Re: [Caml-list] Question about objects and method overriding

[Goswin von Brederlow <goswin-v-b@web.de>]

On Mon, Feb 03, 2014 at 12:24:51PM +0000, Tom Ridge wrote:
> The scenario might be: I don't have the ability to change the object
> implementation to use an instance variable (perhaps the object comes
> from a library I am using), or maybe I don't want to decide upfront
> when I create the object, which methods can be overridden in this way
> (i.e. I don't want to add explicit set_call methods for all functions
> that I might want to override).
> 
> 
> On 3 February 2014 10:35, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> > On Sun, Feb 02, 2014 at 11:55:02AM +0000, Tom Ridge wrote:
> >> Dear caml-list,
> >>
> >> With records, one can functionally update a field e.g. as
> >>
> >> { r with some_field=new_value }
> >>
> >> And new_value may, of course, be a function.
> >>
> >> With objects, is there similar functionality? e.g. can I write something like
> >>
> >> {{ myobj with method some_method=new_method }}
> >>
> >> ?
> >>
> >> Of course, I could copy the methods from myobj explicitly into a new
> >> object (and set some_method to new_method), but I might not know all
> >> the methods available on myobj, and even if I do this becomes
> >> textually extremely verbose.
> >>
> >> Of course, new_method cannot directly refer to self etc. Basically I
> >> am using objects in a similar way to records, and would like to use
> >> this functional record update feature.
> >>
> >> Thanks
> >>
> >> Tom
> >
> > On the source level you can change a method by inheriting the old
> > class and implementing the method again. But you seem to want to
> > change the method at runtime. Problem there is that all instances of a
> > class afaik have the same virtual table to dispatch methods. So
> > changing a methong in one instance would change it in all.
> >
> > But why not dispatch the method through a value of the insance using a
> > closure?
> >
> > class myobj = object
> >   val mutable call_fn = fun () -> ()
> >   method call = call_fn ()
> >   method set_call fn = call_fn <- fn
> > end
> >
> > MfG
> >         Goswin

class myobj = object
  inherit libobj as parent
  val mutable call_fn = fun () -> parent#call
  method call = call_fn ()
  method set_call fn = call_fn <- fn
end

You can always override the method of a given class.

But see other mail for the award deserving Obj.magic hack to change a
method on the fly.

MfG
	Goswin