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Bug? Constraints get ignored in methods
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Date: -- (:)
From: Martin Jambon <martin.jambon@e...>
Subject: Re: [Caml-list] Bug? Constraints get ignored in methods
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/