中没有调整 API，不幸的是。我有时使用像以下这样的功能（在Haskell的 Data.Map.adjust ，具有不同的参数顺序）：

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

现在 adjust（m，Mark）（_ - 50）做你想要的您还可以使用 pimp-my-library模式获得更自然的 m.adjust（Mark）（_ - 50）语法，如果你真的想要一些更清洁的东西。

（注意如果 k 不在地图中，与Haskell行为不同，也可能是您想要在实际代码中修复的内容，上述短版本会抛出异常。）

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?

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.)