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Phoenix framework project generator. Provides a `mix phx.new` task to bootstrap a new Elixir application with Phoenix dependencies.

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phx_new templates phoenix-usage-rules elixir.md
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templates/phoenix-usage-rules/elixir.md 

## 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



## Test guidelines



- **Always use `start_supervised!/1`** to start processes in tests as it guarantees cleanup between tests

- **Avoid** `Process.sleep/1` and `Process.alive?/1` in tests

  - Instead of sleeping to wait for a process to finish, **always** use `Process.monitor/1` and assert on the DOWN message:



      ref = Process.monitor(pid)

      assert_receive {:DOWN, ^ref, :process, ^pid, :normal}



   - Instead of sleeping to synchronize before the next call, **always** use `_ = :sys.get_state/1` to ensure the process has handled prior messages