Using Scala’s String Interpolation to Access a Map

Published 2013-03-15.
Time to read: 1 minutes.

This page is part of the posts collection, categorized under Scala.

Given a map, would it not be nice to have a shorthand way of looking up a value from a key, and to provide a default value? I’ve wanted to be able to do this for a long time. Scala 2.10 makes this really easy!

The MapLookup class below contains a Map[String, Int] that can be looked up by using a dollar sign ($) from code that contains a reference to the implicit class. If the lookup key is not defined by the map, a zero is returned.

implicit class MapLookup(val sc: StringContext) {
  val map = Map(("a", 1), ("b", 2), ("c", 3)).withDefaultValue(0)

  def $(args: Any*): Int = {
    val orig = sc.s (args : _*)
    map.get(orig)
  }
}

Assuming that the above is stored in a file called strInterp.scala, here are some examples of usage:

Shell

$ scala -i strInterp.scala
Loading strInterp.scala...
defined class MapLookup

Welcome to Scala version 2.10.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_17).
Type in expressions to have them evaluated.
Type :help for more information.

scala> $"a"
res2: Int = 1

scala> $"z"
res3: Int = 0

Short and sweet!