Re: new method dispatch rule (matz' proposal)

On 6/13/07, David Flanagan <[e-mail]> wrote:
> This is a late response to the very long thread that started back in
> January with this message:
>
> http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/9996
>
> As I understand it, the problem Matz is seeking to address is that
> private methods are inherited by and may be overridden by subclasses.
> This means that a subclass writer can inadvertently alter the behavior
> of its superclass simply by defining a method that has the same name as
> an internal helper method of the superclass.  The only way to defend
> against this is for subclass writers to be familiar with the internal
> implementation of the superclass.
>
> Matz's proposal was to resolve this problem by altering the method
> resolution algorithm.  If I understand correctly, the problems were:
>
> 1) It was kind of complicated
> 2) The method lookup algorithm depended on whether a method was called
> functionally without an object prefix or in OO form with an object
> prefix.  (That is called as foo() or o.foo())
> 3) There were backward compatibility issues
>
> Part of the discussion was about the question of whether to alter the
> semantics of private methods, or add a new "local" or "personal"
> visibility level for methods that were truely local to the defining
> class and could not be inherited or overridden.
>
> With all that as review, here are my thoughts on the matter.
>
> 1) It is reasonable to expect private methods (or local methods if we
> call them that) to be looked up and dispatched differently than
> non-private methods.
>
> 2) The problem, though, is that since Ruby is a dynamic language, the
> interpreter can't tell whether a method is private or not until it has
> looked it up.
>
> 3) Therefore, solutions like Matz's proposal depend on the method
> invocation syntax (functional style versus object-oriented style) to
> determine which method lookup algorithm is to be used.  This leads to
> the confusing and hard-to-justify situation in which the invocation
> "foo" might do something different than the invocation "self.foo".
>
> I propose, therefore, that some new syntax be introduced (avoiding
> backward incompatibility) either for making local method names obviously
> different from other method names.  Or that a  new method invocation
> syntax be introduced and that this new syntax be used when we want to
> invoke a local method defined in (or inherited by) the caller without
> considering any methods defined further down the class hierarchy.
>
> I don't grok parse.y well enough to know whether any syntax I would
> propose would actually result in an unambiguous grammar, but my
> suggestion is that local methods have an @ prefix just like instance
> variables do.  Then the interpreter knows from the name of the method
> that it is to be dispatched differently.
>
> In fact, I might actually go further than this and propose that local
> methods are not actually methods, but are syntactic sugar for lambdas.
>
> In Ruby 1.8, we can write code like this:
>
> class Test
>    def initialize(greeting)
>      @greeting = greeting
>      @greeter = lambda { |x| puts "#@greeting #{x}" }
>    end
>
>    def greet(x)
>      @greeter[x]
>    end
> end
>
> t = Test.new("hello")
> t.greet("world")
>
> In this code, the instance variable @greeter refers to a local function
> that is completely hidden from subclasses and cannot be altered.

I strongly disagree here. While I think it is good to protect private
methods against inadvertently overriding them, I consider it harmful
to completely hide them from subclasses. If I really want to change
something I should be able to do so (i.e. via reflection). In
languages with strict privacy concepts like Java it is a pain to
extend a class in ways other than intended by the author of that
class.

-- henon

>
> Perhaps Ruby 1.9 could take this idiom and add syntax sugar to make the
> definition and invocation of local functions more like the definition
> and invocation of regular methods.
>
>    David Flanagan
>
>