How to hide type internals

["Gerd" <GerdM.O@t-online.de>]

Hi,

I would like to write a package that exports functions using a special
type, without showing the details in the spec (even not in the private
part).
In Modula-2 one can declare opaque types (which mostly are pointers but
also can be any type of word size).

Like this:

package keys is
  type key;
  function GetKey return key;
  procedure FetchElement (k : key);
-- implementation of key not visible here
end keys;

package body keys
 ...
  type Data is record
                     ...
                    end record;
  type key is access Data;
...
end keys;