learning_phoenix/AGENTS.md

This is a web application written using the Phoenix web framework.

Project guidelines

• Use mix precommit alias when you are done with all changes and fix any pending issues
• Use the already included and available :req (Req) library for HTTP requests, avoid :httpoison, :tesla, and :httpc. Req is included by default and is the preferred HTTP client for Phoenix apps

Phoenix v1.8 guidelines

• Always begin your LiveView templates with <Layouts.app flash={@flash} ...> which wraps all inner content
• The MyAppWeb.Layouts module is aliased in the my_app_web.ex file, so you can use it without needing to alias it again
• Anytime you run into errors with no current_scope assign:
  • You failed to follow the Authenticated Routes guidelines, or you failed to pass current_scope to <Layouts.app>
  • Always fix the current_scope error by moving your routes to the proper live_session and ensure you pass current_scope as needed
• Phoenix v1.8 moved the <.flash_group> component to the Layouts module. You are forbidden from calling <.flash_group> outside of the layouts.ex module
• Out of the box, core_components.ex imports an <.icon name="hero-x-mark" class="w-5 h-5"/> component for for hero icons. Always use the <.icon> component for icons, never use Heroicons modules or similar
• Always use the imported <.input> component for form inputs from core_components.ex when available. <.input> is imported and using it will will save steps and prevent errors
• If you override the default input classes (<.input class="myclass px-2 py-1 rounded-lg">)) class with your own values, no default classes are inherited, so your custom classes must fully style the input

Elixir guidelines

• Elixir lists do not support index based access via the access syntax

  Never do this (invalid):

  i = 0
  mylist = ["blue", "green"]
  mylist[i]
  

  Instead, always use Enum.at, pattern matching, or List for index based list access, ie:

  i = 0
  mylist = ["blue", "green"]
  Enum.at(mylist, i)
  

• Elixir variables are immutable, but can be rebound, so for block expressions like if, case, cond, etc you must bind the result of the expression to a variable if you want to use it and you CANNOT rebind the result inside the expression, ie:

  # INVALID: we are rebinding inside the `if` and the result never gets assigned
  if connected?(socket) do
    socket = assign(socket, :val, val)
  end
  
  # VALID: we rebind the result of the `if` to a new variable
  socket =
    if connected?(socket) do
      assign(socket, :val, val)
    end
  

• Never nest multiple modules in the same file as it can cause cyclic dependencies and compilation errors

• Never use map access syntax (changeset[:field]) on structs as they do not implement the Access behaviour by default. For regular structs, you must access the fields directly, such as my_struct.field or use higher level APIs that are available on the struct if they exist, Ecto.Changeset.get_field/2 for changesets

• Elixir's standard library has everything necessary for date and time manipulation. Familiarize yourself with the common Time, Date, DateTime, and Calendar interfaces by accessing their documentation as necessary. Never install additional dependencies unless asked or for date/time parsing (which you can use the date_time_parser package)

• Don't use String.to_atom/1 on user input (memory leak risk)

• Predicate function names should not start with is_ and should end in a question mark. Names like is_thing should be reserved for guards

• Elixir's builtin OTP primitives like DynamicSupervisor and Registry, require names in the child spec, such as {DynamicSupervisor, name: MyApp.MyDynamicSup}, then you can use DynamicSupervisor.start_child(MyApp.MyDynamicSup, child_spec)

• Use Task.async_stream(collection, callback, options) for concurrent enumeration with back-pressure. The majority of times you will want to pass timeout: :infinity as option

Mix guidelines

• Read the docs and options before using tasks (by using mix help task_name)
• To debug test failures, run tests in a specific file with mix test test/my_test.exs or run all previously failed tests with mix test --failed
• mix deps.clean --all is almost never needed. Avoid using it unless you have good reason

Phoenix guidelines

• Remember Phoenix router scope blocks include an optional alias which is prefixed for all routes within the scope. Always be mindful of this when creating routes within a scope to avoid duplicate module prefixes.

• You never need to create your own alias for route definitions! The scope provides the alias, ie:

  scope "/admin", AppWeb.Admin do
    pipe_through :browser
  
    live "/users", UserLive, :index
  end
  

  the UserLive route would point to the AppWeb.Admin.UserLive module

• Phoenix.View no longer is needed or included with Phoenix, don't use it

Ecto Guidelines

• Always preload Ecto associations in queries when they'll be accessed in templates, ie a message that needs to reference the message.user.email
• Remember import Ecto.Query and other supporting modules when you write seeds.exs
• Ecto.Schema fields always use the :string type, even for :text, columns, ie: field :name, :string
• Ecto.Changeset.validate_number/2 DOES NOT SUPPORT the :allow_nil option. By default, Ecto validations only run if a change for the given field exists and the change value is not nil, so such as option is never needed
• You must use Ecto.Changeset.get_field(changeset, :field) to access changeset fields
• Fields which are set programatically, such as user_id, must not be listed in cast calls or similar for security purposes. Instead they must be explicitly set when creating the struct

