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I'm having trouble understanding the difference between / reason for, for example, immutable.Map.transform and immutable.Map.map. It looks like transform won't change the key, but that just seems like a trivial variation of the map method. Am I missing something?

I was expecting to find a method that applied a function to the (key,value) of the map when/if that element was accessed (rather than having to iterate through the map eagerly with the map function). Does such a method exist?

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To me, transform is just a clean way of doing myMap map { case (k, v) => (k, f(k, v)) }. About your second question (which seems not closely related to the first one?), are you just looking for something like updated? – Kane May 24 at 20:38
@kane: yeah, I guess, it just seemed odd to have so many similar variations, especially when I couldn't find the other more useful function I was looking for. The function I want does exist though, 0___'s answer of "andThen" is actually the one I want. Sorry for combining two unrelated questions. – Brian May 28 at 14:06

2 Answers

up vote 2 down vote accepted

If you only need the interface of PartialFunction, you can exploit the fact that Map inherits from PartialFunction:

val m = Map(1 -> "foo", 2 -> "bar")
val n = m.andThen(_.reverse)

n(1)  // --> oof
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hmm.... Maybe I didn't read this clearly enough, now I actually don't think this is what I was looking for.... since this only transforms the value, without access to the key. – Brian May 28 at 22:16
up vote 3 down vote

You can do exactly that with mapValues. Here is the explanation from the docs:

def mapValues[C](f: (B) ⇒ C): Map[A, C]

Transforms this map by applying a function to every retrieved value.

f - the function used to transform values of this map.

returns - a map view which maps every key of this map to f(this(key)). The resulting map wraps the original map without copying any elements.

edit:

Although extending classes of the collection API is not often a good idea, it could work like this:

class LazilyModifiedMap[A,B,C](underlying: Map[A,B])(f: (A,B) => C) extends Map[A,C] {
  def get(key: A) = underlying.get(key).map( x => f(key, x))

  def iterator = underlying.iterator.map { case (k,v) => (k, f(k,v)) }

  def -(key: A) = iterator.toMap - key

  def +[C1 >: C](kv: (A,C1)) = iterator.toMap + kv
}
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1  
Yep, but transform takes two-argument function from both key and value, whereas mapValues takes only one argument -- from value. Let's say you want to append key to value and have a new Map(key, key+value) as the result -- you can't archive this with mapValues. (just wondering why transform exists, if mapValues and map cover 99.9% percents of use cases) – om-nom-nom May 24 at 14:25
yeah, as nom nom said, not very useful without the key. So many methods in scala collections and yet... – Brian May 24 at 14:29
@cmbaxter: nah, you can map from a Map to a List or something too: scala> Map(1->"one",2->"two").map{ case (k,v) => "hi" } res9: scala.collection.immutable.Iterable[java.lang.String] = List(hi, hi) – Brian May 24 at 14:30
1  
Sorry, that was not clear from your post: I was expecting to find a method that applied a function to the value of the map when/if that element was accessed – drexin May 24 at 14:30
@drexin: ah, yeah, I'll clarify that then. – Brian May 24 at 14:30
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