From: Robert Klemme <shortcutter@...>
Date: 2006-12-24T19:00:06+09:00
Subject: Re: Help with a ruby idiom

On 23.12.2006 19:55, dblack@wobblini.net wrote:
> Hi --
> 
> On Sun, 24 Dec 2006, Robert Klemme wrote:
> 
>> On 23.12.2006 17:02, Tim Waters wrote:
>>> From the O'Reilly Cookbook there is code that keeps call functions in a
>>> hash as a subscriber/listener type pattern.  What I don't understand (or
>>> I don't understand how it works is the line:
>>> (@EventDispatcher_listeners[event] ||= []) << callback
>>
>> a ||= b
>>
>> is equivalent to
>>
>> a || (a = b)
>>
>> IOW
>>
>> a = b unless a
>>
>> IOW
>>
>> unless a
>>  a = b
>> end
>>
>> IOW
>>
>> "eval b and assign the result to a if a is nil or false".
> 
> It is indeed functionally equivalent to those, though all of them
> would blow up :-)  I believe the reason a ||= b doesn't blow up is
> that it's expanded to:
> 
>   a = a || b
> 
> and when Ruby sees "a =" it allocates a local variable called a -- so
> that means that the rhs will not raise an error, which it otherwise
> would.
> 
> This doesn't add anything to what the idiom does, but I think it's an
> interesting semantic detail.

Absolutely!  I had this slightly nagging feeling that I had the first 
variant wrong, and indeed

$ ruby -e 'a || (a=1); p a'
-e:1: undefined local variable or method `a' for main:Object (NameError)

Thanks for the correction!

	robert