Introduce an Explore section (#310)

* Add explore page

* Move sidebar to a configurable component

* Fix homepage test

* Add images

* Store example notebooks in files and make explore notebooks linkable

* Fix tests

* Raise on invalid notebook slug

* Keep just the file contents in notebook info

* Move notebook lookup to Explore

* Exclude notebooks in progress

README.md

@@ -1,4 +1,4 @@
-<h1><img src="https://github.com/elixir-nx/livebook/raw/main/priv/static/logo-with-text.png" alt="Livebook" width="400"></h1>
+<h1><img src="https://github.com/elixir-nx/livebook/raw/main/priv/static/images/logo-with-text.png" alt="Livebook" width="400"></h1>
 
 Livebook is a web application for writing interactive and collaborative code notebooks. It features:
 

lib/livebook/notebook/explore.ex

@@ -0,0 +1,114 @@
+defmodule Livebook.Notebook.Explore.Utils do
+  @moduledoc false
+
+  @doc """
+  Defines a module attribute `attr` with notebook info.
+  """
+  defmacro defnotebook(attr, props) do
+    quote bind_quoted: [attr: attr, props: props] do
+      {path, notebook_info} = Livebook.Notebook.Explore.Utils.fetch_notebook!(attr, props)
+
+      @external_resource path
+
+      Module.put_attribute(__MODULE__, attr, notebook_info)
+    end
+  end
+
+  def fetch_notebook!(attr, props) do
+    name = Atom.to_string(attr)
+    path = Path.join([__DIR__, "explore", name <> ".livemd"])
+
+    markdown = File.read!(path)
+    # Parse the file to ensure no warnings and read the title.
+    # However, in the info we keep just the file contents to save on memory.
+    {notebook, []} = Livebook.LiveMarkdown.Import.notebook_from_markdown(markdown)
+
+    notebook_info = %{
+      slug: String.replace(name, "_", "-"),
+      livemd: markdown,
+      title: notebook.name,
+      description: Keyword.fetch!(props, :description),
+      image_url: Keyword.fetch!(props, :image_url)
+    }
+
+    {path, notebook_info}
+  end
+end
+
+defmodule Livebook.Notebook.Explore do
+  @moduledoc false
+
+  defmodule NotFoundError do
+    @moduledoc false
+
+    defexception [:slug, plug_status: 404]
+
+    def message(%{slug: slug}) do
+      "could not find an example notebook matching #{inspect(slug)}"
+    end
+  end
+
+  import Livebook.Notebook.Explore.Utils
+
+  defnotebook(:intro_to_livebook,
+    description: "Get to know Livebook, see how it works and explore its features.",
+    image_url: "/images/logo.png"
+  )
+
+  defnotebook(:intro_to_elixir,
+    description: "New to Elixir? Learn about the language and its core concepts.",
+    image_url: "/images/elixir.png"
+  )
+
+  defnotebook(:intro_to_nx,
+    description:
+      "Enter numerical Elixir, experience the power of multi-dimensional arrays of numbers.",
+    image_url: "/images/nx.png"
+  )
+
+  defnotebook(:intro_to_axon,
+    description: "Build Neural Networks in Elixir using a high-level, composable API.",
+    image_url: "/images/axon.png"
+  )
+
+  defnotebook(:intro_to_vega_lite,
+    description: "Learn how to quickly create numerous plots for your data.",
+    image_url: "/images/vega_lite.png"
+  )
+
+  @type notebook_info :: %{
+          slug: String.t(),
+          livemd: String.t(),
+          title: String.t(),
+          description: String.t(),
+          image_url: String.t()
+        }
+
+  @doc """
+  Returns a list of example notebooks with metadata.
+  """
+  @spec notebook_infos() :: list(notebook_info())
+  def notebook_infos() do
+    [
+      @intro_to_livebook
+      # @intro_to_elixir, @intro_to_nx, @intro_to_axon, @intro_to_vega_lite
+    ]
+  end
+
+  @doc """
+  Finds explore notebook by slug and returns the parsed data structure.
+  """
+  @spec notebook_by_slug!(String.t()) :: Livebook.Notebook.t()
+  def notebook_by_slug!(slug) do
+    notebook_infos()
+    |> Enum.find(&(&1.slug == slug))
+    |> case do
+      nil ->
+        raise NotFoundError, slug: slug
+
+      notebook_info ->
+        {notebook, []} = Livebook.LiveMarkdown.Import.notebook_from_markdown(notebook_info.livemd)
+        notebook
+    end
+  end
+end

