it's a method name. In Scala $ is a legal identifier. The method is inherited from the org.apache.spark.ml.param.Param trait.

See the source.

Answer 2

I believe that the dollar sign is usually used for string interpolation. What that means is that if I want to use a val(ue)/var(iable) inside a string (denoted with an s before the first double quotation), I can access such vals/vars using the $ sign. Otherwise, we won't be able to use vals/vars inside a string since it would not know how to escape the string characters.

Example:

val favoriteFruit = "strawberry" val strawberryCount = 12  println(s"My favorite fruit is ${favoriteFruit}. I've had ${strawberryCount} pieces today!") // string interpolation 

In your case, however it seems that $ is a shortcut to getOrDefault the var/val (as sourced by @Michael Zajac and @kingledion...gets the value/variable. If it does not exist, gets the default value/variable). Using getOrDefault may be a more robust solution in cases in which you expect the parameter to not always have a corresponding value, for which you can set a default value.