How to define Macro for a new Type?

Questions / Help
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My Answer

After reading more about macros in Elixir, talking to the community and reading about NewType, I have refined my ideas. While the exact implementation of my original idea is not possible, with some changes you can still get the core benefit of NewType.

Changes to original idea

  • No usage of Name("John") syntax. As explained in this post this syntax is not valid in Elixir.
  • No defguard. Because the type is @opaque it is not possible to have a guard that analyses the internal structure of the data without having dialyzer complaining. Since the main goal here is to have Dialyzer help me detect issues, and since the internal structure of the opaque data can only be analyzed by functions that belong to the module itself, this means this idea is not possible.
  • No verification on data type when invoking new. Originally I thought about having some verification mechanism, but this is not necessary, since dialyzer will let the user know if the user is invoking new with an incorrect parameter.
  • No self-generated functions. Instead of having Age.age? or Name.name? I have opted for the more general NewType.is_type?/2, which will accomplish the same and is more general.

Code

With these changes in mind, this is the macro I came up with:

defmodule NewType do
  defmacro deftype(name, type) do
    quote do
      defmodule unquote(name) do
        @opaque t :: {unquote(name), unquote(type)}

        @spec new(value :: unquote(type)) :: t
        def new(value), do: {unquote(name), value}

        @spec extract(new_type :: t) :: unquote(type)
        def extract({unquote(name), value}), do: value
      end
    end
  end

  @spec is_type?(data :: {atom, any}, new_type :: atom) :: boolean
  def is_type?({type, _data}, new_type) when type == new_type, do: true
  def is_type?(_data, _new_type), do: false
end

Which can be used like:

type.ex:

defmodule Type do
  import NewType

  deftype Name, String.t()
end

test.ex:

defmodule Test do
  alias Type.Name

  @spec print(Name.t()) :: binary
  def print(name), do: Name.extract(name)

  def run do
    arg = 1
    name = Name.new(arg) # dialyzer detects error !
    {:ok, name}
  end
end
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