All entries for Saturday 11 June 2011

June 11, 2011

Further Scala: Implicit conversions

My attempts to learn scala continue…

Implicit conversions are a cool, if slightly unsettling (from a java programmers POV) scala feature. If I have an instance of one class, and I try and call a method on it which is defined in a different class, then if there’s an “implicit” method in scope which will convert between the two, scala will silently use it.

scala> var x = 12
x: Int = 12

scala> x.substring(1,2)
<console>:9: error: value substring is not a member of Int

scala> implicit def foo(i:Int):String={i.toString}
foo: (i: Int)String

scala> 12.substring(1,2)
res10: java.lang.String = 2


This lends itself to a very very useful trick; the ability to enhance classes with additional methods. Say you had a java Map class, and you wanted the ability to merge it with another Map according to some sort of merge function on the values. You’d probably do it like this:

class MergeableMap implements Map{

public MergeableMap(Map delegate){
 this.delegate = delegate

public Map merge(Map otherMap, ValueMergingFunction mergeFunction){

... delegate implementations of all Map methods here...

Trouble is, (a) writing all the delegate methods is tedious, and(b) every time you want to use it, you’ve got to do

MergeableMap m = new MergeableMap(myMapVariable)

Implicits in scala make this a lot easier:

class MergeableMap[A, B](self: Map[A, B]) {
  def merge(m1, merger): Map[A, B] = {
... implementation here...

implicit def map2mergeableMap[A,B](m:Map[A,B]):MergeableMap[A,B] = new MergeableMap(m)

myMap.merge(myOtherMap, myMergeFunction)

there’s no need to implement the other delegate methods, since we can just call them on the original Map class – but when we call merge() compiler-based voodoo works out that we want a mergeable map, and swaps it in for us. Magical.

Most recent entries


Search this blog

on twitter...


    Not signed in
    Sign in

    Powered by BlogBuilder
    © MMXXI