Introduce new lib-function for lists to filter out empty elements

[SebTM]

Description

Related to #2314
Moved function needed by this PR because its providing basic/general functionality and therefore should be located in lib

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[rycee]

I think it's best to avoid introducing this function, I think it is too ambiguous as to what "empty" means in this context. For example, why not remove empty lists as well? And the flatten functionality seems arbitrary. So it seems to me that this function is not actually generally useful but rather intended for a specific use-case.

I'm thinking that perhaps a function like

removeAny [3 4] [ 1 3 4 3 ]
# => [ 1 ]

to match the remove function may be nice. The implementation would be something like

removeAny = l: builtins.filter (x: ! builtins.elem x l)

It wouldn't do any flattening, though, but I think that is an orthogonal functionality.

[SebTM]

Hmm, the function/separation outside the module happened after discussion in the above PR, as the "notEmpty"-function comes from "xidlehook" it felt like a more often use-case with the below examples:

• https://github.com/nix-community/home-manager/blob/master/modules/programs/neovim.nix#L49 / 52
• https://github.com/nix-community/home-manager/blob/master/modules/services/fnott.nix#L9
• https://github.com/nix-community/home-manager/blob/master/modules/programs/mbsync.nix#L161 / 180
• (unsure) https://github.com/nix-community/home-manager/blob/master/modules/programs/skim.nix#L108 (with your adjustments about empty sets)

The goal with providing this function was to reduce the possible duplicate code and more common handling of these recurring situations, as far as I understand.

I understand your point with the naming, we can rename and extend it to also filter empty lists 👍🏻
I'm not sure how to understand "And the flatten functionality seems arbitrary" can you explain a bit more, please?

Thanks for your review and feedback, I appreciate it ⭐

[rycee]

@SebTM About the flatten, I think that the flattening is so different from the notEmpty naming that it would have to be included in the name, similar to how concat is part of concatMap. Specifically, with the existing naming I would expect

notEmpty [ [ "" 34 ] [] null "s" ]

to return [ [ "" 34 ] "s" ], not [ 34 "s" ]. So perhaps something like flattenRemoveEmpty and removeEmpty (for the non-flattening version of the function).

I'm not super convinced about the necessity of these functions, though. When it only is empty strings that are removed then I think remove "" is more straight-forward and clear. I created the #2590 PR to make that change. The filter remains for cases where you want to remove, for example, both null and "" values. For those perhaps the removeAny (alternatively named removes) function would be simplest. I.e., something like

removes [ "" null ] listOfStuff

The flattening could be done explicitly

removes [ "" null ] (flatten listOfStuff)

or with a custom function

flattenRemoves [ "" null ] listOfStuff

[SebTM]

Seems to be not really needed as I could use lists.subtractLists without Problems - see #2314 :)