Re: [Caml-list] does class polymorphism need to be so complicated?

[Benjamin Geer <ben@socialtools.net>]

Jacques Garrigue wrote:
> Here there may be a deeper misunderstanding about the ocaml type
> system: if a subclass does not add methods to its superclass, its type
> does not change.
> That is, I would expect all connections to have the same type, and as
> a result there is no need for considering the more general
> #connection.

I think this would place an undesirable restriction on driver authors. 
They may want to add additional methods to their implementation of 
#connection, for use by other classes in the driver, even if the user 
will never be able to call those methods.

In general, this approach allows for a complete separation between 
interface and implementation: the implementing class can always have 
more methods than the interface, if this makes the implementation more 
convenient to write.

Alternatively, you could use a virtual base class 'connection', and 
always downcast the implementing class before passing it to application 
code.  But this places an additional burden on the library author.

> OK, there's also another way to do it, without inheritance. I just
> tried not to be confusing.
> 
> class type printer = object
>   method virtual print : #printable -> unit
> end
> 
> class my_printer () = object (self : #printer)
>    method print obj = ...
> end
> 
> Looks a bit strange at first, but it does the work.

Is there a way to write a class that implements more than one interface? 
  I've tried the following, but it produces a syntax error:

class type virtual printer = object
    method virtual print : #printable -> unit
end ;;

class type virtual talker = object
    method virtual talk : #printable -> unit
end ;;

class my_printer_talker () = object (self : #printer; #talker)
     method print obj = (* ... *)
     method talk obj = (* ... *)
end ;;

Am I right in guessing that you have to use multiple inheritance to 
achieve this?

Ben

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