mutable.Map.filterKeys should return the instance of mutable.Map instead of collection.Map

[scabug]

In current implementation, mutable.Map.filterKeys returns the instance of collection.Map so we can't update the result of filtering.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9595?orig=1
Reporter: Kousuke Saruta (sarutak)
Affected Versions: 2.11.7

[scabug]

@szeiger said:
This is trickier than it looks. Unlike most other operations (notably including "filter"), "filterKeys" is specified as returning a view of the original map:

bq. @return an immutable map consisting only of those key value pairs of this map where the key satisfies the predicate p. The resulting map wraps the original map without copying any elements.

This is at odds with wanting to get a mutable Map back. Possible ways out:

• Leave "filterKeys" as a special case. When you want an eagerly evaluated operation, just use "filter".
• Remove "filterKeys". The difference in semantics to "filter" is not obvious. Having these methods behave differently can be confusing to users.
• Give "filterKeys" the same semantics as "filter".

Since we're going to focus on the collections library in 2.13, I'm inclined to go with 1) for now and defer a decision until 2.13.

You could easily build a case for 2) or 3) if we had proper Map views. ".filterKeys" with the current view-like semantics should really be ".view.filterKeys" but that's not possible because Map.view only gives you an IterableView.

[szeiger]

filterKeys is deprecated in 2.13 and we do have the replacement view.filterKeys now with a proper MapView. We may be able to reuse the name filterKeys in 2.14 or 3.1 for a strict version.

[som-snytt]

It works as described. Instead of force use toMap.