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ChatGPT? Can you write a simple example for me to work and build off of?

Make it Federated using Activitypub.:

I'd be happy to provide an example of a simple web application built using Phoenix, Elixir, and ActivityPub for federation, but keep in mind this example will not be a full feature rich Mastodon app, it will rather serve as a starting point for building your own application.

Here is a rough outline of what such an application might look like:

1. Create a new Phoenix project

mix phx.new simple_app

1. Install activitypub_ex package, which is an Elixir library for working with the ActivityPub protocol.
2. ***https://hex.pm/packages/activity_pub is actual package *** Chatgpt made an error ***

cd simple_app
mix add activitypub_ex

1. Define a schema for a Post model, this will be the main model for the application, storing the text of a post, the user who created it, and any other information you might want to include.

defmodule SimpleApp.Post do
  use Ecto.Schema

  schema "posts" do
    field :text, :string
    belongs_to :user, SimpleApp.User

    timestamps()
  end

  @impl true
  def to_activitypub(post) do
    %{
      "@context" => "<https://www.w3.org/ns/activitystreams>",
      "id" => "<https://example.com/posts/#{post.id}>",
      "type" => "Create",
      "actor" => post.user.to_activitypub,
      "object" => %{
        "id" => "<https://example.com/posts/#{post.id}>",
        "type" => "Note",
        "content" => post.text
      }
    }
  end
end

1. Implement a controller to handle the create action for the post

defmodule SimpleApp.PostController do
  use SimpleApp.Web, :controller

  def create(conn, %{"post" => post_params}) do
    post = %Post{text: post_params["text"], user: current_user(conn)}

    case SimpleApp.Repo.insert(post) do
      {:ok, post} ->
        # Broadcasts the post to followers
        SimpleApp.ActivityPub.broadcast(post)
        json(conn, %{post: post})
      {:error, changeset} ->
        json(conn, %{error: "Post could not be created"})
    end
  end
end

1. Implement the Federation part, this could be achieved by:
   1. Discovering the other users' server on the network and exchange public keys.
   2. Storing the discovered servers and their public key.
   3. When a post is created, query the followers to get the delivery urls and send the post object using the stored public key.

This is just a rough idea of what an application built using Phoenix, Elixir, and ActivityPub might look like. There are many more considerations, such as security, scalability, and user management that would need to be implemented to create a production-ready application, but this example should give you an idea of how you might go about building one.

As I mentioned before, creating Mastodon will be a non trivial task, a lot of features and considerations that need to be taken care of, this is just a starting point for you to work with.