From: dblack@...
Date: 2002-11-20T08:22:39+09:00
Subject: Re: Symbol usage help

Hi --

On Wed, 20 Nov 2002, ahoward wrote:

> On Wed, 20 Nov 2002 dblack@candle.superlink.net wrote:
>
> D> Actually this:
> D>
> D>   Class === obj.type
> D>
> D> is always true, because it's testing to see whether obj.type is a
> D> Class.
> D>
> D> Maybe you could do:
> D>
> D>   if Class === const_get(s)
> D>     obj = s.new
> D>   end
>
> it seems to be a valid test for me?
>
>   irb(main):002:0>  Class === (eval s).type
>   NameError: (eval):1:in `irb_binding': undefined local variable or method
>   `foobar' for #<Object:0x40190ce0> from (irb):2
> 	  from (irb):2

Well, admittedly Class === obj.type is only true if obj is a Ruby
object :-)  In your example, the eval raises a NameError, so the
=== test is never even performed.

>   irb(main):003:0> s = 'String'
>   "String"
>   irb(main):004:0>  Class === (eval s).type
>   true

The #type method always returns a Class object, and this:

   Class === SomeClass

is always true.  So

   Class === some_object.type

will always be true.

In the example we're discussing, the syntax sort of chases its own
tail, in the sense that we've got a String "String", eval'ing which
yields the Class object String -- which, of course, is the class of
the thing we just eval'd....  See if this little battery of tests
helps clarify it:

  irb(main):001:0> s = 'String'
  "String"
  irb(main):002:0> s.type
  String
  irb(main):003:0> eval(s)
  String
  irb(main):004:0> eval(s).type
  Class
  irb(main):005:0> Class === s
  false
  irb(main):006:0> Class === s.type
  true
  irb(main):007:0> Class === eval(s)
  true
  irb(main):008:0> Class === eval(s).type
  true
  irb(main):009:0> s.type == eval(s)
  true

Note the one false result: s itself is a String, not a Class.  But the
class of s is a Class, and the class of String (namely, Class) is also
a Class.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav