Zero-cost abstraction for NewTypes in Elixir

Hmmm, zero-runtime cost mean somehow the work is done on compile time? i wonder if it’s possible to do this via macro, which run at compile time. but i think in that case the obvious limitation would be that the checking would only work if when the value is known at compile time (static).

While I can see the usefulness of the general pattern one has to acknowledge that elixir is not a statically typed language. The compiler only has limited knowledge around the types of data at compile time (especially around everything message passing). So the remaining options are runtime checks.

As you noted things won’t be “zero cost” at runtime. The smallest way to add information to a piece of data would be a number per type or on the beam an atom, which at runtime is also basically a number. Erlang usually uses records for that {:user, "someone"}. Tuples afaik come with very little overhead in actual memory over just the two values itself (1). In elixir we usually don’t use tuples/records that much, we use maps/structs. Luckily small maps (<32 keys) are stored in (again afaik) a similar memory layout to tuples. Still a bit more overhead, but less than for large maps.

So those are the options to look at imo for deciding if the runtime hit is worthwhile. Generally I feel like structs are a good way to “type” data, but I wouldn’t do it for every scalar floating around in your system, but rather things which are reasonable entities or values in your system. E.g. Location in an event booking system makes sense. Wrapping every city string probably not.

[1] Memory Usage — Erlang System Documentation v29.0.2

This is very interesting. But if I were to use a tuple, my signature would have to be:

@spec new(artist :: String.t, country :: {:name, String.t}, ... ) :: Artist.t

Instead of:

@spec new(artist :: String.t, country :: Location.t, ...) :: Artist.t

Right?

I would also need to extract the value via elem/2.

An idea worth exploring though, thanks !

defmodule Location do
  @type t :: {:location, String.t}
end

But yes you’d have to unwrap the value - just like with a struct. This is runtime data we’re dealing with. You cannot implicitly wrap a string to be tagged “a location” and the runtime would infer that tag from the plain string received. There’s things you can do in a statically typed language you simply cannot do if you don’t have a statically typed language.

I will try to help, but after so many months out of Elixir my understanding may be cloudy…

I kind of tried to achieve this on my own in the past, but then resorted to use the Domo library.

I also know of the Typed Struct library that I have not tried yet.

Does any of this libs can help you achieve what you want?

Thank you for trying!

When I want structs about something, I usually use TypedStruct. Some people I know use Embedded Ecto schemas.

However, here the purpose is different. Even though I am using a struct, my objective is not to make “using structs easier” (like it is with TypedStruct). My objective here is to simply wrap a primitive value into an abstraction that allows dialyzer (or gradient) to complain.

If anything, structs are an implementation detail that I would hide under the hood of a macro

I usually use a single argument that’s a keyword list, map or struct in such cases, like

@spec new(artist: String.t, country: String.t, genre: String.t) :: Artist.t

or

@spec new(%{artist: String.t, country: String.t, genre: String.t}) :: Artist.t

or

@spec new(%ArtistConfig{artist: String.t, country: String.t, genre: String.t}) :: Artist.t

Though it’s more verbose, I find it more readable and Dialyxir is theoretically able to find bugs there

So, what I use currently to avoid bad data popping in at runtime is this approach:

defmodule TypeIt.Progress do

  use Domo

  @all_states %{
    backlog: "Backlog",
    todo: "Todo",
    doing: "Doing",
    pending: "Pending",
    done: "Done",
    archived: "Archived",
  }

  @states Map.keys(@all_states)

  typedstruct do
    field :state, :backlog | :todo | :doing | :pending | :done | :archived
    field :title, String.t()
    field :since, NaiveDateTime.t()
  end

  def default(), do: new_for!(:todo)

  def next_state(:backlog), do: :todo
  def next_state(:todo), do: :done
  def next_state(:done), do: :todo

  def new_for!(state), do: new!(state: state, title: @all_states[state], since: NaiveDateTime.utc_now())
  def new_for!(state, since: since), do: new!(state: state, title: @all_states[state], since: since)
  def new_for!(state, title: title), do: new!(state: state, title: title, since: NaiveDateTime.utc_now())

  def states() do
    @states
  end

  def all() do
    @all_states
  end
end

For what I understand you want to make it possible only with compile time checks, but from my understanding that’s not possible in the BEAM, but I really hope you find a solution to be only compile time check. Let me know when you find it that I can help you testing it.

The Domo library I use here adds type specs for me when the code is compiled to help Dialyzer to catch as much as possible, and the rest I have to code it by my self to be checked at runtime, but with the caveat that one using the Struct can always bypass what I implemented by not calling the provided functions to create and manipulate the struct.

Another approach would be to use a 2-tuple with the first element denoting the “newtype”:

defmodule Location do
  def new(name), do: {:location, name}
end

(resemblance to Erlang records not entirely accidental)

Then the callsite looks the same as in the struct case:

MyModule.new("Metallica", Location.new("U.S."), Genre.new("Heavy Metal"))

But the implementation is a little different:

@spec new(artist :: Artist.t, country :: Country.t, genre :: Genre.t) :: Artist.t
def new({:artist, n}, {:country, c}, {:genre, g}) do
  %Artist{
    name: n, country: c, genre: g
  }
end

An additional thought: that signature for new/3 looks a lot like a keyword list without the list-ness.

Named arguments wouldn’t prevent mis-configuration quite as well as types, but would produce a moderately-string error signal since writing artist: params[:country] looks weird

Mixing your suggestion with @LostKobrakai suggestion, a possible implementation would be:

defmodule Location do
   @type t :: {:location, String.t}

   def new(name), do: {:location, name}
  
   def get({:location, name}), do: name
end

User code:

loc = Location.new("U.S.")

# Instead of Scala's `loc.name` we would do `Location.get(loc)`
locataion_name = Location.get(loc) 

In MyModule:

@spec new(artist_name :: String.t, country :: Country.t, genre :: Genre.t) :: Artist.t
def new(artist_name, country, genre) do
  %Artist{
    name: artist_name, country: Country.get(country), genre: Genre.get(genre)
  }
end

MyModule.new("Metallica", Location.new("U.S."), Genre.new("Heavy Metal"))

I can honestly see both options working.
In both cases, we would get a compiler warning (via Dialyzer or Gradient) for calling the function with parameters swapped.