From: Phrogz <gavin@...>
Date: 2007-04-04T22:45:07+09:00
Subject: Re: Everything is a object?

On Apr 4, 7:19 am, Jamal Soueidan <jkhaledsouei...@gmail.com> wrote:
> >> if a.nil?
> >>   p 'thanks';
> >> end
>
> > Indeed, if a has not been given a value yet, it cannot be "nil" ("nil"
> > *is* a value, and an object), so this will fail.
>
> That made me confused a little bit, How can nil be a class? and then a
> method in object base class (nil?)

  Slim:~ gkistner$ irb

  irb(main):001:0> a = nil
  => nil

  irb(main):002:0> a.class
  => NilClass

  irb(main):003:0> nil.class
  => NilClass

  irb(main):008:0> NilClass.instance_methods(false)
  => ["to_s", "|", "to_f", "to_a", "&", "^", "nil?", "to_i",
"inspect"]

Just like "true" is the literal way to express an instance of the
TrueClass and "[]" is the literal way to express an instance of the
Array class, "nil" is the literal way to express an instance of the
NilClass. That class has methods, like any other. One of those methods
is named "nil?". (Not that it makes much difference, but note that
"nil?" is a different name from "nil"; the question mark is part of
the name.)

I say that it doesn't make much difference because you can have method
names that are the same as a built-in literal. For example:

  irb(main):001:0> class Foo
  irb(main):002:1>   def true
  irb(main):003:2>     "maybe"
  irb(main):004:2>   end
  irb(main):005:1> end
  => nil

  irb(main):006:0> f = Foo.new
  => #<Foo:0x3847bc>

  irb(main):007:0> puts f.true
  maybe
  => nil

Heck, lots of built-in classes have a method named "[]", usually used
for accessing methods. Indeed, the Array class, which can be
instantiated by the literal [], is the primary example of it. So if
you write:

  irb(main):008:0> [][]
  ArgumentError: wrong number of arguments (0 for 1)
        from (irb):8:in `[]'
        from (irb):8

...what's going on is the first [] instantiate a new array instance,
and the second [] call the "[]" method on them...which requires a
single argument, hence the error.

  irb(main):009:0> [ 'a', 'b', 'c', 'd' ][ 2 ]
  => "c"