Section 7.9.2 of the manual gives this example:<div><div><a href="http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc76">http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc76</a></div></div><div>

<br></div><div><div># module N : sig</div><div>     type t = private int</div><div>     val of_int: int -&gt; t</div><div>     val to_int: t -&gt; int</div><div>   end</div><div>  = struct</div><div>       type t = int</div>

<div>       let of_int n = assert (n &gt;= 0); n</div><div>       let to_int n = n</div><div>    end;;</div><div><br></div><div>module N :  sig type t = private int val of_int : int -&gt; t val to_int : t -&gt; int end</div>

</div><div><br></div><div>Deep coercion to a list of integers works as expected:</div><div><br></div><div><div><div># let l = List.map N.of_int [1;2;3];;</div><div>val l : N.t list = [1; 2; 3]</div></div><div><br></div><div>

# (l :&gt; int list);;</div><div>- : int list = [1; 2; 3]</div></div><div><br></div><div>But for arrays it doesn&#39;t work:</div><div><br></div><div><div># let a = Array.of_list l;;</div><div>val a : N.t array = [|1; 2; 3|]</div>

<div><br></div><div># (a :&gt; int array);;</div><div>Error: Type N.t array is not a subtype of int array</div></div><div><br></div><div>Is this because the array type does not have a variance annotation? If so, why doesn&#39;t it?</div>

<div><br></div><div>I can get around this by mapping over the elements:</div><div><br></div><div># Array.map (fun (x : N.t) -&gt; (x :&gt; int)) a;;</div><div><br></div><div>But am I right that coercions have no run-time cost, as where the mapping will?</div>

<div><br></div>