Fill gap of functions between `List` and `NonEmptyList`

[Evangelink]

I am using the NonEmptyList collection in some codebases and I have noticed that some functions available in List (e.g., collect, groupBy...).

I am wondering if these functions are missing for a good reason or just because they weren't yet implemented. In case of the latter I'd be happy to provide a PR.

[github-actions]

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Thanks for raising this — missing NonEmptyList functions are definitely worth filling in, and contributions are welcome!

What already exists

The current NonEmptyList module has: create, singleton, cons, head, tail, length, toList, toArray, toSeq, ofArray, ofList, ofSeq, map, append, appendList, reduce, last, rev, collect, zip.

Functions that return NonEmptyList (safe to add)

These preserve non-emptiness and can return NonEmptyList<_> directly:

Function	Notes
sort / sortBy / sortWith / sortDescending / sortByDescending	Preserves length
map ✓	Already there
collect ✓	Already there (mapping must return NonEmptyList)
zip ✓	Already there
mapi	Like map with index
iter / iteri	Side effects
exists / forall / contains	Predicates
maxBy / minBy / max / min	Safe because non-empty
sum / sumBy / average / averageBy	Same
pairwise	Returns list (can be empty for length 1)
groupBy	Returns NonEmptyList<'Key * NonEmptyList<'T>> — non-empty by definition

Functions that return list (because result can be empty)

Function	Notes
filter / choose	Result can be empty → 'T list
skip n	Can produce empty → 'T list
pairwise	Length-1 input → empty → ('T * 'T) list

Suggested approach

• Functions that cannot make the result empty → return NonEmptyList<_>
• Functions that can make the result empty → return 'T list (document this in the XML doc)

PR #229 already adds iter, iteri, mapi, exists, forall, contains, sort, sortBy, sortWith, maxBy, minBy to the module — that PR addresses the most common missing functions mentioned here.

Happy to review a PR adding groupBy, sumBy, averageBy, pairwise etc. if you'd like to contribute!

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