[ruby-core:23745] Re: Ducktyping interface

I would hope that it's a spec.

It's not coercion, but here's another area we rely on respond_to being
the way ruby determines if a method exists.

When using Marshal.dump you can add a _dump method to do the dumping
yourself.  But in some kind of lazy loading scenario it is convenient
to use this method to cause a full load, and to then call
Marshal.dump(self).  But that won't work because _dump is called again
- unless you add a respond_to and return false for :_dump once the
object is fully loaded.

I don't know another way of doing this easily.

And in general I think it is very helpful to be able to rely on ruby
calling respond_to.

- Stephen



On Sun, Jun 7, 2009 at 3:30 AM, Yehuda Katz<[e-mail]> wrote:
> I should add that further examination reveals that people who make proxy
> objects (Drb, Rails Association Proxy) expect that their respond_to? will
> work correctly. If implementations were allowed to do internal checks only,
> proxies would need to explicitly define all coercion methods, something they
> do not currently do.
> -- Yehuda
>
> On Sat, Jun 6, 2009 at 5:12 PM, Yehuda Katz <[e-mail]> wrote:
>>
>> Matz,
>> I've been working on adding some specs to RubySpec related to the
>> ducktyping interface. I have observed the following behavior and want to
>> know whether or not it's intentional. Let's use the #to_s coercion as an
>> example:
>> 1. If the object has a method #to_s, it's called.
>> 2. If #to_s returns something that is not a String, a TypeError is
>> raised.
>> In MRI, the mechanism for determining if an object has a method #to_s is
>> to call respond_to?. This means that even if the method does not exist,
>> #to_s will get called, and fall through to #method_missing, if the
>> #respond_to method returns true for :to_s. As a result, when using MRI, it
>> is possible to implement the duck-typing interface without defining #to_s,
>> but instead defining a combination of #respond_to? and #method_missing that
>> returns a String.
>> Is this intentional? Would it be ok for an alternative implementation to
>> look up the #to_s method using an internal check instead of calling the
>> user-defined #respond_to?, as MRI does?
>> In other words, is the only "correct" way to enlist in Ruby coercion to
>> define to_s explicitly, or is another correct way to define respond_to? and
>> method_missing to return an appropriate response?
>> --
>> Yehuda Katz
>> Developer | Engine Yard
>> (ph) 718.877.1325
>
>
>
> --
> Yehuda Katz
> Developer | Engine Yard
> (ph) 718.877.1325
>