Deploy the scala collection to map, filter, etc. Created the right type

Question

I am trying to implement a map with default ratings , and I would like filters, maps, etc. they DefaultingMapalso produced DefaultingMapwhenever possible. Here is my initial implementation:

class DefaultingMap[K, V](defaultValue: => V)
extends mutable.HashMap[K, V]
with mutable.MapLike[K, V, DefaultingMap[K, V]] {

  override def empty = new DefaultingMap[K, V](defaultValue)

  override def default(key: K): V = {                 
    val result = this.defaultValue
    this(key) = result
    result                                            
  }
}

I get type objects DefaultingMapwhen I use filter, but not when I use map:

scala> val counter = new DefaultingMap[Char, Int](0)
counter: DefaultingMap[Char,Int] = Map()

scala> for (c <- "ababcbbb") counter(c) += 1

scala> counter.filter{case (k, v) => v > 1}
res1: DefaultingMap[Char,Int] = Map((a,2), (b,5))

scala> counter.map{case (k, v) => (k, v * 2)}
res2: scala.collection.mutable.HashMap[Char,Int] = Map((a,4), (c,2), (b,10))

The difference between the two methods is that it mapaccepts implicit CanBuildFrom. Therefore, I understand that I need to have implicit defsomewhere to provide CanBuildFrom. My first intuition was to do what was done in HashMap:

object DefaultingMap extends generic.MutableMapFactory[DefaultingMap] {

  def empty[K, V]: DefaultingMap[K, V] = // Not possible!

  implicit def canBuildFrom[K, V]:
    generic.CanBuildFrom[Coll, (K, V), DefaultingMap[K, V]] = 
      new MapCanBuildFrom[K, V]
}

, , , empty - , defaultValue. CanBuildFrom , -, , defaultValue.

?

Answer 1

Mutable maps Builder Scala, MapFactory .

, , , factory, collection.generic.MapFactory. , , empty newBuilder defaultValue.

- ( API- Scala 2.8 , , - ):

import collection._                                                                      


class DefaultingMap[K, V](val defaultValue: V)                                                                      
extends mutable.HashMap[K, V]                                                                                       
with mutable.MapLike[K, V, DefaultingMap[K, V]] {                                                                   

  override def empty = new DefaultingMap(defaultValue)                                                              

}                                                                                                                   


object DefaultingMap {                                                                                              
  def newBuilder[K, V](d: V): DefaultingMap[K, V] = new DefaultingMap[K, V](d)                                      

  implicit def canBuildFrom[K, V] =                                                                                 
    new generic.CanBuildFrom[DefaultingMap[K, V], (K, V), DefaultingMap[K, V]] {                                    
      def apply(from: DefaultingMap[K, V]) = newBuilder[K, V](from.defaultValue)                                    
      def apply() = error("unsupported default apply")                                                              
    }                                                                                                               
}                                                                                                                   


object Main {                                                                                                       
  def main(args: Array[String]) {                                                                                   
    println((new DefaultingMap[Int, Int](5)).defaultValue)                                                          
    println(((new DefaultingMap[Int, Int](5)).map(x => x)).defaultValue)                                            
  }                                                                                                                 
}

$ scalac defaulting.scala
$ scala Main
5
5

, apply.

Answer 2

collection.immutable.Map 2.8 , withDefault:

val m = collection.immutable.Map(1->"a", 2->"b", 3->"c")
val n = m withDefaultValue "default"

// n(7) will return "default"

UPDATE

, withDefaultValue :

val o = (for ((k, v) <- m) yield (k, v)) withDefaultValue "default"
// o(0) will return "default"

Answer 3

, , , , :

class DefaultingMap[K, V](defaultValue: => V)
extends mutable.HashMap[K, V]
with mutable.MapLike[K, V, DefaultingMap[K, V]] {

  override def empty = new DefaultingMap[K, V](defaultValue)

  override def default(key: K): V = {                 
    val result = this.defaultValue
    this(key) = result
    result                                            
  }

  implicit def canBuildFrom[NK] =
    new generic.CanBuildFrom[DefaultingMap[K, V], (NK, V), DefaultingMap[NK, V]] {
      def apply(from: DefaultingMap[K, V]) =
        new DefaultingMap[NK, V](from.newDefaultValue)
      def apply() =
        new DefaultingMap[NK, V](defaultValue)
    }

  def newDefaultValue = defaultValue
}

, CanBuildFrom , :

scala> val counter = new DefaultingMap[Char, Int](0)
counter: DefaultingMap[Char,Int] = Map()

scala> for (c <- "ababcbbb") counter(c) += 1

scala> import counter._
import counter._

scala> counter.map{case (k, v) => (k, v * 2)}
res1: DefaultingMap[Char,Int] = Map((a,4), (c,2), (b,10))

scala> for ((k, v) <- counter; if v > 1) yield (k.toString, v * 2)
res2: DefaultingMap[java.lang.String,Int] = Map((a,4), (b,10))

, import counter._, , . , implicit def canBuildFrom, ...

Answer 4

Scala 2.8 API - , , , , , t . , ...

Answer 5

This will not help you with the return type map(but then again, the default value of a new collection usually cannot be determined by conversion map). But to give you a different approach:

class DefaultHashMap[A, B](dflt: => B) extends scala.collection.mutable.Map[A, B] {
  val underlying = new scala.collection.mutable.HashMap[A, B]()

  def get(key: A) = underlying.get(key) orElse Some(default(key))
  def iterator: Iterator[(A, B)] = underlying.iterator
  def +=(kv: (A, B)) = { underlying += kv; this }
  def -=(key: A) = { underlying -= key; this }
  override def empty: DefaultHashMap[A, B] = new DefaultHashMap[A, B](dflt)
  override def default(key: A) = dflt
}