Passing around procs between different objects

I'm trying to make the following work but I'm obviously missing something:

class Person
  def fetch
    puts 'Fetch it boy!'
  end

  def action(data)
    data.call
  end
end

class Animal
  def play
    Person.new.action(proc { fetch })
  end
end

Animal.new.play  # undefined local variable or method `fetch' for #<Animal:0x0000000001b172d0>

My goal is to get Person execute a method Animal has chosen. I've also tried using instance_exec but to no avail.

Now I know I can use eval and just pass the method name as a string but I'd like to avoid that. Any ideas?

Thanks in advance

Disclaimer (of sorts):
Please don't propose code restructuring or rewrites. The purpose of the question is to better understand how blocks behave when passed around between objects.

Solution

You are looking for:

instance_eval(&data)

object.instance_eval evaluates block, but replaces self within that block (which would normally be self of the context block was created in) with object:

whoami = proc { self }

whoami.call => main
1.instance_eval(&whoami) => 1

Note however, that instance_eval also passes an object as an argument to the block, which might be problematic if you want to pass a lambda:

whoami = lambda { self }
1.instance_eval(&whoami) #=> ArgumentError (wrong number of arguments (given 1, expected 0))

There is another similar method: instance_exec. Not only it does not pass self as an argument:

whoami = lambda { self }
1.instance_exec(&whoami) #=> 1

but it additionally allows you to pass other arguments:

add_number = lambda { |number| self + number }
1.instance_exec(3, &add_number) #=> 4

Naturally, both methods need to be used with extra caution and very sparingly - they are nice when you want to declare class-wide hooks in a declarative manner to be executed on each instance. They should not be used as a mean of interaction between two objects, unless you really know what you are ding and can justify it does not validate encapsulation.