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

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

I'm not sure what version of Ruby this was added in (at least 1.8.7), but there seems to be an even simpler way of doing this:

str1 = "Hello"str2 = "Goodbye"def str1.to_spanish  "Hola"endputs str1 # => Helloputs str1.to_spanish # => Holaputs str2 # => Goodbyeputs str2.to_spanish # => Throws a NoMethodError

Learnt about this whilst reading the Ruby Koans (about_class_methods.rb lesson).I'm still not entirely sure what the purpose of this is since it seems a bit dangerous to me.

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'  endend

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