Cheating at Railway Oriented Programming in Elixir

Over the past four months I’ve been writing Elixir full-time for backend systems. From this I’ll be releasing a series of small blog posts about techniques we utilise, and the advantages and disadvantages that we’ve found whilst using them.

The first as the title suggests relates to Railway Oriented Programming as presented by Scott Wlaschin here. If you haven’t watched the video and you are interested in functional programming techniques, then I would highly recommend you stop right here and go watch it.

To summarise, Railway Oriented Programming is a technique designed to allow a program to proceed through to completion should it fail at any point, with a uniform error handling that allows you to compose functions as you would aim to in a “perfect” world. The pain-points to consider with functional programming are around the lack of early returns, exceptions, and modelling stateful data. A blog post around directly implementing those design patterns can be found here.

The initial example from the talk is the following hypothetical:

def update_user_preferences(conn, request)
  request
  |> validate_request
  |> get_user
  |> update_db_from_request
  |> send_email
  |> return_http_message
end

A close equivalent set of functionality utilising the with macro would be:

def update_user_preferences(conn, request)
  with {:ok, request}    <- validate(request),
       {:ok, user}       <- fetch_user(request.user_id),
       {:ok, _}          <- update_preferences(user, request.preferences),
       {:ok, sent_email} <- send_email(user, :preferences_updated)
  do
    return_http_message(conn, user, sent_mail)
  end
end

This functionality is dependent on the return “shape” to be pattern matched, using the standard convention of returning either an ok tuple ({:ok, result}), or an error tuple ({:error, # ...}). It provides us a uniform error handling by propagating the error that occurred back up the call chain, or allows us to pattern match the error in a provided else-clause, like so:

def update_user_preferences(conn, request)
  with {:ok, request}    <- validate(request),
       {:ok, user}       <- User.fetch(request.user_id),
       {:ok, _}          <- User.update_preferences(user, request.preferences),
       {:ok, sent_email} <- send_email(user, :preferences_updated)
  do
    return_http_message(conn, user, sent_mail)
  else
    # Three-form error tuples will be the topic for another blog-post
    {:error, :validation, errors} -> render_as_json(errors)
    {:error, :user, :not_found}   -> # ...
  end
end

We also regain the notion of stateful data, in that each variable assigned from the result of a previous function call is also accessible in any subsequent one. This provides us with the flexibility to design functions focused on their purpose, rather than needing to consider the input and output of a function in the greater scope.

In order to achieve the same kind of focused and well-defined functions outside of the with macro, we would need dedicated utility/wrapper functions that dilute or otherwise distract from the readability/intent of the code.

A correct-in-spirit-but-not-quite way to imagine it is as a set of composed case statements, with the error handling injected as pattern matches in each of the cases. This provides us with access to any of the parent case scopes.

case validate(request) do
  {:ok, request} ->
    case User.fetch(request.user_id) do
      # Rinse and repeat
      {:ok, user} ->
        case User.update_preferences(user, request.preferences) do
          {:ok, _} ->
  # Inject the error handling
  {:error, :validation, errors} ->
    render_as_json(errors)
  # ...
end

In summary, the with macro is a nice abstraction that handles the initial error examples demonstrated by Scott Wlaschin in an elegent and powerful way, as promoted by Elixirs awesome pattern-matching functionality. It “cheats” at the R.O.P concept by providing us an impure-yet-elegent way of composing high-level functions, resulting in a clean and powerful abstraction that gets the job done.

References