Hi,

why do I often get this error message? 

( type xxx is a class type and the other class implements this interface)

----------------------Error:
This expression has type
  < (* many methods *); .. >
but is here used with type xxx
Self type cannot be unified with a closed object type
---------------------
I try to get rid of them by adding random type declarations which I mostly 
don't understand :-), but that's not the solution.

What is the reason for that message?

cheers,
 Michael