Question

I see ruby code that looks like the following. It seems to be some sort of idiom for creating configuration or settings but I don’t really understand it. Also, how would the Application.configure part of this code look?

MyApp::Application.configure do
  config.something = foo
  config.....
  config....
  .
  config....
end

Answer 1

First of all, that configuration way is not specific to Ruby ; it’s the applications (or libraries, gems) that choose to use it or not.

To explain you what does that code do, I’ll take your snippet as an example:

MyApp::Application.configure do
  config.something = foo
end

Here, you are calling MyApp::Application.configure method, with no parameter. After the call, you’re giving it a block.

You can think of blocks as a piece of code that you can use however you want.
They can be written in one single line or many:

{ puts 'hello' }
{ |param| puts param } # with passing it a param

# or 

do |param|
  puts param
end

(remember my_array.each do ... end? It’s a block you pass it. 😉 )

Now, that block would be called inside the configure method thanks to yield.

yield uses (or executes) the instructions of the block that has been passed to the method.

Example: Let’s define a method with a yield inside of it:

def hello
  puts "Hello #{yield}"
end

If you call this method, you’d get a 'hello': no block given (yield) (LocalJumpError)'.

You need to pass it a block: hello { :Samy }.
The result would then be Hello Samy. As you can see, it simply used what was in the block passed to the method.

That’s exactly what’s happening in the Rails configuration code. You simply set config.something (config is a method) to some value, and that same config.something = foo is execute inside configure.

You can learn more about yield and blocks here, and on this great book.