Kim Joar Bekkelund

Playing with instance_eval to create a syntax similar to routes in Rails 3

Routes in Rails 3 looks amazing compared to Rails 2. One of the great aspects in the revamped routes is the removed map. An example from Rails 2:

map.resources :assemblies do |assemblies|
  assemblies.resources :parts
end

In Rails 3 we would write this as:

resources :assemblies do
  resources :parts
end

So how can we get this syntax going for ourselves? Playing around in my new Ruby wrapper for the Instapaper API I wanted to go from:

InstaPush.connect "kjbekkelund@gmail.com" do |conn|
  conn.add "http://kimjoar.net"
  conn.add "http://bjorkoy.com"
end

to:

InstaPush.connect "kjbekkelund@gmail.com" do
  add "http://kimjoar.net"
  add "http://bjorkoy.com"
end

To solve this problem we can use some Ruby metaprogramming goodness, namely instance_eval. By changing from yielding to the block, to using instance eval on the block — to get the correct scope — we don’t longer need the conn. Here’s how my code ended up:

def self.connect(username, password = nil, &block)
  conn = new username, password
  
  if block_given?
    conn.instance_eval(&block)
  else
    conn
  end
end

Now all the methods defined in the conn instance will be available in the block. Such a simple trick, but very powerful. Yet another great day to be a Rubyist.

Edit (August 3. 2010):

Judofyr commented on this technique on Hacker News with a slight modification. Let’s change the code to:

def self.connect(username, password = nil, &block)
  conn = new username, password
  
  if block_given?
    if block.arity == 1
      block.call(conn)
    else
      conn.instance_eval(&block)
    end
  else
    conn
  end
end

Now we are able to do both

InstaPush.connect "mail@kimjoar.net" { authenticate } 

and

InstaPush.connect "mail@kimjoar.net" { |conn| conn.authenticate }