On 2011/08/27, at 19:53, Chris Yocum wrote:
> A friend of mine is giving a talk about "monkey patching" entitled
> "Monkey patching, subclassing, and accidental overriding"
> (http://aaroncrane.co.uk/talks/monkey_patching_subclassing/paper.html).
> I was wondering how Ocaml deals with this situation or if it is even a
> problem at all in Ocaml? I mocked up some code:
>
> class base =
> object
> method meth x =
> print_endline "base";
> print_endline (string_of_int x)
> end
>
> class deriv =
> object
> inherit base
> method meth x =
> print_endline "deriv";
> print_endline (string_of_int x)
> end
>
> which kind of(?) shows the problem in Ocaml. He suggests in his paper
> that using a Meta-Object Protocol is the way around this. What do you
> think?
You can require overriding to be explicit by activating warning 7:
$ ocaml -w +7
OCaml version 3.13.0+dev6 (2011-07-29)
# class c = object method x = 1 end;;
class c : object method x : int end
# class d = object inherit c method x = 2 end;;
Warning 7: the method x is overridden.
class d : object method x : int end
Note also that in ocaml a method only has one signature,
so the problems with API changes doesn't apply here:
you get an error if the type changed in the base class,
and a warning if the method name was changed in the base class.
However, in this respect I think that a "final" keyword is missing in ocaml.
Namely, you have no way to say that you don't want subclasses to modify
a method (because you want to be able to redefine it yourself, for instance).
But OCaml is not an object-only language, so if you want this kind of
guarantee you can use functors: specify exactly what the user should provide,
without allowing a posteriori overriding.
Jacques Garrigue
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