Usage Rules

IMPORTANT: Consult these usage rules early and often when working with the packages listed below. Before attempting to use any of these packages or to discover if you should use them, review their usage rules to understand the correct patterns, conventions, and best practices.

phoenix:ecto usage

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:elixir usage

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 docker compose exec -T app yarn test test/my_test.exs or run all previously failed tests with docker compose exec -T app yarn test --failed
mix deps.clean --all is almost never needed. Avoid using it unless you have good reason

phoenix:html usage

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 usage

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 demo/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

phoenix:phoenix usage

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

igniter usage

A code generation and project patching framework

igniter usage rules

usage_rules usage

A dev tool for Elixir projects to gather LLM usage rules from dependencies

Using Usage Rules

Many packages have usage rules, which you should thoroughly consult before taking any action. These usage rules contain guidelines and rules directly from the package authors. They are your best source of knowledge for making decisions.

Modules & functions in the current app and dependencies

When looking for docs for modules & functions that are dependencies of the current project, or for Elixir itself, use mix usage_rules.docs

# Search a whole module
mix usage_rules.docs Enum

# Search a specific function
mix usage_rules.docs Enum.zip

# Search a specific function & arity
mix usage_rules.docs Enum.zip/1

Searching Documentation

You should also consult the documentation of any tools you are using, early and often. The best way to accomplish this is to use the usage_rules.search_docs mix task. Once you have found what you are looking for, use the links in the search results to get more detail. For example:

# Search docs for all packages in the current application, including Elixir
mix usage_rules.search_docs Enum.zip

# Search docs for specific packages
mix usage_rules.search_docs Req.get -p req

# Search docs for multi-word queries
mix usage_rules.search_docs "making requests" -p req

# Search only in titles (useful for finding specific functions/modules)
mix usage_rules.search_docs "Enum.zip" --query-by title

usage_rules:elixir usage

Elixir Core Usage Rules

Pattern Matching

Use pattern matching over conditional logic when possible
Prefer to match on function heads instead of using if/else or case in function bodies
%{} matches ANY map, not just empty maps. Use map_size(map) == 0 guard to check for truly empty maps

Error Handling

Use {:ok, result} and {:error, reason} tuples for operations that can fail
Avoid raising exceptions for control flow
Use with for chaining operations that return {:ok, _} or {:error, _}

Common Mistakes to Avoid

Elixir has no return statement, nor early returns. The last expression in a block is always returned.
Don't use Enum functions on large collections when Stream is more appropriate
Avoid nested case statements - refactor to a single case, with or separate functions
Don't use String.to_atom/1 on user input (memory leak risk)
Lists and enumerables cannot be indexed with brackets. Use pattern matching or Enum functions
Prefer Enum functions like Enum.reduce over recursion
When recursion is necessary, prefer to use pattern matching in function heads for base case detection
Using the process dictionary is typically a sign of unidiomatic code
Only use macros if explicitly requested
There are many useful standard library functions, prefer to use them where possible

Function Design

Use guard clauses: when is_binary(name) and byte_size(name) > 0
Prefer multiple function clauses over complex conditional logic
Name functions descriptively: calculate_total_price/2 not calc/2
Predicate function names should not start with is and should end in a question mark.
Names like is_thing should be reserved for guards

Data Structures

Use structs over maps when the shape is known: defstruct [:name, :age]
Prefer keyword lists for options: [timeout: 5000, retries: 3]
Use maps for dynamic key-value data
Prefer to prepend to lists [new | list] not list ++ [new]

Mix Tasks

Use mix help to list available mix tasks
Use mix help task_name to get docs for an individual task
Read the docs and options fully before using tasks

Testing

Run tests in a specific file with docker compose exec -T app yarn test test/my_test.exs and a specific test with the line number docker compose exec -T app yarn test path/to/test.exs:123
Limit the number of failed tests with docker compose exec -T app yarn test --max-failures n
Use @tag to tag specific tests, and docker compose exec -T app yarn test --only tag to run only those tests
Use assert_raise for testing expected exceptions: assert_raise ArgumentError, fn -> invalid_function() end
Use mix help test to for full documentation on running tests

Debugging

Use dbg/1 to print values while debugging. This will display the formatted value and other relevant information in the console.

usage_rules:otp usage

OTP Usage Rules

GenServer Best Practices

Keep state simple and serializable
Handle all expected messages explicitly
Use handle_continue/2 for post-init work
Implement proper cleanup in terminate/2 when necessary

Process Communication

Use GenServer.call/3 for synchronous requests expecting replies
Use GenServer.cast/2 for fire-and-forget messages.
When in doubt, use call over cast, to ensure back-pressure
Set appropriate timeouts for call/3 operations

Fault Tolerance

Set up processes such that they can handle crashing and being restarted by supervisors
Use :max_restarts and :max_seconds to prevent restart loops

Task and Async

Use Task.Supervisor for better fault tolerance
Handle task failures with Task.yield/2 or Task.shutdown/2
Set appropriate task timeouts
Use Task.async_stream/3 for concurrent enumeration with back-pressure

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