Question

I am new to ruby and rails. Can someone please point out what is the below construct. I see the respond_to constructs in methods (def). So is it a method inside a method ? I mean I am trying to understand the language construct here.

I do understand its functionality, that is send the response in a given format.

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @posts }
    end

Answer 1

The language construct is a ruby block. But if you really want to understand respond_to here’s a blog post that provides a good overview

How does respond_to work?

The key thing to understand is that respond_to is a method attached to
your controllers superclass: ActionController, and we are passing in
as an argument something called a block:

…

inside the respond_to method, we end up with a Proc that takes one
argument. This argument has the .xml and .html methods called on it.
What do we pass into the Proc as an argument when we call it from
inside respond_to? We pass in an instance of the Responder class.

So we end up calling .html and .xml on an instance of the responder
class as it is passed into the block (that’s been converted to a Proc)
inside the respond_to method… Phew.