From: Rick DeNatale <rick.denatale@...>
Date: 2007-10-11T21:48:09+09:00
Subject: Re: will #in? be in ruby2? (was Re: Finding shared elements between to arrays.)

On 10/11/07, Robert Klemme <shortcutter@googlemail.com> wrote:


> While reading this thread another possible solution came to my mind:
> what about an operator like "def_once" or "def!" which will raise an
> exception if the method is defined for that class already?  Of course,
> we could cook our own using define_method but there are some
> restrictions so it's not exactly equivalent to def.
>
> Semantics would have to be carefully chosen.  At the moment it seems
> that "def_once" should raise only if a) the instance's class has an
> instance method of the same name already or b) there is a singleton
> method of the same name.  However, I am not sure how to deal with the
> situation when there is an instance method in the class and then a
> singleton method definition is attempted.

Jim Weirich uses something like this in rake.  He defines a method
which takes a block containing a method definition

class Module

  # Check for an existing method in the current class before extending.  IF
  # the method already exists, then a warning is printed and the extension is
  # not added.  Otherwise the block is yielded and any definitions in the
  # block will take effect.
  #
  # Usage:
  #
  #   class String
  #     rake_extension("xyz") do
  #       def xyz
  #         ...
  #       end
  #     end
  #   end
  #
  def rake_extension(method)
    if instance_methods.include?(method)
      $stderr.puts "WARNING: Possible conflict with Rake extension:
#{self}##{method} already exists"
    else
      yield
    end
  end
end

Note that it produces a warning rather than an exception.

Also I guess he figured that Module#rake_extension wouldn't already exist! <G>

It pays to read the classics!
-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/