Is it possible to define a Ruby singleton method using a block?

Question

So, I want to define a singleton method for an object, but I want to do it using a closure.

For example,

def define_say(obj, msg)
  def obj.say
    p         


        

Answer 1

Here's an answer which does what you're looking for

def define_say(obj, msg)
  # Get a handle to the singleton class of obj
  metaclass = class << obj; self; end 

  # add the method using define_method instead of def x.say so we can use a closure
  metaclass.send :define_method, :say do
    puts msg
  end
end

Usage (paste from IRB)

>> s = "my string"
=> "my string"
>> define_say(s, "I am S")
=> #<Proc:0xb6ed55b0@(irb):11>
>> s.say
I am S
=> nil

For more info (and a little library which makes it less messy) read this:

http://viewsourcecode.org/why/hacking/seeingMetaclassesClearly.html

As an aside, If you're a ruby programmer, and you HAVEN'T read that, go do it now~!

Answer 2

Object#define_singleton_method was added to ruby-1.9.2 by the way:

def define_say(obj, msg)
  obj.define_singleton_method(:say) do
    puts msg
  end
end