Iterating over Map with Scala

Here is a summary of the iteration over a Scala Map discussion, started by Ikai at the Scala mailing list.
Here is the Map we wish to iterate over.

scala> val m1 = Map[Int, Int](1->2, 2->3, 4->50)                
m1: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 2 -> 3, 4 -> 50)

The options are:
Accessing the Tuple2 object structure.

scala> m1 foreach ( (t2) => println (t2._1 + "-->" + t2._2))
1-->2
2-->3
4-->50

Using case on the Tuple2

scala> m1 foreach {case (key, value) => println (key + "-->" + value)}
1-->2
2-->3
4-->50

Using a for loop

scala> for ((key, value) <- m1) println (key + "-->" + value) 
1-->2
2-->3
4-->50

Advanced: using unapply to expand the Tuple and use it in the match:

scala> object -> { def unapply[A,B](x : (A,B)) : Option[(A,B)] = Some(x) }
defined module $minus$greater

scala> m1 foreach {case k->v => println (k + "-->" + v)}      
1-->2
2-->3
4-->50

I don't know why the next one does not work, any idea?

scala> m1 foreach (println (_._1 + "-->" + _._2))           
:6: error: missing parameter type for expanded function ((x$1, x$2) => x$1._1.$plus("-->").$plus(x$2._2))
       m1 foreach (println (_._1 + "-->" + _._2))
                            ^

If you know of other options, please add.