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copied!<p>It seems the only way to make such recursive list is by using <code>Obj</code> module.</p> <h2>Copy the list and make it recursive</h2> <pre><code>let cycle lst = match lst with | [] -> [] | _ -> let rec get_last_cell = function | [] -> assert false | _::[] as last -> last | _::tl -> (get_last_cell tl) in let new_lst = List.map (fun x -> x) lst in let last_cell = get_last_cell new_lst in Obj.set_field (Obj.repr last_cell) 1 (Obj.repr new_lst); new_lst </code></pre> <h2>Create recursive list and then insert new cons cells</h2> <pre><code>let cycle lst = match lst with | [] -> [] | hd::tl -> let rec loop cell lst = match lst with | [] -> () | hd::tl -> let new_cell = [hd] in let new_cell_obj = Obj.repr new_cell in let cell_obj = Obj.repr cell in Obj.set_field new_cell_obj 1 (Obj.field cell_obj 1); Obj.set_field cell_obj 1 new_cell_obj; loop new_cell tl in let rec cyc_lst = hd::cyc_lst in loop cyc_lst tl; cyc_lst </code></pre> <p>The idea is pretty straightforward:</p> <ol> <li>Create recursive list <code>cyc_lst</code> with only one element.</li> <li>Insert one or more new cons cells immediately before tail of <code>cyc_lst</code>.</li> </ol> <p>Example</p> <p><code>cycle [1;2]</code></p> <ol> <li><p>Create recursive list <code>cyc_lst</code>. It is represented in memory as a self-recursive cons cell</p> <pre> let rec cyc_lst = hd::cyc_lst .--------. | | | +---+-|-+ `->| 1 | * | +---+---+ </pre></li> <li><p>Create <code>new_cell</code> using 2 as the only element</p> <pre> let new_cell = [hd] cell new_cell .--------. | | | +---+-|-+ +---+---+ `->| 1 | * | | 2 | X | +---+---+ +---+---+ </pre></li> <li><p>Set <code>new_cell</code> tail pointer to first cell</p> <pre> Obj.set_field new_cell_obj 1 (Obj.field cell_obj 1) cell new_cell .--------.--------------. | | | | +---+-|-+ +---+-|-+ `->| 1 | * | | 2 | * | +---+---+ +---+---+ </pre></li> <li><p>Set <code>cell</code> tail pointer to <code>new_cell</code></p> <pre> Obj.set_field cell_obj 1 new_cell_obj cell new_cell .-----------------------. | | | +---+---+ +---+-|-+ `->| 1 | *------->| 2 | * | +---+---+ +---+---+ </pre></li> </ol> <p>I hope GC is ok with such list manipulations. Let me know if it is not.</p>

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