From: "David McClain" <dmcclain@azstarnet.com>

> Juan got me thinking about this problem... So here is a solution:
> 
> external rplacd : 'a list -> 'a list -> unit = "rplacd"
>
> -- and the external C code is
> 
> value rplacd(value cell, value item)
> {
>   Store_field(cell,1,item);
>   return Val_unit;
> }

While this is undocumented, there is a slightly simpler way to define
rplacd, wihout using C. This should be faster in most cases
(particularly if you inline it).

let rplacd (cell : 'a list) (item : 'a) =
  Obj.set_field (Obj.repr cell) 1 (Obj.repr item)

As for using the null pointer to have more efficient representations
in data-structures, this is theoretically possible (and I believe that
Xavier Leroy had an implementation with it), but this is not in the
current version of ocaml.

	Jacques

---------------------------------------------------------------------------
Jacques Garrigue      Kyoto University     garrigue at kurims.kyoto-u.ac.jp
		<A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>