Ruby class_eval vs instance_eval

ruby_meta.rb

# Two great articles allow to understand Ruby better
# http://yugui.jp/articles/846
# http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/

class A; end

A.instance_eval { define_method(:hoge) { "hoge" } }
A.class_eval    { define_method(:fuga) { "fuga" } }
A.instance_eval { def piyo ;  "piyo"; end }
A.class_eval    { def foo ;  "foo"; end }

p A.new.hoge #=> "hoge"
p A.new.fuga #=> "fuga"
p A.piyo     #=> "piyo" 
p A.new.foo  #=> "foo"

# Here 'define_method' can leverage closures and wider scope,  
# where 'def' can serve the opposite goal and provide better isolation.

singleton_class_example.rb

class X ; end

X.singleton_class.instance_eval do
  def y
    :y
  end  

  define_method :w do
    :w
  end
  
  define_singleton_method :z do
    :z
  end  
end

X.singleton_class.y
X.w
X.singleton_class.z