Ranges and their operators

[isti115]

I have just recently started playing around with Containers (and OCaml in general) and have found a few improvements I would consider for the range functions and their operators. I have already submitted #480 to fix some comments around these areas, but have some more ideas that I'd like to get an approval for before developing them into pull requests. (They currently lack tests, documentation, etc.)

Stepped range for Seq

I have implemented a range function (actually, two versions) for Seq that supports setting the step size. I wonder if this would be something that could be considered for inclusion in the library, and if so, which version would be preferred?

• V1:

let range_with_step i j s =
  let rec aux i' () =
    if (i <= j && i' > j) || (i >= j && i' < j) then Seq.Nil
    else if i < j then Seq.Cons (i', aux (i' + s))
    else Seq.Cons (i', aux (i' - s))
  in
  aux i

• V2:

let range_with_step i j s =
  if i = j then CCSeq.singleton i
  else
    let direction = compare i j |> CCInt.sign in
    let rec aux i' () =
      if compare i' j |> CCInt.sign != direction then Seq.Nil
      else Seq.Cons (i', aux (i' - (direction * s)))
    in
    aux i

(Now that I think about it, it might even make sense to have stepped ranges for infinite sequences as well. 🤔)

Operator for stepped ranges

While using the previously mentioned functions I came up with a way to fake a "ternary" operator by accepting a tuple as its first argument. After implementing it for myself I have realized that there was precedent for such an operator in Batteries, implemented using the exact same trick, just with a different symbol. Sticking to their notation, it could be implemented as:

(* Example usage: (3, 5) --. 20 |> List.of_seq = [3; 8; 13; 18] *)
let ( --. ) (i, s) j = range_with_step i j s

After a while I have also noticed the existence of CCList.range_by, which could use a similar operator form as well.
In case we'd really like to cover everything, --.^ could be added too.

---

PS. I also have some ideas around refactoring existing code, but this has already became quite a long post, so I'll wait for feedback before adding more of my thoughts.

[c-cube]

Hi! :)

For Seq, the CCSeq module is honestly here to be somewhat minimal, the fun stuff is all in oseq. Refactoring can be welcome (especially since the lower bound for OCaml was raised to 4.08, some parts of the code weren't updated to benefit from that). I have no opinion on infix operators for stepped ranges honestly.