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]})
}

Thanks for the implementation! few questions regarding the implementation:

1. Do you think it is reasonable to assume that the get function always returns a non nullable value?
   At the moment it can return the B? but we always default back to the empty value which is type B.
   So the following change is possible and makes the get method safe.
   override fun get(key: A): B = underlying[key] ?: empty

2. It seems like the plus operator behaves as an identity function and it does not combine the underlying map.

  val first = DefaultDictionary.of(0, "one" to 1)
  val other = DefaultDictionary.of(0, "two" to 2)
  println(first.plus(other)) //DefaultDictionary(empty=0, underlying={two=2})

Is it the expected behaviour?
What I would expect in this case is that the underlying value would be merged and the result of the plus above would be DefaultDictionary(empty=0, underlying={one=1, two=2}) with

operator fun plus(other: DefaultDictionary<A, B>): DefaultDictionary<A, B> =
    DefaultDictionary(empty,  underlying + other.underlying)

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