Re: [Caml-list] Why List.map does not be implemented tail-recursively?

[Erkki Seppala <flux-caml@inside.org>]

Martin Jambon <martin.jambon@ens-lyon.org> writes:

> Unfortunately the practice obfuscates the source code. It makes it
> tricky to add or remove an operation in the chain, and overall it's an
> unnecessary cognitive burden for anyone reading the code.

While I do agree with this, perhaps this kind of trick could be used
where it is crucial to eliminate excess reversing:

type forwards = Forwards
type backwards = Backwards

let reverse (order1, order2, list) = (order2, order1, List.rev list)

let mk list = (Forwards, Backwards, list)

let forward_order = function
  | Forwards, _, list -> list
  | _, Forwards, list -> List.rev list

let foo () = 
  let xs = mk [1; 2; 3] in
  let xs = reverse xs in
  let xs = reverse xs in
  (* let xs = reverse xs in*)
  let (Forwards, _, xs) = xs in xs
  (* forward_order xs*)
  (* or: let (Forwards, _, xs) = xs in xs for compile-time assertion *)
 
Or is it excessive?-)

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