Idea: Rust-like syntactic support for :ok/:error

Rust is too low-level for most of my work, but I’m envious of the strong fp influence in its design. E.g., the verbose way to react to an error — and bubble it up — looks like this. The gen2() function also returns this Result Union/Either type:

fn caller() -> Result<String, String> {
    let r = gen2();
    match r {
        Ok(i) => Ok(i.to_string()),
        Err(e) => Err(e),
    }
}

To me, that looks a lot like the usual Elixir way of matching on {:ok, ...} and {:error, ...}. Rust, though, has this nice UX syntactic support with the ? operator. This code is equivalent to the above:

fn caller() -> Result<String, String> {
    let n = gen2()?;
    Ok(n.to_string())
}

(Source: https://betterprogramming.pub/rust-error-handling-84e7bd169e47)

Pretty damn nice, IMO. When I was doing a lot of Rust coding, error handling was a breezy, low-boilerplate experience — yet the code was as robust as ever. Invocations with ? like above can be chained together with the . operator.

I’m imagining syntactic support like this that’d work seamlessly with the existing :ok/:error convention, just like the Rust syntax works with Result.

Brainstorming and adapting the example from Writing Predictable Elixir Code with Reducers | AppSignal Blog … here’s the original code:

options = %{conn: conn}
 
with {:ok, payment} <- fetch_payment_information(params, options),
     {:ok, user}    <- fetch_user(conn),
     {:ok, address} <- fetch_address(%{user: user, params: params}, options),
     {:ok, order}   <- create_order(%{user: user, address: address, payment: payment}, options)
  do
  conn
  |> put_flash(:info, "Order completed!")
  |> redirect(to: Routes.order_path(conn, order))
else
  {:error, error_description} ->
    conn
    |> put_flash(:error, parse_error(error_description))
    |> render("checkout.html")
end

Let’s say we want to refactor this, extracting the with clause to a new function, but use a new => operator:

def process_payment(params, options) do
  payment <= fetch_payment_information(params, options),
  user    <= fetch_user(conn),
  address <= fetch_address(%{user: user, params: params}, options),

  order   <= create_order(%{user: user, address: address, payment: payment}, options)
end

Like Rust’s ?, this <= would automatically “unwrap” the return values, directly giving access to the :ok result. But if any of these returns :error it’ll automatically return the first found :error result. In a nutshell, it’d de-sugar to a with/else-construct.

The refactored caller could be standard un-sugared Elixir since it’s top-level and wants to handle the error, not bubble it up. But now it’s more readable:

options = %{conn: conn}
 
case process_payment(params, options) do
  {:ok, order} do
    conn
    |> put_flash(:info, "Order completed!")
    |> redirect(to: Routes.order_path(conn, order))
  {:error, error_description} ->
    conn
    |> put_flash(:error, parse_error(error_description))
    |> render("checkout.html")
end

This has been vetted and used in Rust for years. So even though it hides some info and does some things automatically, I think the Rust experience has validated it as a positive addition to the language.

Along with <= I was thinking about an unwrapping version of the pipe |>: This new version would transparently handle functions that return the :ok/:error tuple, enabling them to be used in pipelines with functions that don’t. IMO this second syntax could make code even cleaner, e.g. if the new pipe is !>:

username
|> to_lower
!> get_user_from_database
|> format_address