From: Dan Doel <djd15@...>
Date: 2003-08-12T10:42:46+09:00
Subject: Re: using case with classes

Michael Garriss wrote:

> Ok, this did not work like I expected (because I know too little about 
> Ruby):
>
> def test( obj )
>    case obj.class
>       when Array then puts "Array"
>       when Hash  then puts "Hash"
>       when Class then puts "Class"
>       else puts "ugh"
>    end
> end
>
> irb(main):009:0> test Array.new
> Class
> => nil
> irb(main):010:0> test Hash.new
> Class
> => nil
> irb(main):011:0> test Class.new
> Class
> => nil
>
> Hmmm.  Ok.  So I figured out that case/when uses === and not ==.  I'm 
> sure there is for a good reason but....why?
>
> irb(main):012:0> Array === Array
> => false
> irb(main):013:0> Class === Class
> => true
>
> So it turns out that === for Class checks if the rvalue is an instance 
> of the lvalue and === of Object is a just ==.  Is this right?  
> Obviously Array is not an instance of Array, but not so obviously, 
> Class IS an instance of Class.  This makes sense but my brain is 
> starting to hurt.  So I can write 'test' this way:
>
> def test( obj )
>    case obj  # <---- removed the '.class'
>       when Array then puts "Array"
>       when Hash  then puts "Hash"
>       when Class then puts "Class"
>       else puts "ugh"
>    end
> end
>
> irb(main):022:0> test Array.new
> Array
> => nil
> irb(main):023:0> test Hash.new
> Hash
> => nil
> irb(main):024:0> test Class.new
> Class
> => nil
>
> Werid.  Can someone help me figure out the === operator?  When should 
> I be redefining it?  In what cases should I be using it?
>
> Michael
>
Looking at the Ruby Class/Library reference at:

http://www.rubycentral.com/ref/

Which might not be updated for 1.8, but whatever...

=== is the case equality operator.  So you should redefine it to make 
your class act appropriately in case statements.

For example, the Range class redefines it so that  Range === obj tests 
to see if obj is contained in range, so you can do:

case 50
    when 1..10 then foo
    when 11..100 then bar
end

For Regexps, === is the same as =~.

Most of the time === is the same as ==, but Module redefines === to mean 
is rvalue an instance of lvalue, or
of a descendant of lvalue, which is where Class gets it from, I assume.

Hope this helps.

- Dan