Type-classes for Prepend (Cons) and Append (Snoc) operations about cats HOT 5 OPEN

We have prependK and appendK on Alternative with those signatures. Sergey has optimized their implementations for standard collections in #4052 and #4055. Does that address your usecase? Thanks!

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Oh that's nice, didn't notice, thanks to @satorg 🙏

That seems to address my wish list above, and the intent of #1360 ✅

While on this topic, what about the co-operations to pull items out of containers, perhaps called "uncons" and "unsnoc", F[A] => (F[A], A)?

I see we have an implementation for Chain, but no abstraction for it afaict.

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While on this topic, what about the co-operations to pull items out of containers

You can use Comonad#extract to e.g. get the head of a NonEmptyList. But off the top of my head I'm not aware of any operations for getting the tail, or the (init, last).

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Also, perhaps as a work-around, there's Foldable#toIterable that can provide access to head and tail, but the latter will be just Iterable of course, not F[_] anymore.

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• When I tried rewriting code written in terms of List to M: Alternative, I got stuck where the lists (used as stacks) get destructured to remove the head. Naturally Foldable#toIterable doesn't express the efficient destructure of List/Chain -like containers.

• I do think the complimentary co- operations Uncons and Unsnoc are valuable in some contexts, but should I create a separate issue as its drifted from the title?

• What would be the correct type-class for destructuring duals to prependK & appendK?

  Comonad seems too specific. Would a CoSemigroupK define some splitK dual to combineK? And CoNonEmptyAlternative define a unprependK and unappendK (perhaps with aliases uncons and unsnoc)?

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