[LRUG] To debug the impossible bug

Murray Steele murray.steele at gmail.com
Wed Aug 3 05:44:34 PDT 2011 

Ah, well, probably not then :)  Although once ActiveRecord has got it's
grubby mitts on things who knows what might happen given that what you are
delegating to might be an association object which is never really nil
(proxy objects woo!) until you least expect it.

Speaking of AR, is it possible that @question is a scope call that hasn't
resolved yet?  e.g. it's not nil, because it's a scope proxy object, but
when you try to call a method on it, it resolves the scope to fetch the
object, which then turns out to be nil?  (I am clutching at straws here I
think).

On 3 August 2011 13:29, Viktor Tron <viktor.tron at gmail.com> wrote:

> Muz, explicit delegation gives you runtime error
>
> ruby-1.9.2-p180 :078 > class T; delegate :t, :to => :nilf; def nilf; end
> end
>  => nil
> ruby-1.9.2-p180 :079 > @q=T.new
>  => #<T:0x9887800>
> ruby-1.9.2-p180 :080 > case when @q then @q.t end
> RuntimeError: T#t delegated to nilf.t, but nilf is nil: #<T:0x9887800>
>
> although yes can think of a few convoluted ways to get the impossible bug
> for lols:
>
> ruby-1.9.2-p180 :066 > class T; def self.t; @t.t end end
>  => nil
> ruby-1.9.2-p180 :067 > @q=T
>  => T
> ruby-1.9.2-p180 :068 > case when @q then @q.t end
> NoMethodError: undefined method `t' for nil:NilClass
>
>
> ruby-1.9.2-p180 :069 > class T; def self.t; raise(NoMethodError,"undefined
> method `t' for nil:NilClass") end end
>  => nil
> ruby-1.9.2-p180 :070 > @q=T
>  => T
> ruby-1.9.2-p180 :071 > case when @q then @q.t end
> NoMethodError: undefined method `t' for nil:NilClass
>        from (irb):69:in `t'
>
>
> but seriously Tats got a point, look at the backtrace to make sure
> NoMethodError is coming from
> the case statement itself, not from inside the WhateverQuestionClass#**background
> method.
>
> hth vik
>
>
>
> On Wed, 03 Aug 2011 13:21:21 +0100, Murray Steele <murray.steele at gmail.com>
> wrote:
>
>  Hi Simon,
>>
>> I don't suppose that the background of a question is a delegate method?
>>  E.g. the question is not nil, so the case fires that branch, but the
>> object
>> that the question delegates background to *is* nil so you get the
>> NoMethodError.
>>
>> Muz
>>
>> On 3 August 2011 12:59, Tatsuya Ono <ononoma at gmail.com> wrote:
>>
>>  Hi Simon,
>>>
>>> It is a quite interesting problem. What I suspected were ...
>>>
>>> 1) Multithread issue
>>> Daniel pointed out already and you seemed confident that it is not the
>>> issue.
>>>
>>> 2) Error happened different place.
>>> e.g.
>>> class Question
>>>  def background
>>>    return @something.background # <= NoMethod error @something is nil
>>> here
>>>  end
>>> end
>>>
>>> I know that unless you are bit careless, you wouldn't miss the backtrace.
>>> However there might be a situation stack trace is not clear when the
>>> background method is implemented with some kinds of meta programming
>>> trick.
>>>
>>> I could not get any other idea apart from abobe for now. I would output
>>> log
>>> to get more information.
>>>
>>> e.g.
>>>
>>> def page_description
>>>  dump = @question.inspect
>>>  begin
>>>   case
>>>     when @question then @question.background            # exception
>>> raised
>>> here
>>>     <snip other cases>
>>>   end
>>>  rescue NoMethodError=>e
>>>    logger.info "Gotcha!!!!"
>>>    logger.info "BEFORE: #{dump}
>>>    logger.info "AFTER: #{@question.inspect}"
>>>    logger.info "backtrace: #{e.backtrace}"
>>>    logger.info "exception: #{e.message}"
>>>    raise e
>>>  end
>>> end
>>>
>>> Maybe we have to wait another one week, though, I am interested in what
>>> it
>>> tells.
>>>
>>>
>>> Tatsuya
>>> twitter.com/ono
>>>
>>> On 3 August 2011 12:28, Jordi Noguera Leon <jordinoguera83 at gmail.com>**
>>> wrote:
>>>
>>>  I'd rather use:
>>>>
>>>> def some_other_method
>>>>  return foo.whatever if foo
>>>>  return bar.whatever if bar
>>>>  "some message"
>>>> end
>>>>
>>>>
>>>> On 3 August 2011 12:23, Viktor Tron <viktor.tron at gmail.com> wrote:
>>>>
>>>>  yes with no value, case becomes a concise way of writing if .. elsif ..
>>>>> elsif conditionals
>>>>> since simon's code is of this type, the bug he gets is a bug you expect
>>>>> with the
>>>>> case nil variant unless somewhere case is redefined to fall back to nil
>>>>> with no args
>>>>> Simon, is this not the 'case'? :)
>>>>>
>>>>> a little simplified irb for you:
>>>>>
>>>>> ruby-1.9.2-p180 :001 > case nil when nil then true else false end
>>>>>  => true
>>>>> ruby-1.9.2-p180 :002 > case when nil then true else false end
>>>>>  => false
>>>>>
>>>>>
>>>>>
>>>>> On Wed, 03 Aug 2011 12:15:38 +0100, Tom Stuart <tom at therye.org> wrote:
>>>>>
>>>>>
>>>>>  On 3 Aug 2011, at 12:12, Matthew Rudy Jacobs wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> On 3 August 2011 12:09, Tom Stuart <tom at therye.org> wrote:
>>>>>>>
>>>>>>> See page 141 of the pickaxe for this use of case.
>>>>>>>
>>>>>>> Cheers,
>>>>>>>
>>>>>>> Tom
>>>>>>>
>>>>>>> Please enlighten me, then.
>>>>>>> What's the usecase?
>>>>>>>
>>>>>>> Have never seen this done.
>>>>>>>
>>>>>>>
>>>>>> According to the Pickaxe '[this] form is fairly close to a series of
>>>>>> if
>>>>>> statements; it lets you list a series of conditions and execute a
>>>>>> statement
>>>>>> corresponding to the first one that's true."
>>>>>>
>>>>>> The code example there is:
>>>>>>
>>>>>> case
>>>>>> when song.name == "Misty"
>>>>>>  puts "Not again!"
>>>>>> when song.duration > 120
>>>>>>  puts "Too long!"
>>>>>> when Time.now.hour > 21
>>>>>>  puts "It's too late"
>>>>>> else
>>>>>>  song.play
>>>>>> end
>>>>>>
>>>>>> It's pretty ugly, but it's valid Ruby and has nothing to do with the
>>>>>> more usual use of case for comparison with === against multiple
>>>>>> values.
>>>>>>
>>>>>> Cheers,
>>>>>>
>>>>>> Tom
>>>>>>
>>>>>>
>>>>>>
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