From: Robert Klemme <shortcutter@...>
Date: 2010-06-07T18:24:26+09:00
Subject: Re: Processing (potentially) nil method without using nested if

2010/6/5 Brian Candler <b.candler@pobox.com>:
> Ben Vishny wrote:
>> Ideally I'd like to do:
>>
>> if parent.respond_to? "children" && parent.children.size != 0
>>    puts "Has Children"
>> end
>>
>> But I get an error if parent.children is undefined. Isn't there a way of
>> stopping if the first condition is false?
>
> Yes, but you're being bitten by the ambiguity of what you've written,
> which is resolved by precedence rules. I'm not sure exactly how your
> code was being parsed, but I guess something like
>
> if parent.respond_to?("children" && (parent.children.size != 0))
>
> The rule is, if in doubt, add your own parentheses.
>
> The following all work as you expect:
>
> if (parent.respond_to? "children") && (parent.children.size != 0)
>  puts "Has Children"
> end
>
> if parent.respond_to?("children") && parent.children.size != 0
>  puts "Has Children"
> end
>
> if parent.respond_to? "children" and parent.children.size != 0
>  puts "Has Children"
> end
>
> The last of these works because 'and' has very low precedence, but I'd
> say it's much better to be explicit with parentheses than clever with
> your knowledge of precedence rules.
>
> Incidentally, it's more idiomatic to use a symbol rather than a string
> for respond_to?
>
> if parent.respond_to?(:children) ...

It's even more idiomatic to invoke a method and deal with the
exception.  Checking with #respond_to? does not guarantee that the
method can actually be executed.

In this case you could do

puts "Has Children" unless parent.children.empty? rescue nil

Or for a more targeted catch:

begin
  puts "Has Children" unless parent.children.empty?
rescue NoMethodError
  # OK, no output
end

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/