From: "David A. Black" <dblack@...>
Date: 2010-08-19T06:33:27+09:00
Subject: Re: Check existence of object and it's property at the same time

On Thu, 19 Aug 2010, David A. Black wrote:

> Hi --
>
> On Thu, 19 Aug 2010, Alex Stahl wrote:
>
>> I've run into a similar situation before, and found that
>> short-circuiting in an 'if' statement is inconsistent.
>> 
>> For instance, this will short-circuit and prevent evaluation of
>> subsequent statements if the first one is false (which is desired
>> obviously):
>> 
>> unless creds.class.to_s =~ /Hash/ && creds.has_key?("user") && ...
>> 
>> 
>> But this won't:
>> 
>> unless creds.instance_of?(Hash) && creds.has_key?("user") && ...
>
> They both short-circuit. Note that the puts("here") statement never gets
> executed.
>
>>> creds = {}
> => {}
>>> unless creds.class.to_s =~ /Hash/ && creds.has_key?("user") && 
>>> puts("here"); 1; end
> => 1
>>> unless creds.instance_of?(Hash) && creds.has_key?("user") && puts("here"); 
>>> 1; end
> => 1

(P.S. I've made the second condition fail, rather than the first, but if
you make creds a non-hash it will short-circuit after the first one.)

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

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