lib/livebook/notebook/explore/intro_to_axon.livemd

@@ -0,0 +1,3 @@
+# Neural Networks with Axon
+
+TODO: content 🐈

lib/livebook/notebook/explore/intro_to_elixir.livemd

@@ -0,0 +1,3 @@
+# Introduction to Elixir
+
+TODO: content 🐈

lib/livebook/notebook/explore/intro_to_livebook.livemd

@@ -0,0 +1,213 @@
+# Welcome to Livebook
+
+## Introduction
+
+We are happy you decided to give Livebook a try, hopefully it empowers
+you to build great stuff! 🚀
+
+Livebook is a tool for crafting **interactive** and **collaborative** code notebooks.
+It is primarily meant as a tool for rapid prototyping - think of it as an IEx session
+combined with your editor.
+You can also use it for authoring shareable articles that people can easily
+run and play around with.
+Package authors can write notebooks as interactive tutorials
+and include them in their repository, so that users can easily download
+and run them locally.
+
+## Basic usage
+
+Each notebook consists of a number of cells, which serve as primary building blocks.
+There are **Markdown** cells (such as this one) that allow you to describe your work
+and **Elixir** cells where the magic takes place!
+
+To insert a new cell move your cursor between cells and click one of the revealed buttons. 👇
+
+```elixir
+# This is an Elixir cell - as the name suggests that's where the code goes.
+# To evaluate this cell, you can either press the "Evaluate" button above
+# or use `Ctrl + Enter` (or Cmd + Enter on a Mac)!
+
+message = "hey, grab yourself a cup of 🍵"
+```
+
+Subsequent cells have access to the bindings you've defined:
+
+```elixir
+String.replace(message, "🍵", "☕")
+```
+
+Note however that bindings are not global, so each cell *sees* only stuff that goes
+above itself. This approach helps to keep the notebook clean and predictable
+as you keep working on it!
+
+## Sections
+
+You can leverage so called **sections** to nicely group related cells together.
+Click on the "Book" icon in the sidebar to reveal a list of all sections.
+As you can see, this approach helps to easily jump around the notebook,
+especially once it grows.
+
+Let's make use of this section to see how output is captured!
+
+```elixir
+cats = ~w(😼 😹 😻 😺 😸 😽)
+
+for _ <- 1..3 do
+  cats
+  |> Enum.take_random(3)
+  |> Enum.join(" ")
+  |> IO.puts()
+end
+```
+
+## Notebook files
+
+By default notebooks are kept in memory, which is fine for interactive hacking,
+but oftentimes you will want to save your work for later. Fortunately, notebooks
+can be persisted by clicking on the "Disk" icon in the bottom-right corner
+and selecting the file location.
+
+Notebooks are stored in **live markdown** format, which is essentially the markdown you know,
+with just a few assumptions on how particular elements are represented. Thanks to this
+approach you can easily keep notebooks under version control and get readable diffs.
+You can also easily preview those files, reuse for blog posts, and even edit in a text editor.
+
+## Modules
+
+As we have seen, Elixir cells can be used for working on tiny snippets,
+but you may as well define a module!
+
+```elixir
+defmodule Utils do
+  @doc """
+  Generates a random binary id.
+  """
+  @spec random_id() :: binary()
+  def random_id() do
+    :crypto.strong_rand_bytes(20) |> Base.encode32(case: :lower)
+  end
+end
+```
+
+If you're surprised by the above output, keep in mind that
+every Elixir expression evaluates to some value and as so does module compilation!
+
+Having the module defined, let's take it for a spin.
+
+```elixir
+Utils.random_id()
+```
+
+## Imports
+
+You can import modules as normally to make the imported functions visible
+to all subsequent cells. Usually you want to keep `import`, `alias` and `require`
+in the first section, as part of the notebook setup.
+
+```elixir
+import IEx.Helpers
+```
+
+```elixir
+h(Enum.map())
+```
+
+```elixir
+# Sidenote: http://www.numbat.org.au/thenumbat
+i("I ❤️ Numbats")
+```
+
+## Runtimes
+
+Livebook has a concept of **runtime**, which in practice is an Elixir node responsible
+for evaluating your code.
+
+By default, a new Elixir node is started (similarly to starting `iex`),
+but you can also choose to run inside a Mix project (as you would with `iex -S mix`)
+or even manually attach to an existing distributed node!
+You can configure the runtime by clicking the "Runtime" icon on the sidebar.
+
+## Using packages
+
+Sometimes you need a dependency or two and notebooks are no exception to this.
+
+One way to work with packages is to create a Mix project and configure the notebook
+to run in its context (as pointed out above). This approach makes sense if you already have
+a Mix project that you are working on, especially because this makes all project's
+modules available as well.
+
+But there are cases when you just want to play around with a new package
+or quickly prototype some code that relies on such. Fortunately, Elixir v1.12+ ships with
+[`Mix.install/2`](https://hexdocs.pm/mix/Mix.html#install/2) that allows you to install
+dependencies into your Elixir runtime! This approach is especially useful when sharing notebooks
+because everyone will be able to get the same dependencies. Let's try this out:
+
+**Note:** compiling dependencies may use a reasonable amount of memory. If you are
+hosting Livebook, make sure you have enough memory allocated to the Livebook
+instance, otherwise the command below will fail.
+
+```elixir
+Mix.install([
+  {:jason, "~> 1.2"}
+])
+```
+
+```elixir
+%{elixir: "is awesome"}
+|> Jason.encode!()
+|> IO.puts()
+```
+
+It is a good idea to specify versions of the installed packages,
+so that the notebook is easily reproducible later on.
+
+Also keep in mind that `Mix.install/2` can be called only once
+per runtime, so if you need to modify the dependencies, you should
+go to the notebook runtime configuration and **reconnect** the current runtime.
+
+## Running tests
+
+It is also possible to run tests directly from your notebooks.
+The key is to disable `ExUnit`'s autorun feature and then explicitly
+run the test suite after all test cases have been defined:
+
+```elixir
+ExUnit.start(autorun: false)
+
+defmodule MyTest do
+  use ExUnit.Case, async: true
+
+  test "it works" do
+    assert true
+  end
+end
+
+ExUnit.run()
+```
+
+## Math
+
+Livebook uses $\\TeX$ syntax for math.
+It supports both inline math like $e^{\\pi i} + 1 = 0$, as well as display math:
+
+$$
+S(x) = \\frac{1}{1 + e^{-x}} = \\frac{e^{x}}{e^{x} + 1}
+$$
+
+You can explore all supported expressions [here](https://katex.org/docs/supported.html).
+
+## Stepping up your workflow
+
+Once you start using notebooks more, it's gonna be beneficial
+to optimise how you move around. Livebook leverages the concept of
+**navigation**/**insert** modes and offers many shortcuts for common operations.
+Make sure to check out the shortcuts by clicking the "Keyboard" icon in
+the sidebar or by typing `?`.
+
+## Final notes
+
+Livebook is an open source project, so feel free to look into
+[the repository](https://github.com/elixir-nx/livebook)
+to contribute, report bugs, suggest features or just skim over the codebase.
+
+Now go ahead and build something cool! 🚢

