Sinatra router implementation - not working

gistfile1.rb

# This implementation causes error:
# ArgumentError: wrong number of arguments (0 for 1..3)
# config.ru:10:in `instance_eval'
# config.ru:10:in `initialize'
# config.ru:26:in `new'
# config.ru:26:in `block in inner_app'
# /Users/ograbek/.rvm/gems/ruby-2.1.2@owning_rails/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
# /Users/ograbek/.rvm/gems/ruby-2.1.2@owning_rails/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'


class App

  attr_accessor :routes

  def initialize(&block)
    @routes = {}
    instance_eval &block
  end

  def call(env)
    if routes.has_key?(env['PATH_INFO'])
      [200, { "Content-Type" => 'text/html' }, [routes[env['PATH_INFO']].call]]   
    else
      [200, { "Content-Type" => 'text/html' }, ['Default response']]     
    end
  end

  def get(path, &block)
    @routes[path] = block
  end
end

run App.new do

  get '/hi' do
    'Owning!'
  end

end

If I change this lines:

run App.new do

  get '/hi' do
    'Owning!'
  end

end

into:

app = App.new do

  get '/hi' do
    'Owning!'
  end

end

run app

this implementation works. I understand that without this change instance_eval doesn't get block, but I don't know why. Is run method somehow special ?

Short answer: do is always attached the left-most method.

Here's a bit of code to explain.

a 1 do

end
# Same as
a(1) do

end

a b do

end
# Same as
a(b) do

end

a b c do

end
# Same as
a(b(c)) do

end

If you want to force c to take the block:

a b(c do

end)

However, the following works because the do operator takes precedence over =.
Meaning it will be evaluated first.
Like * would take precedence over + in 1 + 2 * 3.

x = b do

end

In your example

run App.new do

end
# Same as
run(App.new) do

end