From: Mage <mage@...>
Date: 2006-02-03T23:11:34+09:00
Subject: Re: nil or NameError

Ross Bamford wrote:

>On Fri, 2006-02-03 at 21:58 +0900, Mage wrote:
>  
>
>>          Hello,
>>
>>(1..2).each { if 1 == 2 then a = 5 else puts a end }
>>nil
>>nil
>>
>>(1..2).each { if 1 == 2 then b = 5 else puts a end }
>>NameError: undefined local variable or method `a' for main:Object
>>        from (irb):8
>>        from (irb):8
>>
>>
>>Okay, I understand what happend, but I am wondering. Is this okay? Is 
>>there any philosophical reason behind it or it's just easier for the 
>>interpreter?
>>    
>>
>
>I think it's about the way Ruby decides whether 'a' is a method or a
>local variable. It uses a heuristic and looks for stuff like argument
>list and so forth, but in cases like this it comes down to whether Ruby
>has 'seen an assignment' to that symbol. Note that this doesn't mean
>*executed* an assignment, just seen (parsed? compiled?) one.
>
>This is a simplified example:
>
>puts a
># => -:2: undefined local variable or method `a' for main:Object
>
>if false then a = 5 end
>puts a
># => nil
>
>In the first case, because Ruby hasn't seen any previous reference to
>'a', the NameError is raised.
>
>In the second case, I guess Ruby creates a local slot for 'a' as it
>compiles the if false... expression, initialised to nil, and since that
>is never executed, the local stays nil (but *is* known as a local, Ruby
>'remembers' seeing it being assigned to, even though it was never
>executed).
>  
>
I thought that it can be purposive. I have written a buggy code, which 
would either throw an exception or generate a bad result in other 
languages. However it works perfectly in Ruby.

While I never want to write inperfect code, I had to notice that Ruby's 
scope and variable handling "fixed" my error.

          Mage