From: "David A. Black" <dblack@...>
Date: 2004-08-07T23:12:44+09:00
Subject: Re: const_defined? raises NameError ?

Hi --

On Sat, 7 Aug 2004, [ISO-8859-2] Marcin Miel�y�ski wrote:

> Hi,
> 
> Given a const how can I check whether the const exists
> without rescuing an exception thrown - if it isn't?
> (using const_set is not elegant since You have to supply a special value)
> 
> if const_defined?('SomeConst')
> 	...
> end

if defined?(SomeConst)

> My next qusetion is:
> Assuming there is a nested const:
> 
> module SomeModule
> 	Const="somevalue"
> end
> 
> the only way to access it by reflection is:
> 
> s='SomeModule::Const'
> mod,con=s.split(/::/).map{|e|e.strip}
> Object.const_get(mod).const_get(con)
> 
> wouldn't it be easier if the following worked avoiding any eval calls:
> 
> object.const_get('SomeModule::Const')

I've seen code floating around that does that.  It's not too hard to
implement (definitely no eval needed):

  def const_get_deep(str)
    str.split("::").inject(Object) {|x,y| x = x.const_get(y)}
  end

  class A
    class B
      C = 1
    end
  end

  p const_get_deep("A::B::C")   # 1

I guess it might be useful to have this in the core.  I don't know
whether it's been considered by Matz in the past.  


David

-- 
David A. Black
dblack@wobblini.net