Elixir Dialyzer error when accessing struct prop using dot syntax

Question

Having updated my project to use Elixir 1.12 I've noticed Dialyzer complaining about certain places where I've accessed the property of a struct using dot notation. For example here's a graphql resolver function in my app:

def update(_root, %{input: input}, %{context: %{current_user: user}}) do
  case user
        |> User.update_changeset(input)
        |> Repo.update() do
    {:ok, updated_user} ->
      if user.email !== updated_user.email do
        Email.email_changed(user.email, updated_user.email)
        |> Mailer.deliver_later()
      end

      {:ok, updated_user}

    err ->
      err
  end
end

Dialyser has highlighted the expression user.email !== updated_user.email with the following error:

The call _.'email'/() requires that _@1 is of type atom(), not {map(), map()}

Any idea what this error means and how to fix it?

(this all compiles and runs fine, I'm just keen to learn why it doesn't seem to satisfy Dialyzer)

score 1 · Accepted Answer · answered Jun 25 '21 at 09:19

1

While it’s hard to tell whether dialyzer is wrong, or your code has glitches inducing this issue, without seeing how context is built before passing to this function, there is a bit of general advice.

Use deep pattern matching everywhere to narrow the issue.

def update(_root, %{input: input},
  %{context: %{current_user: %User{email: email}}}) do
    user
    |> User.update_changeset(input)
    |> Repo.update() 
    |> case do
      {:ok, %User{email: ^email} = user} ->
        {:ok, user}

      {:ok, %User{email: updated_email} = user} ->
        email
        |> Email.email_changed(updated_email)
        |> Mailer.deliver_later()

        {:ok, user}

      err ->
        err
    end
end

That way the code is cleaner, the intent is clearer, and the error (if happens) is more evident and glued to where it actually belongs.

answered Jun 25 '21 at 09:19

Aleksei Matiushkin

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That's a fair comment, but I need access to use the whole `user` anyway, hence why I haven't destructured the whole thing (even though one could do something like `%{context: %{current_user: user = %User{email: email}}}` but this gets pretty verbose). FWIW, this does make the Dialyzer error go away. I'm more interested in what the error is about though. – harryg Jun 25 '21 at 09:22
Then at least show the spec of `update/3`, please. Turns out, [tag:dialyzer] fails to get the type of context correctly. – Aleksei Matiushkin Jun 25 '21 at 09:37
1

Fair enough, yes I think it's a problem of Dialyzer not having enough info to know the type of `user` so it guesses. Adding some hints in the pattern matches solves it. – harryg Jun 25 '21 at 09:40
1

Please also note how I got rid of `if/2` with a direct pattern matching, which makes the code more ahem idiomatic :) – Aleksei Matiushkin Jun 25 '21 at 09:42

Elixir Dialyzer error when accessing struct prop using dot syntax

1 Answers1