[ base ] Deprecate toList functions for sorted sets and maps

[buzden]

Description

Both Data.SortedMap and Data.SortedSet have specialised toList functions.

For Data.SortedSet, I think this is a historical curious case, since its signature and semantics completely copies toList from Prelude, namely from Foldable, which is implemented by SortedSet. I suggest to remove this completely in the future, but suggest now to deprecate this function in favor of prelude's one for backward compatibility.

For Data.SortedMap, toList has different meaning comparing to the one from Foldable, while Foldable is indeed implemented by SortedMap, having Prelude.toList to list all the values, while SortedMap.toList listing all key-value pairs stored in the map. I think having two functions with the same name having different meaning is not a good thing, plus you need to specify namespace each time you use toList for maps (whichever you use). I suggest to introduce a new function for listing key-value pairs list, namely kvList, and to deprecate special key-value toList function in favor of the new one.

To be clear: this PR is a backward-compatible change, but it adds several deprecation warnings, and I suggest the breaking change after the next release (namely, removing deprecated functions).

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).

[dunhamsteve]

There is a fromList which does the opposite of SortedMap.toList. Should we change the name of that (and maybe fromListWith too)?

[buzden]

There is a fromList which does the opposite of SortedMap.toList. Should we change the name of that (and maybe fromListWith too)?

I thought about that but 1) I didn't come up with a better name 2) I thought this may be a too much change which would probably be discarded because of the size. For instance, this would change even the Show instance.

[mattpolzin]

I'm not sure I like the renaming of SortedMap.toList. I like the symmetry that toList/fromList are duals and I find it more useful to consider the action of taking a map to a list to be a key-value list than just a value list so changing the name of SortedMap.toList removes the more intuitive toList in favor of the less intuitive one IMO.

I get the argument that it is unfortunate to have a collision of names with prelude, but I'm not sure I'd vote for resolving it at the cost of the above preferences of mine.

Just my $0.02 though.

I'm not at all opposed to deprecating and removing any specialized implementation that is identical to the generic one (re SortedSet).

[buzden]

I find it more useful to consider the action of taking a map to a list to be a key-value list than just a value list so changing the name of SortedMap.toList removes the more intuitive toList in favor of the less intuitive one IMO

Looks like it's okay to make Foldable imlementation to be named in this case. When the current Foldable is named, them toList would behave exactly as you say. However, null function will require explicit named instance instead, that's, probably, not so desired; but we can do the same as with toList, then, we can create a specialised function for this.

I'm not at all opposed to deprecating and removing any specialized implementation that is identical to the generic one (re SortedSet).

I did #3425 for this