From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2003-05-07T07:52:45+09:00
Subject: Re: Testing for a class existence

In article <20030506213500.GA49605@uk.tiscali.com>,
Brian Candler  <B.Candler@pobox.com> wrote:
>On Wed, May 07, 2003 at 06:23:06AM +0900, Gennady wrote:
>> In both cases Ruby parses Aaa::Bbb::Ccc before calling a method and is
>expected to complain.
>> So, is "defined?" an operator then? I am too lazy today to look into
>the source.
>
>You can always look in the Pickaxe :-) Yes, it's an operator:
>
>   Ruby supports all the standard boolean operators and introduces the  
>   new operator defined?.
>..
>   The defined? operator returns nil if its argument (which can be an
>   arbitrary expression) is not defined, otherwise it returns a
>   description of that argument. If the argument is yield, defined?  
>   returns the string `yield'' if a code block is associated with the   
>   current context.
>   
>   defined? 1             � "expression"
>   defined? dummy         � nil
>   defined? printf        � "method"
>   defined? String        � "constant"
>   defined? $&            � nil
>   defined? $_            � "global-variable"
>   defined? Math::PI      � "constant"
>   defined? ( c,d = 1,2 ) � "assignment"

This one ^^^^^^^^^^^^^^^^  doesn't seem to work under 1.8:

irb(main):002:0> defined?(c,d = 1,2)
SyntaxError: compile error
(irb):2: syntax error
defined?(c,d = 1,2)


>   defined? 42.abs        � "method"
>


It's interesting, I always thought defined? returned true or false, but it 
returns a String (if the argument has been defined) or nil (if it hasn't).
....you learn something new everyday (or should anyway).

Phil