From: "David A. Black" <dblack@...>
Date: 2007-10-23T01:39:56+09:00
Subject: Re: Conflicts between using respond_to? and extending Ruby

Hi --

On Tue, 23 Oct 2007, Rick DeNatale wrote:

> On 10/22/07, David A. Black <dblack@rubypal.com> wrote:
>
>> What I've always called the soft/hard duck typing distinction isn't
>> quite the same as what you're calling the chicken/duck distinction,
>> though, in the sense that I rule is_a? and kind_of? completely out of
>> bounds as duck typing of any kind. Duck typing, as I understand it
>> from various writings by, and exchanges with, Dave Thomas, starts
>> where is_a? leaves off. respond_to? starts to enter the duck-typing
>> orbit, in the sense that at least it's focused on the object and its
>> capabilities, rather than its class/module ancestry (and that's the
>> most decisive line to cross). Since it still falls short of simply
>> asking the object to do something, I consider it a "soft" form of duck
>> typing. It's object-driven, but not atomic.
>
> One difference between kind_of? and respond_to? is that it's slightly
> more likely that a particular method or methods mean what the chicken
> typer expects when using kind_of? than respond_to?,  since kind_of? is
> testing an implementation relationship,  but that's by no means
> guaranteed.
>
> The notion of 'type' held by the user of an object is often more
> complex and or general than just being in a particular implementation
> hierarchy, or responding to a certain set of verbs at the lexical
> level.  So I don't see so much of a distinction between respond_to?
> and kind_of?

The distinction I see is proximity to the object. respond_to? is
asking the object about its capabilities, whereas kind_of? is asking
it about its ancestry. So respond_to? is a tighter fit with the object
as it is at a given point in runtime.


David

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