lib/livebook/notebook/explore/intro_to_nx.livemd

@@ -0,0 +1,3 @@
+# Introduction to Nx
+
+  TODO: content 🐈

lib/livebook/notebook/explore/intro_to_vega_lite.livemd

@@ -0,0 +1,3 @@
+# Plotting with VegaLite and Kino
+
+TODO: content 🐈

lib/livebook/notebook/welcome.ex

@@ -1,225 +0,0 @@
-defmodule Livebook.Notebook.Welcome do
-  livemd = ~s'''
-  # Welcome to Livebook
-
-  ## Introduction
-
-  We are happy you decided to give Livebook a try, hopefully it empowers
-  you to build great stuff! 🚀
-
-  Livebook is a tool for crafting **interactive** and **collaborative** code notebooks.
-  It is primarily meant as a tool for rapid prototyping - think of it as an IEx session
-  combined with your editor.
-  You can also use it for authoring shareable articles that people can easily
-  run and play around with.
-  Package authors can write notebooks as interactive tutorials
-  and include them in their repository, so that users can easily download
-  and run them locally.
-
-  ## Basic usage
-
-  Each notebook consists of a number of cells, which serve as primary building blocks.
-  There are **Markdown** cells (such as this one) that allow you to describe your work
-  and **Elixir** cells where the magic takes place!
-
-  To insert a new cell move your cursor between cells and click one of the revealed buttons. 👇
-
-  ```elixir
-  # This is an Elixir cell - as the name suggests that's where the code goes.
-  # To evaluate this cell, you can either press the "Evaluate" button above
-  # or use `Ctrl + Enter` (or Cmd + Enter on a Mac)!
-
-  message = "hey, grab yourself a cup of 🍵"
-  ```
-
-  Subsequent cells have access to the bindings you've defined:
-
-  ```elixir
-  String.replace(message, "🍵", "☕")
-  ```
-
-  Note however that bindings are not global, so each cell *sees* only stuff that goes
-  above itself. This approach helps to keep the notebook clean and predictable
-  as you keep working on it!
-
-  ## Sections
-
-  You can leverage so called **sections** to nicely group related cells together.
-  Click on the "Book" icon in the sidebar to reveal a list of all sections.
-  As you can see, this approach helps to easily jump around the notebook,
-  especially once it grows.
-
-  Let's make use of this section to see how output is captured!
-
-  ```elixir
-  cats = ~w(😼 😹 😻 😺 😸 😽)
-
-  for _ <- 1..3 do
-    cats
-    |> Enum.take_random(3)
-    |> Enum.join(" ")
-    |> IO.puts()
-  end
-  ```
-
-  ## Notebook files
-
-  By default notebooks are kept in memory, which is fine for interactive hacking,
-  but oftentimes you will want to save your work for later. Fortunately, notebooks
-  can be persisted by clicking on the "Disk" icon in the bottom-right corner
-  and selecting the file location.
-
-  Notebooks are stored in **live markdown** format, which is essentially the markdown you know,
-  with just a few assumptions on how particular elements are represented. Thanks to this
-  approach you can easily keep notebooks under version control and get readable diffs.
-  You can also easily preview those files, reuse for blog posts, and even edit in a text editor.
-
-  ## Modules
-
-  As we have seen, Elixir cells can be used for working on tiny snippets,
-  but you may as well define a module!
-
-  ```elixir
-  defmodule Utils do
-    @doc """
-    Generates a random binary id.
-    """
-    @spec random_id() :: binary()
-    def random_id() do
-      :crypto.strong_rand_bytes(20) |> Base.encode32(case: :lower)
-    end
-  end
-  ```
-
-  If you're surprised by the above output, keep in mind that
-  every Elixir expression evaluates to some value and as so does module compilation!
-
-  Having the module defined, let's take it for a spin.
-
-  ```elixir
-  Utils.random_id()
-  ```
-
-  ## Imports
-
-  You can import modules as normally to make the imported functions visible
-  to all subsequent cells. Usually you want to keep `import`, `alias` and `require`
-  in the first section, as part of the notebook setup.
-
-  ```elixir
-  import IEx.Helpers
-  ```
-
-  ```elixir
-  h(Enum.map())
-  ```
-
-  ```elixir
-  # Sidenote: http://www.numbat.org.au/thenumbat
-  i("I ❤️ Numbats")
-  ```
-
-  ## Runtimes
-
-  Livebook has a concept of **runtime**, which in practice is an Elixir node responsible
-  for evaluating your code.
-
-  By default, a new Elixir node is started (similarly to starting `iex`),
-  but you can also choose to run inside a Mix project (as you would with `iex -S mix`)
-  or even manually attach to an existing distributed node!
-  You can configure the runtime by clicking the "Runtime" icon on the sidebar.
-
-  ## Using packages
-
-  Sometimes you need a dependency or two and notebooks are no exception to this.
-
-  One way to work with packages is to create a Mix project and configure the notebook
-  to run in its context (as pointed out above). This approach makes sense if you already have
-  a Mix project that you are working on, especially because this makes all project's
-  modules available as well.
-
-  But there are cases when you just want to play around with a new package
-  or quickly prototype some code that relies on such. Fortunately, Elixir v1.12+ ships with
-  [`Mix.install/2`](https://hexdocs.pm/mix/Mix.html#install/2) that allows you to install
-  dependencies into your Elixir runtime! This approach is especially useful when sharing notebooks
-  because everyone will be able to get the same dependencies. Let's try this out:
-
-  **Note:** compiling dependencies may use a reasonable amount of memory. If you are
-  hosting Livebook, make sure you have enough memory allocated to the Livebook
-  instance, otherwise the command below will fail.
-
-  ```elixir
-  Mix.install([
-    {:jason, "~> 1.2"}
-  ])
-  ```
-
-  ```elixir
-  %{elixir: "is awesome"}
-  |> Jason.encode!()
-  |> IO.puts()
-  ```
-
-  It is a good idea to specify versions of the installed packages,
-  so that the notebook is easily reproducible later on.
-
-  Also keep in mind that `Mix.install/2` can be called only once
-  per runtime, so if you need to modify the dependencies, you should
-  go to the notebook runtime configuration and **reconnect** the current runtime.
-
-  ## Running tests
-
-  It is also possible to run tests directly from your notebooks.
-  The key is to disable `ExUnit`'s autorun feature and then explicitly
-  run the test suite after all test cases have been defined:
-
-  ```elixir
-  ExUnit.start(autorun: false)
-
-  defmodule MyTest do
-    use ExUnit.Case, async: true
-
-    test "it works" do
-      assert true
-    end
-  end
-
-  ExUnit.run()
-  ```
-
-  ## Math
-
-  Livebook uses $\TeX$ syntax for math. 
-  It supports both inline math like $e^{\\pi i} + 1 = 0$, as well as display math:
-
-  $$
-  S(x) = \\frac{1}{1 + e^{-x}} = \\frac{e^{x}}{e^{x} + 1}
-  $$
-
-  You can explore all supported expressions [here](https://katex.org/docs/supported.html).
-
-  ## Stepping up your workflow
-
-  Once you start using notebooks more, it's gonna be beneficial
-  to optimise how you move around. Livebook leverages the concept of
-  **navigation**/**insert** modes and offers many shortcuts for common operations.
-  Make sure to check out the shortcuts by clicking the "Keyboard" icon in
-  the sidebar or by typing `?`.
-
-  ## Final notes
-
-  Livebook is an open source project, so feel free to look into
-  [the repository](https://github.com/elixir-nx/livebook)
-  to contribute, report bugs, suggest features or just skim over the codebase.
-
-  Now go ahead and build something cool! 🚢
-  '''
-
-  {notebook, []} = Livebook.LiveMarkdown.Import.notebook_from_markdown(livemd)
-
-  @notebook notebook
-
-  def new() do
-    @notebook
-  end
-end

