Middleware like plug

Are you using Phoenix? My memory was that this works out of the box.

But maybe take a look at Plug.ErrorHandler — Plug v1.20.2

Yes, I’m using Phoenix

I already tried the Plug.ErrorHandler, but it re-raises the error as the documentation points

I need a solution to enrich the error message and return a JSON

Why is the error reraising is undesirable for your use case? Your user is still going to get a json response (and the handlers allow that to be anything).

Raising an error is not the way you do things in elixir.

Instead of raising, you can return a tuple in the format {:error, :not_found} and handle it in a fallback controller.

Curious what you mean by this. Certainly runtime errors, and therefore error raising, is a feature of the language? Or maybe you just mean that raising an error in this case is a bad pattern? But Plug literally has built in support for this and Phoenix is configured to take advantage of it by default, so while opinions may differ it is certainly “the way you do things” for many people…

It’s a feature to pass errors up to the parent process, why you would use it in this case is beyond my understanding.

I have a huge code base that some functions raise errors, I didn’t have a choice

Convenience I think would be the main reason. I assume your problem with Plug’s design is that generally it is a bad idea to use errors to control flow. But since we generally want to respond with a user friendly error even when an exception is not expected, this kind of logic seems required here. And once it’s necessary it seems too convenient not to take advantage of in cases where the exception is not strictly speaking unexpected but does not and could not require any special logic other than building the response, because the alternative is needing separate and explicit handling for every single API requirement (like parameter shape/format).

My problem is in general related to using exceptions as flow, this is the same as using goto statements, error prone and totally unreadable.

You don’t want to respond to a user with a specific response when something unexpected happens, as this is a bug in the system, and the user doesn’t need to know the details, as you are potentially exposing a exploitation interface. In the case with not found, that is not a bug though, it is a case that should be handled appropriately and using a fallback controller in conjunction with a with statement is the most appropriate solution.

I don’t see how this is the case here, you just handle the error response coming from your controller:

def call(conn, {:error, :not_found}) do
    conn
    |> put_status(:not_found)
    |> put_view(MyErrorView)
    |> render(:"404")
  end

I didn’t intend to make this a debate about the merits of the approach. Again, I just wanted to point out that, unless I am greatly mistaken, your claim that using Plug.ErrorHandler for this is not “the Elixir way” is almost exactly opposite of the truth, it’s actually the default in Phoenix which I would say makes the most common approach. I don’t think it’s helpful to formulate minority opinions as if they are majority (which of course is not to say that your opinion must be wrong because it’s the minority).