What do you think about adding `back` function in Queue module?

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1

Hi.
While translating some tools written in Python into OCaml, I found that the OCaml standard library itself could become more improved in some way.
One example is the module Queue. It has such a simple and powerful implementation, but it lacks the functionality of back (just like c++'s std::vector::back). I think adding functions like the below could improve this module’s usability hugely.

let back q =
  match q.last with 
  | Nil -> raise Empty
  | Cons { content } -> content

let back_opt q =
  match q.last with 
  | Nil -> None
  | Cons { content } -> Some content

I wonder why this function is missing from stdlib in the first place, and I hope if I can make a contribution related to it.
Thanks.

2

Judging by your code snippet, I think you have confused the module Queue with Seq. For Seq, discussion is ongoing to add more functions to the stdlib (including “tail”), see https://github.com/ocaml/ocaml/pull/10583.

Cheers,
Nicolas

3

Thanks for your comment.

After I’ve read the Seq's tail implementation, I found I’ve confused the name of the tail function. back or last is more suitable for my original intention, similar to the c++'s std::vector::back which returns the last element of the container. (I’ve updated the title and the content)
The current Queue module implementation makes it trivial to add this feature, so I thought it would be nice to get better usability.

4

OCaml’s Queue module is intended as a (single-ended) queue (you write at one end and read at the other end), not a (double-ended) dequeue (you can read and write at both ends). The back function you propose is a dequeue function. This said, this function makes sense, and is trivial to add, so maybe we should consider adding it and other dequeue operations to the Queue module.

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5

Thanks for your comment.
If this proposal about adding double-ended queue features does make sense, and it is ok for me to contribute to it, then I want to make a pull request for this!

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