Version Matrix

Immutable priority maps for Scala

Build Status Coverage Maven Central Scala.js

Priority maps are similar to sorted maps, but while for sorted maps iterator returns an iterator that produces entries sorted by their keys, calling iterator on a priority map returns an iterator that produces entries sorted by their values. Priority maps also offer several range methods, which return a submap with values inside a given range.

Since calling head on a priority map returns a key-value pair with minimal value, priority maps can also be thought of as a more versatile variant of priority queues.

This implementation of priority maps has been inspired by Mark Engelberg's implentation for Clojure.


The latest version is 1.0.0 and supports Scala 2.10–2.12 on the JVM as well as on Scala.js.

Releases are available from Maven Central. If you use sbt, simply add the following dependency to your build file:

libraryDependencies += "de.ummels" %%% "scala-prioritymap" % "1.0.0"

See the release notes if you upgrade from an earlier release.


The easiest way to instantiate a new priority map is to use the apply method in the PriorityMap companion object.

scala> import de.ummels.prioritymap.PriorityMap
import de.ummels.prioritymap.PriorityMap

scala> val m = PriorityMap('a' -> 1, 'b' -> 2, 'c' -> 0)
m: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(c -> 0, a -> 1, b -> 2)

Since priority maps are immutable, updating a key/value pair returns a new map and does not modify the old map.

scala> m + ('b' -> -1)
res0: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(b -> -1, c -> 0, a -> 1)

scala> m
res1: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(c -> 0, a -> 1, b -> 2)

In addition to the methods available for maps, priority maps offer methods for obtaining a submap whose values lie inside a given range.

scala> m.from(1)
res2: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(a -> 1, b -> 2)

scala> m.until(2)
res3: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(c -> 0, a -> 1)

scala> m.range(1, 2)
res4: de.ummels.prioritymap.PriorityMap[Char,Int] = PriorityMap(a -> 1)

The full API docs are available here.