foldMap

gistfile1.scala

import scalaz.Scalaz._

/** The problem: given the following database query results, associate each parentId to
  * a List of filenames.
  */
case class Row(parentId: Int, file: String)
val queryResult = Stream(Row(123, "foo.jpg"), Row(123, "bar.jpg"), Row(234, "foo.png"))

/** Using foldLeft is verbose because we have to specify:
  *   - how to create an empty Map to begin with
  *   - how to create an empty List the first time a key is encountered
  *   - how to prepend a new element to a List
  *   - how to append a new (key, value) pair to a Map
  */
val usingFoldLeft =
  queryResult.foldLeft(Map.empty[Int, List[String]]){ (acc, row) =>
    val existing = acc.get(row.parentId).getOrElse(Nil)
    val newValue = row.file :: existing
    acc + ((row.parentId, newValue))
  }
// Map(123 -> List(bar.jpg, foo.jpg), 234 -> List(foo.png))

/** Using a mutable Map is verbose because we must:
  *   - create an empty Map to begin with
  *   - convert the mutable Map to an immutable one to return it
  *   - and again specify the empty List and prepend operation
  */
val usingMutableMap = {
  val result = collection.mutable.Map.empty[Int, List[String]]
  for (Row(id, file) <- queryResult) {
    val existing = result.get(id).getOrElse(Nil)
    val newValue = file :: existing
    result.update(id, newValue)
  }
  result.toMap
}
// Map(123 -> List(bar.jpg, foo.jpg), 234 -> List(foo.png))

/** Using groupBy is neater, but creates an intermediate Map which must
  * then be traversed once more to clean unwanted data from the values.
  * It also gives us each value as a Stream, which we must fix if we want
  * a List.
  */
val usingGroupBy =
  queryResult groupBy (_.parentId) mapValues (_.map(_.file).toList)
// Map(234 -> List(foo.png), 123 -> List(foo.jpg, bar.jpg))

val usingFoldMap =
  queryResult foldMap { case Row(id, file) => Map(id -> List(file)) }
// Map(234 -> List(foo.png), 123 -> List(foo.jpg, bar.jpg))