In Ruby, how does one verify the identity of a Struct?

Question

I've been trying to make sense of the class of self in the present method that Ryan Bates used in his Presenters RailsCast (#287 Presenters from Scratch (pro) - RailsCasts). In the video, Ryan says, 'Self is the template object which has all the helper methods we want to access', but I wanted to know the class for this object. After reading a range of blog posts, SO threads, and Ruby docs, I'm starting to think that self is a kind of Struct, but I don't know how to confirm this notion.

My questions are: 1) In the present method below, is self a Struct?, and 2) How does one verify that something is a Struct?

module ApplicationHelper
  def present(object, klass = nil)
    klass ||= "#{object.class}Presenter".constantize
    presenter = klass.new(object, self)
    yield presenter if block_given?
    presenter
  end
end

I'm asking this because I don't have much experience working with Struct classes and when I stick binding.pry in the middle of the method above and try to get the name of the class for self, I end up with more questions.

• When I enter self.class, I get, #<Class:0x007fb64f696268> I wondered if getting Class here might indicate that I have a Struct, but I couldn't find any documentation that confirmed this
• When I enter self.class.class, I get Class

• When I enter self, I get an extensive object that starts with the lines of code of code listed below

  @ line 16 ApplicationHelper#present:
  
  14: def present(object, klass = nil)
  15:   klass ||= "#{object.class}Presenter".constantize
  16:   binding.pry
  17:   presenter = klass.new(object, self)
  18:   yield presenter if block_given?
  19: end
  
  [1] pry(#<#<Class:0x007fb64f696268>>)> self
  => #<#<Class:0x007fb64f696268>:0x007fb64f6948f0
  @_assigns={"marked_for_same_origin_verification"=>true},
  @_config={},
  @_controller=
    #<PostsController:0x007fb64f6762d8
     @_action_has_layout=true,
     @_action_name="show",
     @_config={},
     @_db_runtime=0,
     @_lookup_context=
      #<ActionView::LookupContext:0x007fb64f6760d0
        @cache=true,
        @details=
          {:locale=>[:en],
          :formats=>[:html],
          :variants=>[],
          :handlers=>[:raw, :erb, :html, :builder, :ruby]},
        @details_key=#<Concurrent::Map:0x007fb64f697938 entries=0 default_proc=nil>,
        @prefixes=["posts", "application"],
        @rendered_format=:html,
        @view_paths=
          #<ActionView::PathSet:0x007fb64f675fe0
  

This post was helpful in explaining how a Struct works, but doesn't explain how one can confirm that they have a Struct.

Initially, when I started dissecting the present method, I found this answer to be helpful. However, I was thrown off by the comment, saying that the "ModelPresenter is initialized by passing the model, and the ApplicationHelper class", since ApplicationHelper is a module.

Answer 1

Summary

Use is_a?(Struct)

Explanation

A struct is a constructor for an anonymous class:

struct_class = Struct.new(:foo)
# => #<Class:0x007fa7e006ea98>

You can check that an instance of the anonymous class is a struct like so:

inst = struct_class.new
inst.class.superclass
# => Struct

However Object#is_a? checks the parent class as well as superclasses:

inst.is_a?(Struct)
# => true

You can see the same behavior in the following arbitrary example:

# inherits from String
anon_class = Class.new(String) 

inst = anon_class.new
# => ""

inst.class == String
# => false

inst.is_a?(String)
# => true