Hi everyone

I’m trying to improve the typespecs in my application contexts, but I’m running into dialyzer errors when dealing with schemas that have fields that can be nil.

Here’s an example:

defmodule MyApp.User do
  use Ecto.Schema

  import Ecto.Changeset

  @type t :: %__MODULE__{
          id: integer(),
          email: String.t(),
          age: non_neg_integer()
        }

  schema "users" do
    field :email, :string
    field :age, :integer
  end

  def changeset(user, attrs) do
    user |> cast(attrs, [:email, :age]) |> validate_required(:email)
  end
end

defmodule MyApp.UsersContext do
  alias MyApp.User

  @spec change_user(user :: User.t(), attrs :: map()) :: Ecto.Changeset.t()
  def change_user(%User{} = user, attrs \\ %{}) do
    User.changeset(user, attrs)
  end
end

defmodule MyApp.FakeController do
  alias MyApp.User

  def index() do
    MyApp.UsersContext.change_user(%User{}, %{})

    :ok
  end
end

In this schema, there are no default values so %User{} will generate a struct where every field is nil.

Using this setup, dialyzer complains with:

The function call will not succeed.

MyApp.UsersContext.change_user(
  %MyApp.User{
    :__meta__ => %Ecto.Schema.Metadata{
      :context => nil,
      :prefix => nil,
      :schema => MyApp.User,
      :source => <<117, 115, 101, 114, 115>>,
      :state => :built
    },
    :age => nil,
    :email => nil,
    :id => nil
  },
  %{}
)

breaks the contract
(user :: MyApp.User.t(), attrs :: map()) :: Ecto.Changeset.t()

To fix this error I have to explicitly put | nil in MyApp.User.t() type:

  @type t :: %__MODULE__{
          id: integer() | nil,
          email: String.t() | nil,
          age: non_neg_integer() | nil
        }

But here are some things I’m unsure about:

• Is it idiomatic to be so explicit with | nil for every nullable field?
• Is there a better or preferred way to declare the schema type (@type t) that keeps it maintainable and readable, especially in large schemas?

Another possible solution I’ve found is to declare, somewhere in the codebase, a generic schema typespec:

  @type schema_t(schema) :: %{
          optional(atom) => any,
          __struct__: schema,
          __meta__: Ecto.Schema.Metadata.t(schema)
        }

and use it in my specs:

  @spec change_user(user :: schema_t(User), attrs :: map()) :: Ecto.Changeset.t()

Curious how others approach this.

Thanks!