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

[Tom Ridge <tom.j.ridge+list@googlemail.com>]

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
>
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