limitations of passing a block to class_eval

One shortcoming of define_method is that it does not allow you to
specify a method body that expects a block. If you need to dynamically
create a method that accepts a block, you will need to use the def
statement within class_eval. Here's a trivial example:

class Module
  def acts_as_thing(name, &block)
    self.class_eval {
      define_method(name, &block)

class Thing
  #using define_method does not allow us to specify a method body that
expects a block, because the block used in define method becomes the
method body itself, not a block passed to a method body.
  acts_as_thing :animals do
    puts "animal group"

  def initialize(name)
    @name = name

#To address the limitations of define_method, that is, to dynamically
create a method that accepts a block, we need to use the def statement
with class_eval.
Thing.class_eval {
  def spell_out_name
    yield @name

thing ="dog")
thing.animals # => animal group
thing.spell_out_name {|name| puts "The name is #{name}."} # => The
name is dog.

Here's my question. I am reading a book called The Ruby Programming
Language. It says the following:

"If the method you are creating is sufficiently dynamic, you may not
be able to pass a block to class_eval and will instead have to specify
the method definition as a string to be evaluated."

I'm not sure what they are talking about here. What does it mean by
"sufficiently dynamic"? In my example above, I proved that a method
defined within the context of class_eval can accept a block.

Why are you using define_method inside of class eval again? That set
aside I hope you are not learning Ruby on 1.8.7 because that would
defeat the purpose of learning Ruby, since 1.8.7 is pretty much dead
and 1.9.3 is where you should start, since hopefully no company will
ever ask you to do Ruby on 1.8.7 that said, ruby does in-fact support
blocks in define_method see:

class MyClass
  define_method(:my_method1) do |&block|
    if !block
      raise "No block given"
end do
  $stdout.puts "Hello World"