idiomatic “get or else update” for immutable.Map?

Question

What is the idiomatic way of a getOrElseUpdate for immutable.Map instances?. I use the snippet below, but it seems verbose and inefficient

var map = Map[Key, Val         


        

Answer 1

Let me summarise your problem:

• You want to call a method on a immutable data structure
• You want it to return some value and reassign a var
• Because the data structure is immutable, you’ll need to
  • return a new immutable data structure, or
  • do the assignment inside the method, using a supplied closure

So, either your signature has to look like

def getOrElseUpdate(key: K): Tuple2[V, Map[K,V]]
//... use it like
val (v, m2) = getOrElseUpdate(k)
map = m2

or

def getOrElseUpdate(key: K, setter: (Map[K,V]) => Unit): V
//... use it like
val v = getOrElseUpdate(k, map = _)

If you can live with one of these solutions, you could add your own version with an implicit conversion but judging by the signatures alone, i wouldn’t think any of these is in the standard library.