redefining a single ruby method on a single instance with a lambda

In Ruby, is there a way to redefine a method of a particular instance of a class using a proc? For example:

class Foo
  def bar()
    return "hello"
  end
end

x = Foo.new
y = Foo.new

(Something like):

y.method(:bar) = lambda { return "goodbye" }

x.bar
y.bar

Producing:

hello
goodbye

Thanks.

From stackoverflow

• def define_singleton_method_by_proc(obj, name, block)
    metaclass = class << obj; self; end
    metaclass.send(:define_method, name, block)
  end
  p = proc { "foobar!" }
  define_singleton_method_by_proc(y, :bar, p)
  

  or, if you want to monkey-patch Object to make it easy

  class Object
    # note that this method is already defined in Ruby 1.9
    def define_singleton_method(name, callable = nil, &block)
      block ||= callable
      metaclass = class << self; self; end
      metaclass.send(:define_method, name, block)
    end
  end
  
  p = proc { "foobar!" }
  y.define_singleton_method(:bar, p)
  #or
  y.define_singleton_method(:bar) do
     "foobar!"
  end
  

  or, if you want to define your proc inline, this may be more readable

  class << y
    define_method(:bar, proc { "foobar!" })
  end
  

  or,

  class << y
    define_method(:bar) { "foobar!" }
  end
  

  this is the most readable, but probably doesn't fit your needs

  def y.bar
    "goodbye"
  end
  

  This question is highly related

• You can use the syntax class <<object to get an object's "singleton class" (that's a special parent class belonging only to that object) and define methods only for that instance. For example:

  str1 = "Hello"
  str2 = "Foo"
  
  class <<str1
    def to_spanish
      'Hola'
    end
  end
  

  Now if you do str1.to_spanish, it will return "Hola", but str2.to_spanish will give you a NoMethodFound exception.

  John Douthat : if he has an externally defined proc in a simple variable, it's difficult to get it into the new scope created by class<