lib/livebook_web/live/explore_live.ex

@@ -0,0 +1,108 @@
+defmodule LivebookWeb.ExploreLive do
+  use LivebookWeb, :live_view
+
+  import LivebookWeb.UserHelpers
+  import LivebookWeb.SessionHelpers
+
+  alias Livebook.Notebook.Explore
+
+  @impl true
+  def mount(_params, %{"current_user_id" => current_user_id}, socket) do
+    if connected?(socket) do
+      Phoenix.PubSub.subscribe(Livebook.PubSub, "users:#{current_user_id}")
+    end
+
+    current_user = build_current_user(current_user_id, socket)
+
+    [lead_notebook_info | notebook_infos] = Explore.notebook_infos()
+
+    {:ok,
+     assign(socket,
+       current_user: current_user,
+       lead_notebook_info: lead_notebook_info,
+       notebook_infos: notebook_infos
+     )}
+  end
+
+  @impl true
+  def render(assigns) do
+    ~L"""
+    <div class="flex flex-grow h-full">
+      <%= live_component @socket, LivebookWeb.SidebarComponent,
+            id: :sidebar,
+            items: [
+              %{type: :logo},
+              %{type: :break},
+              %{type: :user, current_user: @current_user, path: Routes.explore_path(@socket, :user)}
+            ] %>
+      <div class="flex-grow px-6 py-8 overflow-y-auto">
+        <div class="max-w-screen-md w-full mx-auto px-4 pb-8 space-y-8">
+          <div>
+            <div class="relative">
+              <%= live_patch to: Routes.home_path(@socket, :page),
+                    class: "hidden md:block absolute top-[50%] left-[-12px] transform -translate-y-1/2 -translate-x-full" do %>
+                <%= remix_icon("arrow-left-line", class: "text-2xl align-middle") %>
+              <% end %>
+              <h1 class="text-3xl text-gray-800 font-semibold">
+                Explore
+              </h1>
+            </div>
+            <p class="mt-4 text-gray-700">
+              Check out a number of examples showcasing various parts of the Elixir ecosystem.
+              Click on any notebook you like and start playing around with it!
+            </p>
+          </div>
+          <div class="p-8 bg-gray-900 rounded-2xl flex space-x-4 shadow-xl">
+            <div class="self-end max-w-sm">
+              <h3 class="text-xl text-gray-50 font-semibold">
+                <%= @lead_notebook_info.title %>
+              </h3>
+              <p class="mt-2 text-sm text-gray-300">
+                <%= @lead_notebook_info.description %>
+              </p>
+              <div class="mt-4">
+                <%= live_patch "Let's go", to: Routes.explore_path(@socket, :notebook, @lead_notebook_info.slug),
+                      class: "button button-blue" %>
+              </div>
+            </div>
+            <div class="flex-grow hidden md:flex flex items-center justify-center">
+              <img src="<%= @lead_notebook_info.image_url %>" height="120" width="120" alt="livebook" />
+            </div>
+          </div>
+          <div class="grid grid-cols-1 md:grid-cols-2 gap-4">
+            <%= for {info, idx} <- Enum.with_index(@notebook_infos) do %>
+              <%= live_component @socket, LivebookWeb.NotebookCardComponent,
+                    id: "notebook-card-#{idx}",
+                    notebook_info: info %>
+            <% end %>
+          </div>
+        </div>
+      </div>
+    </div>
+
+    <%= if @live_action == :user do %>
+      <%= live_modal @socket, LivebookWeb.UserComponent,
+            id: :user_modal,
+            modal_class: "w-full max-w-sm",
+            user: @current_user,
+            return_to: Routes.explore_path(@socket, :page) %>
+    <% end %>
+    """
+  end
+
+  @impl true
+  def handle_params(%{"slug" => slug}, _url, socket) do
+    notebook = Explore.notebook_by_slug!(slug)
+    {:noreply, create_session(socket, notebook: notebook)}
+  end
+
+  def handle_params(_params, _url, socket), do: {:noreply, socket}
+
+  @impl true
+  def handle_info(
+        {:user_change, %{id: id} = user},
+        %{assigns: %{current_user: %{id: id}}} = socket
+      ) do
+    {:noreply, assign(socket, :current_user, user)}
+  end
+end

