Make viewness of mapValues / filterKeys more obvious

[lrytz]

@Ichoran suggests to rename them to mappedValueView / filteredKeyView (#4776 (comment)).

For naming, I think mapValuesView / filterKeysView would be more consistent, as the strict methods are called map / filter (not mapped / filtered).

"Alternatively, return a value class with a single method, view, that gives out the view"

[smarter]

Having a strict version of these methods would be great too.

[lrytz]

In principle the operations should be strict on strict collections, and lazy on views, like other operations. We probably cannot just change the semantics though..

[szeiger]

Let's deprecate the operations on MapOps now and undeprecate them on MapView so that you get the desired semantics with .view.mapValues and .view.filterKeys. Maybe we can make the versions on Map strict in 2.14 after a deprecation cycle without removing them first? Otherwise this will have to wait until Scala 3.1.

[lrytz]

This sounds good, but it's annoying if you want a strict mapValues (without deprecation warning): you have to write .view.mapValues(f).toMap. We could add mapValuesStrict? But it's also not great to add a method only to deprecate it in the next release...

[szeiger]

Well, it's already better than 2.12 where there's no obvious way at all to make the result strict.

[joshlemer]

@szeiger do we still want to deprecate MapOps#mapValues?

[szeiger]

After rereading this thread it still looks like a sensible option to me. I'm undecided whether the replacement should be a dedicated mapValuesView method or .view.mapValues.

[julienrf]

Introducing mapValuesStrict or mapValuesView would look quite bad, imho.

[SethTisue]

@joshlemer once you've accepted the invitation to https://github.com/orgs/scala/teams/contributors , comment here and I'll assign this ticket to you.

[joshlemer]

@SethTisue I accepted it

[SethTisue]

👍 thanks for all the good scala/scala PRs lately, much appreciated!

[SethTisue]

it's annoying if you want a strict mapValues (without deprecation warning): you have to write .view.mapValues(f).toMap

And also note that by going through .view like that you lose the original collection type. toMap will only be the correct rewrite if the input type was immutable.Map

fwiw, transform exists as a strict, collection-type-preserving alternative:

scala 2.13.1> collection.immutable.SortedMap(1 -> "foo")
res7: scala.collection.immutable.SortedMap[Int,String] = TreeMap(1 -> foo)

scala 2.13.1> res4.transform((_, v) => v.length)
res8: scala.collection.immutable.SortedMap[Int,Int] = TreeMap(1 -> 3)

thx @OlegYch for the reminder