From: Tim Hunter <TimHunter@...>
Date: 2008-03-30T09:06:24+09:00
Subject: Re: Extrange behaviour using "if XXX ..." and "XXX if ..."

I単aki Baz Castillo wrote:
> El Domingo, 30 de Marzo de 2008, Tim Hunter escribi坦:
> 
>> In case (b), there has not yet been an assignment before the reference
>> to a, so Ruby assumes that "a" is a method call, yet there is no
>> definition for a so you get the message.
> 
>   b)
>   a if a="QWEQWE"
>   NameError: undefined local variable or method `a' for main:Object
> 
> 
> But why the first "a" is interpreted before the "if" stament? Imagine this 
> example:
> 
>   defined?a
>   => nil
>   a="BLABLA" if 2 == 3
>   => nil
>   a
>   => nil
> 
> After this code "a" will remain "nil" (or the previous value it had), so:
>   a="BLABLA"
> hasn't been interpreted since the "if" stament failed.
> 
> Also note this other example:
>   
>   defined?kk
>   => nil
>   kk if nil
>   => nil
> 
> Why in this last case the code doesn't return:
>   "undefined local variable or method kk' "
> ?
> 
> I assume the reason: "kk" is not interpreted if the condition fails.
> In b) the first "a" is then interpreted AFTER the condition success, so in 
> that moment "a" has been already declared (into the condition), so it would 
> exist and not give an error.
> 
> Am I not right?
> 
> 
> 

You must distinguish between parsing and execution. Ruby parses your 
program before Ruby executes your program.

Ruby decides how to interpret the reference to a during parsing, before 
Ruby executes the if statements in your examples. Their success or 
failure has nothing to do with the decision. In this case

a if a = "qweqwe"

Remember parsing occurs from left to right. Ruby sees the reference to a 
  to the left of the assignment to a. When Ruby first encounters a 
reference to a it assumes that since it has not yet seen any assigment 
to a, then a must be a method name.


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