lib/livebook_web/live/home_live.ex

@@ -2,6 +2,7 @@ defmodule LivebookWeb.HomeLive do
   use LivebookWeb, :live_view
 
   import LivebookWeb.UserHelpers
+  import LivebookWeb.SessionHelpers
 
   alias Livebook.{SessionSupervisor, Session, LiveMarkdown, Notebook}
 
@@ -14,12 +15,14 @@ defmodule LivebookWeb.HomeLive do
 
     current_user = build_current_user(current_user_id, socket)
     session_summaries = sort_session_summaries(SessionSupervisor.get_session_summaries())
+    notebook_infos = Notebook.Explore.notebook_infos() |> Enum.take(3)
 
     {:ok,
      assign(socket,
        current_user: current_user,
        path: default_path(),
-       session_summaries: session_summaries
+       session_summaries: session_summaries,
+       notebook_infos: notebook_infos
      )}
   end
 
@@ -27,28 +30,19 @@ defmodule LivebookWeb.HomeLive do
   def render(assigns) do
     ~L"""
     <div class="flex flex-grow h-full">
-      <div class="w-16 flex flex-col items-center space-y-5 px-3 py-7 bg-gray-900">
-        <div class="flex-grow"></div>
-        <span class="tooltip right distant" aria-label="User profile">
-          <%= live_patch to: Routes.home_path(@socket, :user),
-                class: "text-gray-400 rounded-xl h-8 w-8 flex items-center justify-center" do %>
-            <%= render_user_avatar(@current_user, class: "h-full w-full", text_class: "text-xs") %>
-          <% end %>
-        </span>
-      </div>
+    <%= live_component @socket, LivebookWeb.SidebarComponent,
+          id: :sidebar,
+          items: [
+            %{type: :break},
+            %{type: :user, current_user: @current_user, path: Routes.home_path(@socket, :user)}
+          ] %>
       <div class="flex-grow px-6 py-8 overflow-y-auto">
-        <div class="max-w-screen-lg w-full mx-auto p-4 pt-0 pb-8 flex flex-col items-center space-y-4">
-          <div class="w-full flex flex-col space-y-2 items-center sm:flex-row sm:space-y-0 sm:justify-between sm:pb-4 pb-8 border-b border-gray-200">
+        <div class="max-w-screen-lg w-full mx-auto px-4 pb-8 space-y-4">
+          <div class="flex flex-col space-y-2 items-center sm:flex-row sm:space-y-0 sm:justify-between sm:pb-4 pb-8 border-b border-gray-200">
             <div class="text-2xl text-gray-800 font-semibold">
-              <img src="/logo-with-text.png" class="h-[50px]" alt="Livebook" />
+              <img src="/images/logo-with-text.png" class="h-[50px]" alt="Livebook" />
             </div>
             <div class="flex space-x-2 pt-2">
-              <span class="tooltip top" aria-label="Introduction">
-                <button class="button button-outlined-gray button-square-icon"
-                  phx-click="open_welcome">
-                  <%= remix_icon("compass-line") %>
-                </button>
-              </span>
               <%= live_patch to: Routes.home_path(@socket, :import, "url"),
                     class: "button button-outlined-gray whitespace-nowrap" do %>
                 Import
@@ -59,7 +53,7 @@ defmodule LivebookWeb.HomeLive do
               </button>
             </div>
           </div>
-          <div class="w-full h-80">
+          <div class="h-80">
             <%= live_component @socket, LivebookWeb.PathSelectComponent,
               id: "path_select",
               path: @path,
@@ -92,10 +86,29 @@ defmodule LivebookWeb.HomeLive do
               </div>
             <% end %>
           </div>
-          <div class="w-full py-12">
-            <h3 class="text-xl font-semibold text-gray-800 mb-5">
+          <div class="py-12">
+            <div class="mb-4 flex justify-between items-center">
+              <h2 class="text-xl font-semibold text-gray-800">
+                Explore
+              </h2>
+              <%= live_redirect to: Routes.explore_path(@socket, :page),
+                    class: "flex items-center text-blue-600" do %>
+                <span class="font-semibold">See all</span>
+                <%= remix_icon("arrow-right-line", class: "align-middle ml-1") %>
+              <% end %>
+            </div>
+            <div class="grid grid-cols-1 md:grid-cols-3 gap-4">
+              <%= for {info, idx} <- Enum.with_index(@notebook_infos) do %>
+                <%= live_component @socket, LivebookWeb.NotebookCardComponent,
+                      id: "notebook-card-#{idx}",
+                      notebook_info: info %>
+              <% end %>
+            </div>
+          </div>
+          <div class="py-12">
+            <h2 class="mb-4 text-xl font-semibold text-gray-800">
               Running sessions
-            </h3>
+            </h2>
             <%= if @session_summaries == [] do %>
               <div class="p-5 flex space-x-4 items-center border border-gray-200 rounded-lg">
                 <div>
@@ -160,12 +173,8 @@ defmodule LivebookWeb.HomeLive do
     {:noreply, assign(socket, path: path)}
   end
 
