Add a method to list subfolders of a MailFolder recursively

[Moerfi666]

Also as discussed on Matrix with @seijikun,
here the PR that adds an implementation for recursive folder listing.

This takes an optional parent folder, and returns all folders and subfolders in the hierarchy as flattened list.
So a hierarchy:

- myfolder1
  - myfolder12
  - myfolder13
  - myfolder14
    - myfolder141

becomes this flattened sequential list:

- myfolder1
- myfolder1 / myfolder12
- myfolder1 / myfolder13
- myfolder1 / myfolder14
- myfolder1 / myfolder14 / myfolder141

Normally, IMAP supports this operation directly (O(1)), but JavaMail does not seem to provide this functionality, forcing one to traverse the hierarchy manually.

[eikek]

Hm… it seems the folder is never added to the result? So "INBOX" doesn't belong to the result - or the parent folder? Then folderList is unused, the method is not called with other than Nil.

It would be nice, if it would be tail-recursive, wdyt? Probably not so important in practice, but I tend to prefer it in general as it is usually not harder to write/read, imho, and so we get stack safety from scala.

[Moerfi666]

You are right about the folderList parameter, I was a bit tired writing this, sorry.

Not including the parent folder is intended, maybe renaming to ListSubFolders would be more appropriate?

How would someone transform this into a tail-recursive method? Maybe I just can't wrap my head around it, but it seems to be hard because it's a tree like structure - I'm open for ideas!

[eikek]

Not including the parent folder is intended, maybe renaming to ListSubFolders would be more appropriate?

Ah ok - I looked at the example in the description, it seems to be included there. Maybe we can return a NonEmptyList with the input folder as first element? The user can then decide whether to use it or not. I thought it may be convenient when returning the INBOX when giving a None as input.

Regarding the recursive thing. I thought maybe something like this could work:

@annotation.tailrec
def flatten(folder: Vector[Folder], result: Vector[Folder] = Vector.empty): Vector[Folder] = 
  if (folder.isEmpty) result
  else flatten(folder.flatMap(_.list().tovector), folder ++ result)

// flatten(Vector(parent)) 

(modulo all the syntax errors) But I may just miss something here! can totally relate to the tiredness:) then just leave it - it's not important anyways.

[Moerfi666]

We've converted our method to be tail recursive now.
One problem with having the requested folder as part of the result is when calling listRecursive(None) (so, essentially getting a list of top-most folders), because
it would then add the IMAP drive's root folder into the list.
But that folder is transparent and has no path, which is represented as a MailFolder instance with an empty NonEmptyList for path.

[eikek]

We've converted our method to be tail recursive now. One problem with having the requested folder as part of the result is when calling listRecursive(None) (so, essentially getting a list of top-most folders), because it would then add the IMAP drive's root folder into the list. But that folder is transparent and has no path, which is represented as a MailFolder instance with an empty NonEmptyList for path.

Ah yes, that is true! I would be ok with not adding it in the None case. But otoh this feels quite surprising, then I'm also for removing the input folder from the results. My thoughts for including it first, were more from a ergonomic point of view. I think I wouldn't trade it for surprising behaviour 🤔

[eikek]

Hey @Moerfi666 if you want to rather remove the input folder from the results, I'm totally fine with that. I'm also fine with current state, let me know what you think/want to do. No need to hurry, just wanting to let you know …

[Moerfi666]

Hey! Sorry for the long delay.

Conceptually, we thought about the method more as a way to get the subfolders within a given path.
So we would prefer to stay with the current version.
(Also to avoid the IMAP-induced special cases the other option would require.)