Make ex_constructor a TypedStruct plugin

[ejpcmac]

Since upstream TypedStruct 0.2.0 now supports plugins, Contructor could be a plugin. This way, it would be composable with other TypedStruct plugins.

The API would look like:

defmodule ConstructorExampleUser do
  use TypedStruct

  # Use the “standard” typedstruct macro.
  typedstruct do
    plugin Constructor # Register Constructor as a plugin. Code can be injected here.
    plugin TypedStructLens # Let’s say we also want to generate lenses.

    field :id, integer(), constructor: &is_integer/1, enforce: true
    field :role,  :user | :admin, constructor: &is_valid_role/1, enforce: true
    field :first_name, String.t(), default: "", constructor: &is_string/1
    field :last_name, String.t(), default: "", constructor: &is_string/1
  end

  def is_valid_role(value) do
    case value do
      :admin -> {:ok, value}
      :user -> {:ok, value}
      _ -> {:error, "invalid role!"}
    end
  end
end

[uberbrodt]

@ejpcmac Thanks! Sorry, lost track of this for a bit, but I think this should be simple enough to do.

[ejpcmac]

No problem, Open Source on your spare time shouldn’t be a pressure :)