Goswin von Brederlow wrote: > Hi, > > I want to keep a linked list of structures that have a common subset > of functionality. I thought this would be a good use of ocaml objects.
It is not a good use of objects. You'll notice this pretty soon as you'll run into a variety of problems: - polymorphism - initialization - verbosity - performance All of these issues are inexistent if you use records instead of objects for the list structure (or just a classic list). You can still use objects as elements of the list, but the elements would have to share the base type, as you know. (continued below) > A base class with the common subset of functionality and methods to > link them. And then derived classes for the specific types. Most > simplified it looks like this: > > # class type base_type = object val mutable next : base_type option method > set_next : base_type option -> unit end;; > class type base_type = > object > val mutable next : base_type option > method set_next : base_type option -> unit > end > > # class base : base_type = object val mutable next = None method set_next n = > next <- n end;; > class base : base_type > > # class foo = object inherit base method foo = () end;; > class foo : > object > val mutable next : base_type option > method foo : unit > method set_next : base_type option -> unit > end > > # let a = new base in > let b = new foo in > a#set_next (Some (b :> base_type));; > - : unit = () > > # let a = new base in > let b = new foo in > a#set_next (Some b);; > ^ > Error: This expression has type foo but is here used with type base_type > The second object type has no method foo > > This last error isn't nice. I don't want to have to cast the objects > all the time. So I thought there must be a better way using > polymorphic methods with a constraint. But here is where everything > breaks down. First lets look at just the set_next method: > > # class type virtual vbase_type = object method virtual set_next : 'a. 'a > option -> unit constraint 'a = #vbase_type end;; > class type virtual vbase_type = > object method virtual set_next : 'a option -> unit end > > # class virtual vbase : vbase_type = object method virtual set_next : 'a. 'a > option -> unit constraint 'a = #vbase_type end;; > class virtual vbase : vbase_type > > # class base = object inherit vbase method set_next _ = () end;; > class base : object method set_next : 'a option -> unit end > > # let b = new base;; > val b : base = <obj> > > # b#set_next (Some 1);; > - : unit = () > > Huh? That should not work. 1 is not a superset of #vbase_type. The > constraint gets completly ignored by ocaml. Adding back the next gives > further problems: > > # class type virtual vbase_type = object val mutable next : #vbase_type > option method virtual set_next : 'a. 'a option -> unit constraint 'a = > #vbase_type end;; > class type virtual vbase_type = > object > val mutable next : #vbase_type option > method virtual set_next : 'a option -> unit > end > > # class virtual vbase : vbase_type = object val mutable next = None method > virtual set_next : 'a. 'a option -> unit constraint 'a = #vbase_type end;; > class virtual vbase : vbase_type > > # class base = object inherit vbase > method set_next n = next <- (n :> vbase_type option) end;; > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > Error: This method has type #vbase_type option -> unit > which is less general than 'a. 'a option -> unit > > Again I blame ocaml for dropping the constraint. Given the constraint > the type would be correct. > > > > So how do I have to specify the set_next method that any superset of > #base_type will be accepted as argument? Or is that a bug in ocaml and > my syntax is perfectly fine? I have no idea. It looks way too complicated. Use a classic list: class base = ... class derived = ... (* inherits base *) type obj = Base of base | Derived of derived let obj_list = [ Base (new base); Derived (new derived); ... ] let iter_base f l = List.iter (function Base x -> f x | Derived x -> f (x :> base)) l let iter_derived f l = List.iter (function Derived x -> f x | Base _ -> ()) l ... Martin -- http://mjambon.com/ _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
