immutable.Map.filterKeys should return immutable.Map

[scabug]

collection.immutable.Map.filterKeys should return collection.immutable.Map

Now it returns collection.Map.Projection. So

var map = Map[Int, Int] = ...
map = map.filterKeys(...)

is not possible.

(Map is Predef.Map == immutable.Map)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-751?orig=1
Reporter: @stepancheg

[scabug]

@mcdirmid said:
I have no idea why I put filterKeys in, as it is redundant with with filter. The submitter should use filter and just look at the key vs. filterKeys. I need to check why filterKeys is there, and if there is no good reason, deprecate it for eventual removal.

[scabug]

@odersky said:
Milestone next_bugfix deleted

[scabug]

@paulp said:
In r17770 I changed filterKeys to just use filter, so in line with the rest of the new collections it returns the same type of collection. I also suggested it be deprecated, but either way it may as well do the right thing.

[scabug]

@odersky said:
I think it's better to leave it as a view for the moment. If you want the same kind of map, use filter. Once (if?) we have systematic views for maps we can deprecate it.

[scabug]

@dcsobral said:
Seems to work now:

scala> var map = Map(1 -> "a", 2 -> "b", 3 -> "c")
map: scala.collection.immutable.Map[Int,java.lang.String] = Map((1,a), (2,b), (3,c))

scala> map = map.filterKeys(_ % 2 == 0)
map: scala.collection.immutable.Map[Int,java.lang.String] = Map(2 -> b)

Built from: 2.8.0.r0-b20100306211516

[scabug]

@paulp said:
(In r21162) Test case closes #751. No review.