Merging structural changes and unnamed lists

[jankowtf]

Hi!

This is my first pull request here, so hope I'm doing everything correctly. Hope the comments are sufficient to explain my thinking, please reach out if something should be unclear.

• Added functionality to merge structural changes (named to unnamed list) and unnamed lists based on UID convention
• Added unit tests for new merge functionality (probably will have to add these in a separate PR -> #32)

[jjallaire]

@andrie Could you review this when you have some time later this week. My biggest concern would be whether this might break any existing configs -- at first glance I don't see any obvious issues but it's worth additional scruitiny.

[jankowtf]

@andrie @jjallaire: maybe worth mentioning that the else part in

if (!all(is.null(names(merged_list)))) {
  # This implies a structural change from named list to unnamed list and
  # thus the new content should be returned all together
  overlay_list
} else {
  # This is implies no structural break, but an unnamed list needs to be
  # merged with another unnamed list. As this is a non-trivial task with
  # lots of possible solutions, the current merging mechanism is a
  # simple solution that expects the first element of the sublists to
  # act as a unique identifier

  # Merge lists but with 'overlay_list' taking the lead
  merged_list <- append(overlay_list, merged_list)
  # Extract UIDs
  uids <- extract_uids(merged_list)
  # Drop duplicated UIDs and sort based on UIDs
  merged_list[!duplicated(uids)][order(unique(uids))]
}

was only an idea which idepends upon some made-up conventions.

It might make more sense to also go with overlay_list in such cases in order not change too much of the existing behavior. Along the lines of #25

[psychelzh]

Just wonder if this will be merged, for 2 years has passed.