[SOLVED] Why does Object::try work if it's sent to a nil object?

Why does Object::try work if it's sent to a nil object?

If you try to call a method on a nil object in Ruby, a NoMethodError exception arises with the message:

"undefined method ‘...’ for nil:NilClass"

However, there is a try method in Rails which just return nil if it's sent to a nil object:

require 'rubygems'
require 'active_support/all'

nil.try(:nonexisting_method) # no NoMethodError exception anymore

So how does try work internally in order to prevent that exception?

Solution

ActiveSupport 4.0.0 defines two try methods: one is for Object instances:

class Object
  def try(*a, &b)
    if a.empty? && block_given?
      yield self
    else
      public_send(*a, &b) if respond_to?(a.first)
    end
  end
end

the other is for NilClass instances (nil objects):

class NilClass
  def try(*args)
    nil
  end
end

Now, suppose we have an Object instance (excluding nil, which actually inherits from Object, like everything else in Ruby), defining a method which returns nil:

class Test
  def returns_nil
    nil
  end
end

So, running Test.new.try(:returns_nil) or Test.new.not_existing_method, Object#try gets called, which checks if a public method exists (the respond_to?); if it does it calls the method (the public_send), otherwise it returns nil (there aren't other lines).

If we call another try on of these returning nil methods:

Test.new.try(:returns_nil).try(:any_other_method)
Test.new.try(:not_existing_method).try(:any_other_method)

we call NilClass#try, which is nil#try, which simply ignores everything and returns nil. So any other try is called on a nil instance and returns nil.