How to avoid any mutable things in this builder?

I have a simple Scala class like this:

class FiltersBuilder {

  def build(filter: CommandFilter) = {
    val result = collection.mutable.Map[String, String]()
    if (filter.activity.isDefined) {
      result += ("activity" -> """ some specific expression """)
    } // I well know that manipulating option like this is not recommanded, 
      //it's just for the simplicity of the  example
    if (filter.gender.isDefined) {
      result += ("gender" -> """ some specific expression """)
    result.toMap //in order to return an immutable Map

using this class so:

case class CommandFilter(activity: Option[String] = None, gender: Option[String] = None)

The result content depends on the nature of the selected filters and their associated and hardcoded expressions (String).

Is there a way to transform this code snippet by removing this "mutability" of the mutable.Map?


Map each filter field to a tuple while you add the result to a Seq, then filter out the Nones with flatten finally convert the Seq of tuples to a Map with toMap. For adding more fields to filter you just have to add a new line to the Seq

    def build(filter: CommandFilter) = {
      // map each filter filed to the proper tuple
      // as they are options, map will transform just the Some and let the None as None
      val result = Seq( => "activity" -> s""" some specific expression using $value """), => "gender" -> s""" some specific expression using $value """)
      ).flatten // flatten will filter out all the Nones
      result.toMap // transform list of tuple to a map

Hope it helps. Gaston.

Since there are at most 2 elements in your Map:

val activity = => Map("activity" -> "xx"))
val gender = => Map("gender" -> "xx"))
val empty = Map[String, String]()
activity.getOrElse(empty) ++ gender.getOrElse(empty)

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