Phoenix HTML guidelines

• Phoenix templates always use ~H or .html.heex files (known as HEEx), never use ~E

• Always use the imported Phoenix.Component.form/1 and Phoenix.Component.inputs_for/1 function to build forms. Never use Phoenix.HTML.form_for or Phoenix.HTML.inputs_for as they are outdated

• When building forms always use the already imported Phoenix.Component.to_form/2 (assign(socket, form: to_form(...)) and <.form for={@form} id="msg-form">), then access those forms in the template via @form[:field]

• Always add unique DOM IDs to key elements (like forms, buttons, etc) when writing templates, these IDs can later be used in tests (<.form for={@form} id="product-form">)

• For "app wide" template imports, you can import/alias into the my_app_web.ex's html_helpers block, so they will be available to all LiveViews, LiveComponent's, and all modules that do use MyAppWeb, :html (replace "my_app" by the actual app name)

• Elixir supports if/else but **does NOT support if/else if or if/elsif. Never use else if or elseif in Elixir, always use cond or case for multiple conditionals.

  Never do this (invalid):

  <%= if condition do %>
    ...
  <% else if other_condition %>
    ...
  <% end %>
  

  Instead always do this:

  <%= cond do %>
    <% condition -> %>
      ...
    <% condition2 -> %>
      ...
    <% true -> %>
      ...
  <% end %>
  

• HEEx require special tag annotation if you want to insert literal curly's like { or }. If you want to show a textual code snippet on the page in a <pre> or <code> block you must annotate the parent tag with phx-no-curly-interpolation:

  <code phx-no-curly-interpolation>
    let obj = {key: "val"}
  </code>
  

  Within phx-no-curly-interpolation annotated tags, you can use { and } without escaping them, and dynamic Elixir expressions can still be used with <%= ... %> syntax

• HEEx class attrs support lists, but you must always use list [...] syntax. You can use the class list syntax to conditionally add classes, always do this for multiple class values:

  <a class={[
    "px-2 text-white",
    @some_flag && "py-5",
    if(@other_condition, do: "border-red-500", else: "border-blue-100"),
    ...
  ]}>Text</a>
  

  and always wrap if's inside {...} expressions with parens, like done above (if(@other_condition, do: "...", else: "..."))

  and never do this, since it's invalid (note the missing [ and ]):

  <a class={
    "px-2 text-white",
    @some_flag && "py-5"
  }> ...
  => Raises compile syntax error on invalid HEEx attr syntax
  

• Never use <% Enum.each %> or non-for comprehensions for generating template content, instead always use <%= for item <- @collection do %>

• HEEx HTML comments use <%!-- comment --%>. Always use the HEEx HTML comment syntax for template comments (<%!-- comment --%>)

• HEEx allows interpolation via {...} and <%= ... %>, but the <%= %> only works within tag bodies. Always use the {...} syntax for interpolation within tag attributes, and for interpolation of values within tag bodies. Always interpolate block constructs (if, cond, case, for) within tag bodies using <%= ... %>.

  Always do this:

  <div id={@id}>
    {@my_assign}
    <%= if @some_block_condition do %>
      {@another_assign}
    <% end %>
  </div>
  

  and Never do this – the program will terminate with a syntax error:

  <%!-- THIS IS INVALID NEVER EVER DO THIS --%>
  <div id="<%= @invalid_interpolation %>">
    {if @invalid_block_construct do}
    {end}
  </div>
  

Phoenix LiveView guidelines

• Never use the deprecated live_redirect and live_patch functions, instead always use the <.link navigate={href}> and <.link patch={href}> in templates, and push_navigate and push_patch functions LiveViews
• Avoid LiveComponent's unless you have a strong, specific need for them
• LiveViews should be named like AppWeb.WeatherLive, with a Live suffix. When you go to add LiveView routes to the router, the default :browser scope is already aliased with the AppWeb module, so you can just do live "/weather", WeatherLive
• Remember anytime you use phx-hook="MyHook" and that js hook manages its own DOM, you must also set the phx-update="ignore" attribute
• Never write embedded <script> tags in HEEx. Instead always write your scripts and hooks in the assets/js directory and integrate them with the assets/js/app.js file

LiveView streams

• Always use LiveView streams for collections for assigning regular lists to avoid memory ballooning and runtime termination with the following operations:

  • basic append of N items - stream(socket, :messages, [new_msg])
  • resetting stream with new items - stream(socket, :messages, [new_msg], reset: true) (e.g. for filtering items)
  • prepend to stream - stream(socket, :messages, [new_msg], at: -1)
  • deleting items - stream_delete(socket, :messages, msg)

