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
    puts msg
  end
end

o = Object.new
define_say o, "hello world!"
o.say

This doesn't work because defining a singleton method via "def" is not a closure, so I get an exception that "msg" is an undefined variable or method.

What I would like to do is something like using the "define_method" method in the Module class, but as far as I can tell, this can only be used to define a method on a class... but I want a Singleton Method...

So, I would love to write it something like this:

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

Does anyone know how I can achieve this without having to create a method to store a Proc and then use the Proc within a singleton method? (basically, I want a clean, non-hacky way of doing this)

David Moles · Accepted Answer · 2015-05-27 17:45:57Z

15

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

edited May 27, 2015 at 17:45

David Moles

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answered Jun 22, 2011 at 22:18

asymmetric

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Comments

Andrew Grimm · Accepted Answer · 2011-06-22 23:49:54Z

8

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~!

edited Jun 22, 2011 at 23:49

Andrew Grimm

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answered Sep 25, 2008 at 21:15

Orion Edwards

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1 Comment

Andrew Grimm Over a year ago

I changed the link, because _why went AWOL since this answer was posted.

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