How to avoid any mutable things in this builder? - scala

Question

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?

Answer 1

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(
filter.activity.map(value => "activity" -> s""" some specific expression using $value """),
filter.gender.map(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.

Answer 2

Since there are at most 2 elements in your Map:
val activity = filter.activity.map(_ => Map("activity" -> "xx"))
val gender = filter.gender.map(_ => Map("gender" -> "xx"))
val empty = Map[String, String]()
activity.getOrElse(empty) ++ gender.getOrElse(empty)

Answer 3

I've just managed to achieve it with this solution:
class FiltersBuilder(commandFilter: CommandFilter) {
def build = {
val result = Map[String, String]()
buildGenderFilter(buildActivityFilter(result))
}
private def buildActivityFilter(expressions: Map[String, String]) =
commandFilter.activity.fold(expressions)(activity => result + ("activity" -> """ expression regarding activity """))
private def buildGenderFilter(expressions: Map[String, String]) =
commandFilter.gender.fold(expressions)(gender => result + ("gender" -> """ expression regarding gender """))
}
Any better way?