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

by 
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

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