-  def handle_event("open_welcome", %{}, socket) do
-    create_session(socket, notebook: Livebook.Notebook.Welcome.new())
-  end
-
   def handle_event("new", %{}, socket) do
-    create_session(socket)
+    {:noreply, create_session(socket)}
   end
 
   def handle_event("fork", %{}, socket) do
@@ -173,20 +182,20 @@ defmodule LivebookWeb.HomeLive do
     socket = put_import_flash_messages(socket, messages)
     notebook = Notebook.forked(notebook)
     images_dir = Session.images_dir_for_notebook(socket.assigns.path)
-    create_session(socket, notebook: notebook, copy_images_from: images_dir)
+    {:noreply, create_session(socket, notebook: notebook, copy_images_from: images_dir)}
   end
 
   def handle_event("open", %{}, socket) do
     {notebook, messages} = import_notebook(socket.assigns.path)
     socket = put_import_flash_messages(socket, messages)
-    create_session(socket, notebook: notebook, path: socket.assigns.path)
+    {:noreply, create_session(socket, notebook: notebook, path: socket.assigns.path)}
   end
 
   def handle_event("fork_session", %{"id" => session_id}, socket) do
     data = Session.get_data(session_id)
     notebook = Notebook.forked(data.notebook)
     %{images_dir: images_dir} = Session.get_summary(session_id)
-    create_session(socket, notebook: notebook, copy_images_from: images_dir)
+    {:noreply, create_session(socket, notebook: notebook, copy_images_from: images_dir)}
   end
 
   @impl true
@@ -204,7 +213,7 @@ defmodule LivebookWeb.HomeLive do
   def handle_info({:import_content, content}, socket) do
     {notebook, messages} = Livebook.LiveMarkdown.Import.notebook_from_markdown(content)
     socket = put_import_flash_messages(socket, messages)
-    create_session(socket, notebook: notebook)
+    {:noreply, create_session(socket, notebook: notebook)}
   end
 
   def handle_info(
@@ -246,16 +255,6 @@ defmodule LivebookWeb.HomeLive do
     end
   end
 
-  defp create_session(socket, opts \\ []) do
-    case SessionSupervisor.create_session(opts) do
-      {:ok, id} ->
-        {:noreply, push_redirect(socket, to: Routes.session_path(socket, :page, id))}
-
-      {:error, reason} ->
-        {:noreply, put_flash(socket, :error, "Failed to create a notebook: #{reason}")}
-    end
-  end
-
   defp import_notebook(path) do
     content = File.read!(path)
     LiveMarkdown.Import.notebook_from_markdown(content)

lib/livebook_web/live/notebook_card_component.ex

@@ -0,0 +1,22 @@
+defmodule LivebookWeb.NotebookCardComponent do
+  use LivebookWeb, :live_component
+
+  @impl true
+  def render(assigns) do
+    ~L"""
+    <div class="flex flex-col">
+      <div class="flex items-center justify-center p-6 border-2 border-gray-100 rounded-t-2xl h-[150px]">
+        <img src="<%= @notebook_info.image_url %>" class="max-h-full max-w-[75%]" />
+      </div>
+      <div class="px-6 py-4 bg-gray-100 rounded-b-2xl flex-grow">
+        <%= live_redirect @notebook_info.title,
+              to: Routes.explore_path(@socket, :notebook, @notebook_info.slug),
+              class: "text-gray-800 font-semibold cursor-pointer" %>
+        <p class="mt-2 text-sm text-gray-600">
+          <%= @notebook_info.description %>
+        </p>
+      </div>
+    </div>
+    """
+  end
+end

lib/livebook_web/live/session_helpers.ex

@@ -0,0 +1,21 @@
+defmodule LivebookWeb.SessionHelpers do
+  import Phoenix.LiveView
+  alias LivebookWeb.Router.Helpers, as: Routes
+
+  @doc """
+  Creates a new session, redirects on success,
+  puts an error flash message on failure.
+
+  Accepts the same options as `Livebook.SessionSupervisor.create_session/1`.
+  """
+  @spec create_session(Phoenix.LiveView.Socket.t(), keyword()) :: Phoenix.LiveView.Socket.t()
+  def create_session(socket, opts \\ []) do
+    case Livebook.SessionSupervisor.create_session(opts) do
+      {:ok, id} ->
+        push_redirect(socket, to: Routes.session_path(socket, :page, id))
+
+      {:error, reason} ->
+        put_flash(socket, :error, "Failed to create session: #{reason}")
+    end
+  end
+end

