Does not seem possible to use assign in custom error html

[srcrip]

Environment

• Elixir version (elixir -v): 15
• Phoenix version (mix deps): main
• Operating system: mac

Actual behavior

If you try to use assign in a custom error html like this:

defmodule ExampleWeb.ErrorHTML do
  use ExampleWeb, :html

  def render("404.html" = template, assigns) do
    assigns =
      assign(assigns,
        message: hello
      )

    ~H"""
    <h1 class="text-2xl font-bold tracking-tight text-gray-900 sm:text-4xl">
      <%= @status %> <%= @message %>
    </h1>
    """
  end
end

You get an error like:

(ArgumentError) assign/3 expects a socket from Phoenix.LiveView/Phoenix.LiveComponent  or an assigns map from Phoenix.Component as first argument ...

Expected behavior

This should be a valid assigns so I think it should work?

[agonzalezro]

@srcrip assign expects the socket as the first argument. I am not sure if you would be able to get the socket in the custom errors, but for this specific example you are mentioning maybe it's enough with:

defmodule ExampleWeb.ErrorHTML do
  use ExampleWeb, :html

  def render("404.html" = template, assigns) do
    message = "hello"

    ~H"""
    <h1 class="text-2xl font-bold tracking-tight text-gray-900 sm:text-4xl">
      <%= @status %> <%= message %>
    </h1>
    """
  end
end

I know that you pasted just an example so maybe it doesn't fit what you need. But let me know.

[josevalim]

The error message is correct. This is not the render function of a LiveView nor of a component. We could relax it here and allow assign to work with any map, but there is no change tracking happening here whatsoever, so I am unsure if we should do it. It is up to @chrismccord to decide if he wants to allow assigns to be used anywhere, even outside of the regular rendering stack.

[thiagomajesk]

Not directly related to the issue, but check this comment.

I guess this should be addressed once the component model is consolidated... IMHO, there's still some mental friction between how things used to be before (views/templates) vs after (components/templates) Phoenix 1.7.

Since functional components were introduced, people expect functions that return HTML using ~H to behave similarly, and we have some cases where this is handled slightly differently by the framework (like layouts for instance).

[josevalim]

We should set __changed__: nil before calling the error view.