From: m4dc4p <jgbailey@...>
Date: 2005-11-15T07:22:16+09:00
Subject: Re: removing a constant definition from an environment

Ryan,

This is a tricky problem. First, you can't call remove_const directly
as it's a private method on the Module class. In the simple case, that
means you have to call it from within the class or module from which
you want to remove a constant. Further, that call must be made from
within a *class* (as opposed to instance) method. Here is a simple
program which shows a class which can define and undefine a constant.
If you run it with ruby you should see the output in the comments


class Foo
  def self.define_constant
    class_eval "CONST_VAL = 1"
  end

  def self.undef_constant
    remove_const :CONST_VAL
  end
end

puts "CONST_VAL defined?: #{Foo.const_defined?(:CONST_VAL)}"
Foo.define_constant
puts "CONST_VAL defined now?: #{Foo.const_defined?(:CONST_VAL)}"
Foo.undef_constant
puts "CONST_VAL still defined?: #{Foo.const_defined?(:CONST_VAL)}"

# output:
#CONST_VAL defined?: false
#CONST_VAL defined now?: true
#CONST_VAL still defined?: false

There are trickier ways to do this with the "send" method but this be
enough should get you started.

Robert Evans wrote:
> Hi Ryan,
>
> Thanks for your help.
>
> I tried that, and it didn't work. Here is my irb transcript trying
> that. Is there another way to remove a constant? I thought I had seen
> one in the Pickaxe book, but maybe I was thinking of the module method.
>
> irb
> irb(main):001:0> FOO = "my symbol value"
> => "my symbol value"
> irb(main):002:0> remove_const(FOO)
> NoMethodError: undefined method `remove_const' for main:Object
>          from (irb):2
> irb(main):003:0> remove_const(:FOO)
> NoMethodError: undefined method `remove_const' for main:Object
>          from (irb):3
> irb(main):004:0> ??
>
>
> Any other ideas?
>
> Thanks,
> Bob Evans
>
>
> On Nov 14, 2005, at 1:44 PM, Ryan Leavengood wrote:
>
> > On 11/14/05, Robert Evans <robert.evans@acm.org> wrote:
> >>
> >> I expected the unit test to clean the environment, in lieu of that,
> >> how can I undefined const?
> >
> > ----------------------------------------------------
> > Module#remove_const
> >      remove_const(sym)   => obj
> > ----------------------------------------------------------------------
> > --
> >      Removes the definition of the given constant, returning that
> >      constant's value. Predefined classes and singleton objects
> > (such as
> >      _true_) cannot be removed.
> >
> > You pass it a symbol: remove_const(:VARIABLE)
> >
> > Ryan
> >