How to define Macro for a new Type?

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1

Background

So, I am playing around with a concept named “NewType” and I am taking inspiration from languages like F# and Scala.

My objective, for learning purposes mostly, is to build a macro that makes creating this abstraction something that takes no more than a single line of code.

Intended usage

I would like to create a macro that allows me to do something like this:

defmodule User do
  require NewType # an absolutely original name for the macro :D

  deftype Name, String.t() # Usage of said macro. Here I am defining a new type called "Name"

  @enforce_keys [:name, :age]
  defstruct [:name, :age]
  @type t :: %__MODULE__{
          name: Name.t,
          age: integer()
        }

  @spec new(Name.t, integer) :: User.t
  def new(name, age), do: %User{name: name, age, age}  
end

And now, here is how I could create a User:

defmodule Test do
  alias User
  import User.Name

  @spec run :: User.t
  def run do
    name = Name("John")
    User.new(name, 25)
  end
end

How to implement this interface?

This interface might remind you a little of the Record interface. That’s because I think its API has some good ideas I would like to explore.

So, as a starting point I tried reading the source code for Record, but I was not really able to pick it up and use it to create an implementation for my use case, mainly because I don’t need/want to interface with Erlang records at all.

So, an implementation possibility would be to, under the hood, turn this into a tuple:

defmodule NewType do
  defmacro new(name, val) do
    quote do
      NewType.to_tuple(unquote(name), unquote(val))
    end
  end

  def to_tuple(name, val), do: {String.to_atom(name), val}
end

However, this is miles away from the interface I want to create …

Questions

  1. Using Elixir macros, is it possible to create the API I am aiming for?
  2. How can I change my code to achieve something like Name("John")?
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