[ruby-core:23738] Ducktyping interface

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