From: James Edward Gray II <james@...>
Date: 2007-04-04T21:56:48+09:00
Subject: Re: Everything is a object?

On Apr 4, 2007, at 7:43 AM, Jamal Soueidan wrote:

> Eleanor McHugh wrote:
>> On 4 Apr 2007, at 11:02, Jamal Soueidan wrote:
>>> I have been reading about these various boolean returns, how Ruby  
>>> look
>>> at them, as you mentioned false and nil is the only objects which  
>>> are
>>> considered false.
>>>
>>> But I thought you did not need to define/declare any variables  
>>> before
>>> using it, but this has been misunderstood, you should somehow
>>> assign the
>>> variable to something before trying to use it, since Ruby "Still"
>>> don't
>>> know what object it is, right?
>>
>> The point is that you don't have to define a variable before you make
>> an assignment to it, much like in older variants of BASIC. Assignment
>> implicitly declares the variable to exist within the specified scope.
>> However until you make an assignment the interpreter is not aware of
>> the variable's existence (because logically it does not yet exist)
>> and so attempts to reference it will fail to produce a reference to
>> an object. An attempt will also be made to apply the token to the
>> current object as a method, and if this too fails then the
>> interpreter has no choice but to flag an error condition because
>> semantically your program is broken - it makes no sense.
>
> :D
>
> So until I assign something to it, the interpreter would know nothing
> about what object it is.

I think it's probably better to say that until you assign it, it  
doesn't exist.  But yes, you are on the right track here.

> In C# you can declare every variable what type it is like this:
> String mystring;
>
> I assume I could do that in Ruby also:
> @var = String.new
>
> But no need since Ruby would take care of this automatic when you  
> assign
> something to variables.

C# cares what type a variable is.  It uses that information for  
things like method lookup.  Ruby does not.  A variable can hold  
whatever you like:

str = String.new
# ...
str = 5  # Ruby doesn't care that it's not a String now

This is called "dynamic typing" and is one of the great features of  
Ruby.  It does take some getting use to though, if you are joining us  
from a statically typed language.

> When I write attributes in my code:
> attr_assoccer :var
>
> I assume > then < the interpreter knows only that @var is a object  
> (base
> class)?
>
> So before I can use @var.

Actually, the it doesn't exist until you define it rule only applies  
to local variables:

$ ruby -e 'p local_var'
-e:1: undefined local variable or method `local_var' for main:Object  
(NameError)
$ ruby -e 'p @inst_var'
nil
$ ruby -e 'p $global_var'
nil

However, the last two issue warnings when enabled (which I recommend):

$ ruby -we 'p @inst_var'
-e:1: warning: instance variable @inst_var not initialized
nil
$ ruby -we 'p $global_var'
-e:1: warning: global variable `$global_var' not initialized
nil

Hope that helps.

James Edward Gray II