From: Yehuda Katz <wycats@...>
Date: 2009-06-07T09:19:53+09:00
Subject: [ruby-core:23738] Ducktyping interface

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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

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<div>Matz,</div><div><br></div>I&#39;ve been working on adding some specs t=
o RubySpec related to the ducktyping interface. I have observed the followi=
ng behavior and want to know whether or not it&#39;s intentional. Let&#39;s=
 use the #to_s coercion as an example:<div>

<br></div><div>1. If the object has a method #to_s, it&#39;s called.</div><=
div>2. If #to_s returns something that is not a String, a TypeError is rais=
ed.=A0</div><div><br></div><div>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 #meth=
od_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 witho=
ut defining #to_s, but instead defining a combination of #respond_to? and #=
method_missing that returns a String.</div>

<div><br></div><div>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?=A0</div><div><br>
</div>
<div>In other words, is the only &quot;correct&quot; way to enlist in Ruby =
coercion to define to_s explicitly, or is another correct way to define res=
pond_to? and method_missing to return an appropriate response?</div><div>

<br>-- <br>Yehuda Katz<br>Developer | Engine Yard<br>(ph) 718.877.1325<br>
</div>

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