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)

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~!