Make the difference between maps and lists more explicit

[JeroenDeDauw]

Background: the map/list distinction was not present in the very first versions as got hacked in, leaving something that is not explicit and clean.

Terms:

• Map: associative array. Key matters. Diffs can contain change operations and diffs
• List: array with numeric keys. Comparison by values. Diffs can only contain add and remove operations

Pain points:

• Only one Diff class, rather than a MapDiff and ListDiff. (Those classes actually existed at some point, though as subclasses of Diff, inheriting nearly everything)
• The Diff class takes an $isAssociative flag, though constructing it with this set to false AND associative operations results in no error
• The Diff class is a mess
• All differs implement the Differ interface. All services needing a differ just request a Differ. Since the Differ has no info on if it is doing an associative diff, such services cannot restrict the type of differ, know what kind of operations to expect or know what flag to pass to the Diff class.
• Non-associative patchers (and other services) cannot clearly specify they only deal with non-associative diffs

[BoShurik]

What about two empty interfaces MapDifferInterface and ListDifferInterface so the code

return new Diff( $this->listDiffer->doDiff( $old, $new ), $this->listDiffer instanceof MapDifferInterface);

would be appropriate

[JeroenDeDauw]

That is not elegant, but it

• would fix the problem you are running into
• does not break existing behavior
• would be easy to keep around if it is obsoleted by a more comprehensive solution

So yeah, adding a MapDifferInterface seems OK. I'm really not a fan of the name though. You want to make a PR that adds MapDifferInterface? (no need for ListDifferInterface at this point as far as I can tell)