From: "David A. Black" <dblack@...>
Date: 2007-10-23T04:02:24+09:00
Subject: Re: Conflicts between using respond_to? and extending Ruby

Hi --

On Tue, 23 Oct 2007, ara.t.howard wrote:

>
> On Oct 22, 2007, at 11:26 AM, David A. Black wrote:
>
>> 
>>> case obj
>>>  when Array
>>>    ...
>>> 
>>> is safer and more accurate.  i think it really depends on the use case - i 
>>> personally use both.
>> 
>> It does indeed depend; they do totally different things.
>
> not entirely - they both allowing pre-testing to see if code is going to work 
> or explode...

Pre-testing is what you do, not what the method does :-)

>> I was
>> describing the differences, specifically in the context of duck typing
>> (or alternatives to duck typing): respond_to? gives you information
>> about an object at a given moment, and kind_of? tells you about its
>> ancestry.
>
> well sortof:
>
> OpenStruct.new.respond_to?(:foo)  #=> false
>
> OpenStruct.new.foo()              #=> nil

You're a hard task-master :-) OK, I'll try again:

respond_to?(:x) gives you information about an object at a given
moment -- namely, whether or not any of the classes or modules
currently in an object's method lookup-path has a method "x" defined
(including the object's singleton class).

kind_of?(m) tells you about an object's ancestry -- namely, whether or
not a class or module called m is among its ancestral classes and
modules.

And yes, I know you can do:

   s = "hi"
   def s.respond_to?(x)
     true
   end

and I think people have sometimes even advocated this for OpenStructs.
But the above is as watertight as I can get :-)

> as i said, i think it's 100% situational and you really can't make blanket 
> rules.

I think we're talking at cross purposes; I wasn't suggesting rules,
blanket or otherwise, just commenting on the difference(s) between
respond_to? and kind_of?


David

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