List_nonempty module

[rixed]

from https://discuss.ocaml.org/t/share-your-crazy-ocaml-code-snippet/8116/38 :

Sometimes, I think a feature-full List_nonempty module would be cool in batteries, or as a standalone library.
It should support most BatList operations (and only have tail rec. code).

I too think it would be useful.

[UnixJunkie]

Apparently there is one in Core_kernel: https://github.com/janestreet/core_kernel/tree/master/nonempty_list
We could import part of it and use the interface we want (though this implementation file would be using the MIT license then).

[UnixJunkie]

One way to start resolving this issue would be to agree on the interface, which I believe should be a subset of batList.
Once the interface is agreed upon, we could implement all the functions.

[rixed]

Indeed, I see no reason why we should not aim for a subset of BatList as far as API goes.
What seams less obvious to me is what should be the actual encoding. Should a non empty list be an item + a normal list (as in Core's implementation it seems), which has the advantage of offering a free conversion from/to lists ; or should a list be either the last item or an item + a non empty list, which seems marginally cleaner and makes the definition "safer" (it can then be public and allow users to pattern match directly).

[UnixJunkie]

If the data structure definition is like Core's one, the advantage I see is that the implementation for many functions is pretty much just calling the corresponding function from BatList. And since BatList has a lot of functions, that would be nice to simplify/speedup the implementation.

…

On Wed, Jan 19, 2022 at 10:53 PM Cedric Cellier ***@***.***> wrote: Indeed, I see no reason why we should not aim for a subset of BatList as far as API goes. What seams less obvious to me is what should be the actual encoding. Should a non empty list be an item + a normal list (as in Core's implementation it seems), which has the advantage of offering a free conversion from/to lists ; or should a list be either the last item or an item + a non empty list, which seems marginally cleaner and makes the definition "safer" (it can then be public and allow users to pattern match directly). — Reply to this email directly, view it on GitHub <#1057 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFUFADMVFPPJJXSCG3FIWTUW2647ANCNFSM5HBDTMJA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>. You are receiving this because you commented.Message ID: ***@***.*** com>

[rixed]

This makes a lot of sense.

[UnixJunkie]

I have started some work here: https://github.com/UnixJunkie/batteries-included/tree/non_empty_list
The idea is to uncomment little by little the bottom of the file, implementing things along the way;
keeping BatList's unit tests as much as possible