From: Martin Jambon <martin_jambon@emailuser.net>

> Here is my problem:
> 
> # let obj =
> object
>   method a = ()
>   method b = ()
> end;;
>         val obj : < a : unit; b : unit > = <obj>
> 
> (* That is nice: *)
> # ([ obj ] :> < a : unit > list);;
> - : < a : unit > list = [<obj>]
> 
> (* But why doesn't it work with arrays? *)
> # ([| obj |] :> < a : unit > array);;
> Characters 1-10:
>   ([| obj |] :> < a : unit > array);;
>    ^^^^^^^^^
> This expression cannot be coerced to type < a : unit > array; it has type
>   < a : unit; b : unit > array
> but is here used with type < a : unit > array
> Only the first object type has a method b

Suppose that it worked.
Then you could have this scenario.
  let arr = [|obj|];;
  let arr' = (arr :> <a:unit> array);;
  arr'.(0) <- object method a = () end;;
  arr.(0)#b;;
  Segmentation fault.

Such subtyping is allowed in Java, but this is an unsafe part of the
type system, which requires time-consuming runtime checks.

> In practice I have this problem with a hash table of objects, and I
> expected it to work since it works fine with lists of the same
> type of objects...
> Is there any workaround?

If you don't need to add objects to this hash table, this is possible
with the following approach:

  class ['a,+'b] table tbl =
    object
      val tbl : ('a,'b) Hashtbl.t = tbl
      method find = Hashtbl.find tbl
      method find_all = Hashtbl.find_all tbl
      method mem = Hashtbl.mem tbl
    end
  (* class ['a, 'b] table :
    ('a, 'b) Hashtbl.t ->
    object
      val tbl : ('a, 'b) Hashtbl.t
      method find : 'a -> 'b
      method find_all : 'a -> 'b list
      method mem : 'a -> bool
    end *)
  let coerce tbl = (tbl : ('a,<a:int;b:int>) table :> ('a,<a:int>) table)

See that 'b appears only in covariant positions, allowing its
subtyping.

Jacques Garrigue