gistfile1.rb

class Class
  attr_reader :conditional_methods
  
  def defined_when(name, condition, &method)
    @conditional_methods ||= {}
    @conditional_methods[name.to_sym] = condition
    
    define_method(name) do
      if self.instance_eval(&condition)
        self.instance_eval(&method)
      else
        raise NoMethodError.new("undefined method `#{name}' for #{self.inspect}:#{self.class}")
      end
    end
  end
end

class Object
  alias :defined_when_original_respond_to? :respond_to?
  def respond_to?(sym)
    defined_when_original_respond_to?(sym) and self.instance_eval(&self.class.conditional_methods[sym])
  end
end

class Array
  defined_when(:sum, lambda {
                 all? { |element|
                   element.is_a? Numeric
                 }
               }) do
    self.inject(0) {|a, b| a + b}
  end
end

p [1, 2, 3].respond_to? :wibble     # => false
p [1, 2, "WUT"].respond_to? :wibble # => false

p [1, 2, 3].respond_to? :sum        # => true
p [1, 2, "WUT"].respond_to? :sum    # => false

p [1, 2, 3].sum                     # => 6
p [1, 2, "WUT"].sum                 # => NoMethodError