Question

Poking around Map.withDefault, I couldn’t figure out where “get” is defined. Although default seems to pretty clearly not memoize1, it seems like some implementations of “get” could memoize behind the scenes.

Basically, what I want to know is, if I have a computation that I want to only have to do once, will a map with a default do it many times, or only once?

Answer 1

Not a problem. I’ve been doing a very similar trick for a long time.

scala> :paste
// Entering paste mode (ctrl-D to finish)

def someFunction ( s : String ) = {
  println("someFunction called")
  s.size
}
val someFunctionMemoized : String => Int
  = new collection.mutable.HashMap[String, Int] {
      override def default ( key : String )
        = {
          val value = someFunction(key)
          update(key, value)
          value
        }
    }

// Exiting paste mode, now interpreting.

someFunction: (s: String)Int
someFunctionMemoized: String => Int = Map()

scala> someFunctionMemoized("abc")
someFunction called
res0: Int = 3

scala> someFunctionMemoized("abc")
res1: Int = 3