DefaultDictionary

DefaultDicitionary.kt

package dictionary

/**
 * A default dictionary is a map that provides a default concrete Monoid empty value for [B]
 * Inspired by some of the examples of https://kotlinexpertise.com/default-map-in-kotlin/
 * https://twitter.com/raulraja/status/1141697899383992325
 */
data class DefaultDictionary<A, B>(val empty: B, val underlying: Map<A, B> = emptyMap()) : Map<A, B> by underlying {

  /**
   * The empty value is always contained
   */
  override fun containsValue(value: B): Boolean =
    empty == value || underlying.containsValue(value)

  /**
   * if no value is found for key [A] defaults to the Monoid empty [B]
   */
  override fun get(key: A): B? =
    underlying[key] ?: empty

  /**
   * Combines this Dictionary with other Dictionary.
   * Implements Semigroup.combine for all [A] under the assumption that the empty [B]
   * is a constant indentity value such as 0 for Int, "" for String, emptyList() for List, etc.
   */
  operator fun plus(other: DefaultDictionary<A, B>): DefaultDictionary<A, B> =
    DefaultDictionary(empty, underlying + other.underlying)

  companion object {
    fun <A, B> of(empty: B, vararg pairs: Pair<A, B>): DefaultDictionary<A, B> =
      DefaultDictionary(empty, pairs.toList().toMap())

    fun <A> int(): DefaultDictionary<A, Int> = of(0)

    /**
     * Behavior as shown in the Python example
     */
    fun <A, B> groupOcurrences(values: Collection<Pair<A, B>>): DefaultDictionary<A, List<B>> =
      DefaultDictionary(emptyList(), values.toList().fold(emptyMap()) { acc, (a, b) ->
        val listOfB = acc[a]
        if (listOfB != null) acc + (a to (listOfB + b))
        else acc + (a to listOf(b))
      })

  }
}

fun main() {
  val d = DefaultDictionary.int<String>()
  println(d["someKey"]) //0

  val data = listOf("red" to 1, "blue" to 2, "red" to 3, "blue" to 4, "red" to 1, "blue" to 4)
  println(DefaultDictionary.groupOcurrences(data)) //DefaultDictionary(empty=[], underlying={red=[1, 3, 1], blue=[2, 4, 4]})
}

Hi @kioba.
Is it still a valid override for Map if it just returns B? if it is then yes that signature is more accurate.
You are correct, plus is not well implemented. I'll edit it