From: Eleanor McHugh <eleanor@...>
Date: 2009-05-06T19:54:29+09:00
Subject: Re: '=||'

On 6 May 2009, at 02:31, Rick DeNatale wrote:
> On Tue, May 5, 2009 at 8:45 PM, Eleanor McHugh
> <eleanor@games-with-brains.com> wrote:
>> On 6 May 2009, at 00:09, 7stud -- wrote:
>>> The statement:
>>>
>>> x ||= val
>>>
>>> is actually equivalent to:
>>>
>>> x = val unless x
>>
>>
>> It seems you've misunderstood what happens under the hood when using
>> augmented assignment with tables as '||=' then becomes syntactic  
>> sugar for
>> 'x[] = x[] || some_other_value' and the assignment is performed via  
>> '[]='
>> rather than '='. '[]=' will not create a key if it believes it  
>> already
>> exists and this is the cause of the behaviour you're seeing.
>
> No, if x is truthy then
>
>  x ||= expression
>
> will NOT do any assignment.
>
> The real equivalent to x ||= y
>
> is
>
> x || x = y
>
> The assignment is short-circuited.
>
> For the proof see:
>
> http://talklikeaduck.denhaven2.com/2008/04/26/x-y-redux

Yes, for assignment that's the case. But 'x[n] ||= y' isn't an  
instance of assignment in that case as can easily be demonstrated:

class Test
   def method_missing symbol, *args
     puts "calling method #{symbol}"
   end
end

t = Test.new
t[:a] ||= 17

output:	calling method []
		calling method []=

Notice how even though method_missing returns a value and is thus  
'true' the sequence still attempts all parts of the expression, and

t[:a] = t[:a] || 17

output:	calling method []
		calling method []=
		=> 17

confirms that no short-circuited evaluation occurs.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-brains.net
----
raise ArgumentError unless @reality.responds_to? :reason