Common Elixir 1.3 Deprecation Warning Fix

Elixir 1.3 was released at the end of June. Personally and professionally, the upgrade went smoothly without any hiccups. However, upgrading caused deprecation warnings to pop up when running my tests or starting my server.

The deprecation warning looked a little like this:

warning: the variable "my_params" is unsafe as
it has been set inside a case/cond/receive/if/&&/||.
Please explicitly return the variable value instead.
For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

The awesome part about this deprecation warning is that it is really straight forward about how to fix it. Moreover, reading the release notes gives a great background about why this deprecation warning exists.

The code that I needed to refactor that caused this warning looked a little bit like this:

if links = my_params["links"] do
  foreign_keys = %{
    "foreign_key_a_id" => links["foreign_key_a"],
  }

  my_params =
    my_params
    |> Map.delete("links")
    |> Map.merge(foreign_keys)
end

The code looks at the JSON from my request and sanitizes it so that it can be nicely inserted into the database later on. The problem with this code (and why Elixir 1.3 has a deprecation warning about it), is that it implicitly changes the value of my_params inside of the if block and my_params is being accessed outside of this scope. This is not very functional and can lead to unintended consequences.

To fix this, Elixir prefers that I explicitly return the variable that is changing so that it can be safely used outside of the scope.

The fix code is below:

my_params =
  case my_params["links"] do
    nil ->
      my_params
    links ->
      foreign_keys = %{
        "foreign_key_a_id" => links["foreign_key_a"],
      }

      my_params
      |> Map.delete("links")
      |> Map.merge(foreign_keys)
  end

Now, I am explicitly setting the variable my_params that I was previously changing inside of the if block. If my_params["links"] does not exist (is nil), I will return the initial value. If the my_params["links"] does exist, it will return links. Now both cases are explicitly laid out in the code.

This change makes updating the variable safer and, in my opinion, makes the code more readable and more explicit.