Typespec matching any struct or map with a id key

Questions / Help
1

Hi I got Dialyzer from is_session_creator function until I added %App.Account.User{id: integer} to typespec of second argument. Is there way to make type spec match any struct or map with id key? Or is there something wrong with my typespecs?

If you remove %App.Account.User{id: integer} from is_session_creator typespec you get this warning

The call 'Elixir.App.Workshop':'is_session_creator?'(_session@1::any(),#{'__meta__':=_, '__struct__':='Elixir.App.Account.User', 'disabled':=boolean(), 'email':=binary(), 'id':=integer(), 'inserted_at':=_, 'role':=binary(), 'updated_at':=_, 'verified':=boolean()}) breaks the contract ('nil' | #{'creator_id':=integer()},#{'id':=integer()} | integer()) -> boolean()

Code

  @spec is_session_creator?(
          nil | %{creator_id: integer},
          %App.Account.User{id: integer} | %{id: integer} | integer
        ) :: boolean
  def is_session_creator?(nil, _), do: false

  def is_session_creator?(session, %{id: user_id}) when is_integer(user_id) do
    is_session_creator?(session, user_id)
  end

  def is_session_creator?(%{creator_id: creator_id}, user_id)
      when is_integer(creator_id) and is_integer(user_id) do
    creator_id == user_id
  end

Here is User’s Ecto schema

defmodule App.Account.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :email, :string, null: false
    field :role, :string, null: false
    field :disabled, :boolean, null: false, default: false
    field :verified, :boolean, null: false, default: false

    timestamps()
  end

  @doc false
  def changeset(user, attrs) do
    user
    |> cast(attrs, [:email, :role, :disabled, :verified])
    |> validate_required([:email, :role, :disabled, :verified])
    |> unique_constraint(:email)
  end
end

This is the function that returned that User struct.

@spec get_current_user(Plug.Conn.t()) :: App.Account.User.t()
def get_current_user(conn), do: conn.assigns[:user]
2

Isn’t it complaining about the first argument you are passing to is_session_creator?/2? You specify the type for that should be nil | %{creator_id: integer}, yet the type (as far as dialyzer is concerned) of what you’re passing in appears to be any().

Make sure the result spec of what you’re passing in to is_session_creator?/2 agrees with what’s expected. This is the type of problem I believe you’re running into:

@spec foo(atom()) :: any()
def foo(:one), do: nil
def foo(:two), do: %{creator_id: 4}

@spec is_session_creator?(nil | %{creator_id: integer}) :: boolean()
def is_session_creator?(nil), do: false
def is_session_creator?(%{creator_id: id}) when is_integer(id), do: true

def test() do
  :two
  |> foo()
  |> is_session_creator?()
end

In the example above, although foo/1 will always return nil or a map, the spec says it can return anything. Then dialyzer looks at the specs and sees you’re trying to send “any type imaginable” to is_session_creator?/1 which definitely doesn’t accept every type. Therefore it raises an error.

As a rule of thumb, if you’re going to have “or types” representing the same “thing”, you’ll create fewer mistakes (and improve readability) if you declare a type for them. For example, if you declare a optional_session type to be nil | %{creator_id: integer}, all of a sudden your code will be clearer to the reader AND you’ll probably make fewer mistakes while writing the typespecs, because you’d use that same optional_session type as the return value of foo/1 AND is_session_creator?/1 in the example above. In other words, this will avoid mistakes of mismatched types.

Hope this helps!

2 Likes
3

No it’s not complaining about first argument. Without any code change when I add %App.Account.User{id: integer} to second argument’s typespec all Dialyzer errors disappear. I’m using Visual Studio Code in Windows with ElixirLS plugin version v0.2.25.

4

Also I have no idea why it’s showing any() for first argument. I have this code that returns a session

@spec cache_fetch_session_by_hex_id(binary) :: App.Workshop.Session.t() | nil
def cache_fetch_session_by_hex_id(session_id_string) do
  session_id_string
  |> id_from_hex_string()
  |> App.Workshop.cache_fetch_session()
end

Then I use it like this

session = cache_fetch_session_by_hex_id(session_id_str)

if App.Workshop.is_session_creator?(session, get_current_user(conn)) do
  participant_id = participant_id_str |> id_from_hex_string()
  App.Workshop.delete_session_participant?(session.id, participant_id)
  conn |> resp_empty_object()
else
  conn |> resp_forbidden_status()
end
5

Something must be wrong with my typespecs.

6

Can you please tell us how you actually call the function?

Also perhaps show us the output of dialyxir latest release candidates, it usually points out things better than Elixir-LS does.

Be also aware of the fact, that in some cases you need to shut down the LS, delete the .elixir_ls folder and start it again, to actually get its internal dialyzer DB in sync with the code again. This is the cost we pay for its speed…

1 Like
7

Can you try changing the 2nd function declaration to:

def is_session_creator?(%{creator_id: creator_id}, %{id: user_id}) when is_integer(creator_id) and is_integer(user_id) do

That could be where the any type comes from (because you’re not limiting the type of session with guards in your original code). Also, you should be able to remove %App.Account.User{id: integer} from the spec.

8

I found a solution, I needed to change my typespec to this

@spec is_session_creator?(
        %{required(:creator_id) => integer, optional(atom()) => any()} | nil,
        %{required(:id) => integer, optional(atom()) => any()} | integer
      ) :: boolean

Now it works with both maps and structs that have those fields. Thanks for your help!

3 Likes