• When using the stream/3 interfaces in the LiveView, the LiveView template must 1) always set phx-update="stream" on the parent element, with a DOM id on the parent element like id="messages" and 2) consume the @streams.stream_name collection and use the id as the DOM id for each child. For a call like stream(socket, :messages, [new_msg]) in the LiveView, the template would be:

  <div id="messages" phx-update="stream">
    <div :for={{id, msg} <- @streams.messages} id={id}>
      {msg.text}
    </div>
  </div>
  

• LiveView streams are not enumerable, so you cannot use Enum.filter/2 or Enum.reject/2 on them. Instead, if you want to filter, prune, or refresh a list of items on the UI, you must refetch the data and re-stream the entire stream collection, passing reset: true:

  def handle_event("filter", %{"filter" => filter}, socket) do
    # re-fetch the messages based on the filter
    messages = list_messages(filter)
  
    {:noreply,
    socket
    |> assign(:messages_empty?, messages == [])
    # reset the stream with the new messages
    |> stream(:messages, messages, reset: true)}
  end
  

• LiveView streams do not support counting or empty states. If you need to display a count, you must track it using a separate assign. For empty states, you can use Tailwind classes:

  <div id="tasks" phx-update="stream">
    <div class="hidden only:block">No tasks yet</div>
    <div :for={{id, task} <- @stream.tasks} id={id}>
      {task.name}
    </div>
  </div>
  

  The above only works if the empty state is the only HTML block alongside the stream for-comprehension.

• Never use the deprecated phx-update="append" or phx-update="prepend" for collections

LiveView tests

• Phoenix.LiveViewTest module and LazyHTML (included) for making your assertions

• Form tests are driven by Phoenix.LiveViewTest's render_submit/2 and render_change/2 functions

• Come up with a step-by-step test plan that splits major test cases into small, isolated files. You may start with simpler tests that verify content exists, gradually add interaction tests

• Always reference the key element IDs you added in the LiveView templates in your tests for Phoenix.LiveViewTest functions like element/2, has_element/2, selectors, etc

• Never tests again raw HTML, always use element/2, has_element/2, and similar: assert has_element?(view, "#my-form")

• Instead of relying on testing text content, which can change, favor testing for the presence of key elements

• Focus on testing outcomes rather than implementation details

• Be aware that Phoenix.Component functions like <.form> might produce different HTML than expected. Test against the output HTML structure, not your mental model of what you expect it to be

• When facing test failures with element selectors, add debug statements to print the actual HTML, but use LazyHTML selectors to limit the output, ie:

  html = render(view)
  document = LazyHTML.from_fragment(html)
  matches = LazyHTML.filter(document, "your-complex-selector")
  IO.inspect(matches, label: "Matches")
  

Form handling

Creating a form from params

If you want to create a form based on handle_event params:

def handle_event("submitted", params, socket) do
  {:noreply, assign(socket, form: to_form(params))}
end

When you pass a map to to_form/1, it assumes said map contains the form params, which are expected to have string keys.

You can also specify a name to nest the params:

def handle_event("submitted", %{"user" => user_params}, socket) do
  {:noreply, assign(socket, form: to_form(user_params, as: :user))}
end

Creating a form from changesets

When using changesets, the underlying data, form params, and errors are retrieved from it. The :as option is automatically computed too. E.g. if you have a user schema:

defmodule MyApp.Users.User do
  use Ecto.Schema
  ...
end

And then you create a changeset that you pass to to_form:

%MyApp.Users.User{}
|> Ecto.Changeset.change()
|> to_form()

Once the form is submitted, the params will be available under %{"user" => user_params}.

In the template, the form form assign can be passed to the <.form> function component:

<.form for={@form} id="todo-form" phx-change="validate" phx-submit="save">
  <.input field={@form[:field]} type="text" />
</.form>

Always give the form an explicit, unique DOM ID, like id="todo-form".

Avoiding form errors

Always use a form assigned via to_form/2 in the LiveView, and the <.input> component in the template. In the template always access forms this:

<%!-- ALWAYS do this (valid) --%>
<.form for={@form} id="my-form">
  <.input field={@form[:field]} type="text" />
</.form>

And never do this:

<%!-- NEVER do this (invalid) --%>
<.form for={@changeset} id="my-form">
  <.input field={@changeset[:field]} type="text" />
</.form>

• You are FORBIDDEN from accessing the changeset in the template as it will cause errors
• Never use <.form let={f} ...> in the template, instead always use <.form for={@form} ...>, then drive all form references from the form assign as in @form[:field]. The UI should always be driven by a to_form/2 assigned in the LiveView module that is derived from a changeset