Re: [Caml-list] Function returning recursive lists

[Philippe Wang <mail@philippewang.info>]

Hi,

I have a (somehow silly) answer to your question:

let gen_make_circ n =
  Printf.printf "let make_circ = function [] -> []\n";
  for i = 1 to n do
    Printf.printf "| [";
    for j = i to n do
      Printf.printf "x%d;" j
    done;
    Printf.printf "] -> let rec res = ";
    for j = i to n do
      Printf.printf "x%d :: " j
    done;
    Printf.printf "res in res\n"
  done;
  Printf.printf "| _ -> failwith \"not implemented\"\n%!"

let _ = gen_make_circ 4;;

prints this :

let make_circ = function [] -> []
| [x1;x2;x3;x4;] -> let rec res = x1 :: x2 :: x3 :: x4 :: res in res
| [x2;x3;x4;] -> let rec res = x2 :: x3 :: x4 :: res in res
| [x3;x4;] -> let rec res = x3 :: x4 :: res in res
| [x4;] -> let rec res = x4 :: res in res
| _ -> failwith "not implemented"

And then can be used:

let _ = make_circ [23;45];;
- : int list =
[23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45;
 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23;
 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; 45; 23; ...]


Cheers,
Philippe Wang



On Tue, Dec 18, 2012 at 9:37 AM, Eric Jaeger <eric.jaeger@ssi.gouv.fr> wrote:
> Hi everyone,
>
>
>
> There are various discussions on recursive lists in the archive, yet I was
> wondering whether or not it was possible in pure OCaml to write a function
> returning non-constant recursive lists.
>
>
>
> For example, I would like to have a function “docycle:’a list->’a list” that
> takes a non recursive list and transforms it into a recursive list
> containing the same elements. That is, “docycle [1;2;3]” would return a list
> structurally equivalent to “let rec c=1::2::3::c in c”. So far, my various
> attempts (OCaml 3.12) have not been successful. Another good example is to
> have a List.map compatible with recursive lists.
>
>
>
> Please note that it is, in a way, a theoretical (and possibly naïve)
> question :
>
> -          I do not consider recursive lists as the perfect implementation
> for my problem
>
> -          I do not care about efficiency
>
> -          I do not want to use an ad hoc mutable/lazy list datatype (unless
> I’ve also a conversion function toward standard lists)
>
> -          I do not want to use Obj or other similar tricks
>
> It’s just that I’m curious whether or not what I’m trying to achieve is
> possible.
>
>
>
>   Regards, Eric
>
>



-- 
Philippe Wang
   mail@philippewang.info