lib/livebook_web/live/session_live.ex

@@ -71,40 +71,41 @@ defmodule LivebookWeb.SessionLive do
       id="session"
       data-element="session"
       phx-hook="Session">
-      <div class="w-16 flex flex-col items-center space-y-5 px-3 py-7 bg-gray-900">
-        <%= live_patch to: Routes.home_path(@socket, :page) do %>
-          <img src="/logo.png" height="40" width="40" alt="livebook" />
-        <% end %>
-        <span class="tooltip right distant" aria-label="Sections (ss)">
-          <button class="text-2xl text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center" data-element="sections-list-toggle">
-            <%= remix_icon("booklet-fill") %>
-          </button>
-        </span>
-        <span class="tooltip right distant" aria-label="Connected users (su)">
-          <button class="text-2xl text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center" data-element="clients-list-toggle">
-            <%= remix_icon("group-fill") %>
-          </button>
-        </span>
-        <span class="tooltip right distant" aria-label="Runtime settings (sr)">
-          <%= live_patch to: Routes.session_path(@socket, :runtime_settings, @session_id),
-                class: "text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center #{if(@live_action == :runtime_settings, do: "text-gray-50 bg-gray-700")}" do %>
-            <%= remix_icon("cpu-line", class: "text-2xl") %>
-          <% end %>
-        </span>
-        <div class="flex-grow"></div>
-        <span class="tooltip right distant" aria-label="Keyboard shortcuts (?)">
-          <%= live_patch to: Routes.session_path(@socket, :shortcuts, @session_id),
-                class: "text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center #{if(@live_action == :shortcuts, do: "text-gray-50 bg-gray-700")}" do %>
-            <%= remix_icon("keyboard-box-fill", class: "text-2xl") %>
-          <% end %>
-        </span>
-        <span class="tooltip right distant" aria-label="User profile">
-          <%= live_patch to: Routes.session_path(@socket, :user, @session_id),
-                class: "text-gray-400 rounded-xl h-8 w-8 flex items-center justify-center" do %>
-            <%= render_user_avatar(@current_user, class: "h-full w-full", text_class: "text-xs") %>
-          <% end %>
-        </span>
-      </div>
+      <%= live_component @socket, LivebookWeb.SidebarComponent,
+            id: :sidebar,
+            items: [
+              %{type: :logo},
+              %{
+                type: :button,
+                data_element: "sections-list-toggle",
+                icon: "booklet-fill",
+                label: "Sections (ss)",
+                active: false
+              },
+              %{
+                type: :button,
+                data_element: "clients-list-toggle",
+                icon: "group-fill",
+                label: "Connected users (su)",
+                active: false
+              },
+              %{
+                type: :link,
+                icon: "cpu-line",
+                path: Routes.session_path(@socket, :runtime_settings, @session_id),
+                label: "Runtime settings (sr)",
+                active: @live_action == :runtime_settings
+              },
+              %{type: :break},
+              %{
+                type: :link,
+                icon: "keyboard-box-fill",
+                path: Routes.session_path(@socket, :shortcuts, @session_id),
+                label: "Keyboard shortcuts (?)",
+                active: @live_action == :shortcuts
+              },
+              %{type: :user, current_user: @current_user, path: Routes.session_path(@socket, :user, @session_id)}
+            ] %>
       <div class="flex flex-col h-full w-full max-w-xs absolute z-30 top-0 left-[64px] shadow-xl md:static md:shadow-none overflow-y-auto bg-gray-50 border-r border-gray-100 px-6 py-10"
         data-element="side-panel">
         <div data-element="sections-list">

lib/livebook_web/live/sidebar_component.ex

@@ -0,0 +1,77 @@
+defmodule LivebookWeb.SidebarComponent do
+  use LivebookWeb, :live_component
+
+  import LivebookWeb.UserHelpers
+
+  # ## Attributes
+  #
+  #   * `:items` - a list of sidebar items
+
+  @impl true
+  def render(assigns) do
+    ~L"""
+    <div class="w-16 flex flex-col items-center space-y-5 px-3 py-7 bg-gray-900">
+      <%= for item <- @items do %>
+        <%= render_item(@socket, item) %>
+      <% end %>
+    </div>
+    """
+  end
+
+  defp render_item(socket, %{type: :logo} = item) do
+    assigns = %{item: item}
+
+    ~L"""
+    <%= live_patch to: Routes.home_path(socket, :page) do %>
+      <img src="/images/logo.png" height="40" width="40" alt="livebook" />
+    <% end %>
+    """
+  end
+
+  defp render_item(_socket, %{type: :button} = item) do
+    assigns = %{item: item}
+
+    ~L"""
+    <span class="tooltip right distant" aria-label="<%= item.label %>">
+      <button class="text-2xl text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center"
+        data-element="<%= item.data_element %>">
+        <%= remix_icon(item.icon) %>
+      </button>
+    </span>
+    """
+  end
+
+  defp render_item(_socket, %{type: :link} = item) do
+    assigns = %{item: item}
+
+    ~L"""
+    <span class="tooltip right distant" aria-label="<%= item.label %>">
+      <%= live_patch to: item.path,
+            class: "text-gray-400 hover:text-gray-50 focus:text-gray-50 rounded-xl h-10 w-10 flex items-center justify-center #{if(item.active, do: "text-gray-50 bg-gray-700")}" do %>
+        <%= remix_icon(item.icon, class: "text-2xl") %>
+      <% end %>
+    </span>
+    """
+  end
+
+  defp render_item(_socket, %{type: :break}) do
+    assigns = %{}
+
+    ~L"""
+    <div class="flex-grow"></div>
+    """
+  end
+
+  defp render_item(_socket, %{type: :user} = item) do
+    assigns = %{item: item}
+
+    ~L"""
+    <span class="tooltip right distant" aria-label="User profile">
+      <%= live_patch to: item.path,
+            class: "text-gray-400 rounded-xl h-8 w-8 flex items-center justify-center" do %>
+        <%= render_user_avatar(item.current_user, class: "h-full w-full", text_class: "text-xs") %>
+      <% end %>
+    </span>
+    """
+  end
+end

