From: dblack@...
Date: 2006-12-24T03:55:08+09:00
Subject: Re: Help with a ruby idiom

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.


David

-- 
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