scala - Idiomatic way to update value in a Map based on previous value

Question

Let's say I store bank accounts information in an immutable Map:

val m = Map("Mark" -> 100, "Jonathan" -> 350, "Bob" -> 65)

and I want to withdraw, say, $50 from Mark's account. I can do it as follows:

val m2 = m + ("Mark" -> (m("Mark") - 50))

But this code seems ugly to me. Is there better way to write this?

Answer 1

There's no adjust in the Map API, unfortunately. I've sometimes used a function like the following (modeled on Haskell's Data.Map.adjust, with a different order of arguments):

def adjust[A, B](m: Map[A, B], k: A)(f: B => B) = m.updated(k, f(m(k)))

Now adjust(m, "Mark")(_ - 50) does what you want. You could also use the pimp-my-library pattern to get the more natural m.adjust("Mark")(_ - 50) syntax, if you really wanted something cleaner.

(Note that the short version above throws an exception if k isn't in the map, which is different from the Haskell behavior and probably something you'd want to fix in real code.)