lib/livebook_web/router.ex

@@ -23,6 +23,9 @@ defmodule LivebookWeb.Router do
     live "/home/user-profile", HomeLive, :user
     live "/home/import/:tab", HomeLive, :import
     live "/home/sessions/:session_id/close", HomeLive, :close_session
+    live "/explore", ExploreLive, :page
+    live "/explore/user-profile", ExploreLive, :user
+    live "/explore/notebooks/:slug", ExploreLive, :notebook
     live "/sessions/:id", SessionLive, :page
     live "/sessions/:id/user-profile", SessionLive, :user
     live "/sessions/:id/shortcuts", SessionLive, :shortcuts

lib/livebook_web/templates/auth/index.html.eex

@@ -1,7 +1,7 @@
 <div class="h-screen flex items-center justify-center bg-gray-900">
   <div class="flex flex-col space-y-4 items-center">
     <a href="/">
-      <img src="/logo.png" height="128" width="128" alt="livebook" />
+      <img src="/images/logo.png" height="128" width="128" alt="livebook" />
     </a>
     <div class="text-2xl text-gray-50">
       Authentication required

lib/livebook_web/templates/error/401.html.eex

@@ -11,7 +11,7 @@
     <div class="h-screen flex items-center justify-center bg-gray-900">
       <div class="flex flex-col space-y-4 items-center">
         <a href="/">
-          <img src="/logo.png" height="128" width="128" alt="livebook" />
+          <img src="/images/logo.png" height="128" width="128" alt="livebook" />
         </a>
         <div class="text-2xl text-gray-50">
           Authentication required

lib/livebook_web/templates/error/404.html.eex

@@ -11,7 +11,7 @@
     <div class="h-screen flex items-center justify-center bg-gray-900">
       <div class="flex flex-col space-y-4 items-center">
         <a href="/">
-          <img src="/logo.png" height="128" width="128" alt="livebook" />
+          <img src="/images/logo.png" height="128" width="128" alt="livebook" />
         </a>
         <div class="text-2xl text-gray-50">
           No Numbats here!

lib/livebook_web/templates/error/500.html.eex

@@ -11,7 +11,7 @@
     <div class="h-screen flex items-center justify-center bg-gray-900">
       <div class="flex flex-col space-y-4 items-center">
         <a href="/">
-          <img src="/logo.png" height="128" width="128" alt="livebook" />
+          <img src="/images/logo.png" height="128" width="128" alt="livebook" />
         </a>
         <div class="text-2xl text-gray-50">
           Something went wrong.

test/livebook/explore_test.exs

@@ -0,0 +1,18 @@
+defmodule Livebook.ExploreTest do
+  use ExUnit.Case, async: true
+
+  alias Livebook.Notebook
+  alias Livebook.Notebook.Explore
+
+  describe "notebook_by_slug!/1" do
+    test "returns notebook structure if found" do
+      assert %Notebook{} = Explore.notebook_by_slug!("intro-to-livebook")
+    end
+
+    test "raises an error if no matching notebook if found" do
+      assert_raise Explore.NotFoundError, fn ->
+        Explore.notebook_by_slug!("invalid-slug")
+      end
+    end
+  end
+end

test/livebook/notebook/welcome_test.exs

@@ -1,10 +0,0 @@
-defmodule Livebook.Notebook.WelcomeTest do
-  use ExUnit.Case, async: true
-
-  alias Livebook.Notebook
-  alias Livebook.Notebook.Welcome
-
-  test "new/0 correctly builds a new notebook" do
-    assert %Notebook{} = Welcome.new()
-  end
-end

test/livebook_web/live/explore_live_test.exs

@@ -0,0 +1,19 @@
+defmodule LivebookWeb.ExploreLiveTest do
+  use LivebookWeb.ConnCase
+
+  import Phoenix.LiveViewTest
+
+  test "link to introductory notebook correctly creates a new session", %{conn: conn} do
+    {:ok, view, _} = live(conn, "/explore")
+
+    assert {:error, {:live_redirect, %{to: to}}} =
+             view
+             |> element(~s{a}, "Let's go")
+             |> render_click()
+
+    assert to =~ "/sessions/"
+
+    {:ok, view, _} = live(conn, to)
+    assert render(view) =~ "Welcome to Livebook"
+  end
+end

test/livebook_web/live/home_live_test.exs

@@ -163,8 +163,9 @@ defmodule LivebookWeb.HomeLiveTest do
 
     assert {:error, {:live_redirect, %{to: to}}} =
              view
-             |> element(~s{[aria-label="Introduction"] button})
+             |> element(~s{a}, "Welcome to Livebook")
              |> render_click()
+             |> follow_redirect(conn)
 
     assert to =~ "/sessions/"