defmodule TypeClass do
  # (...)

  defmacro definst(class, opts, do: body) do
    [for: datatype] = opts

    quote do

      # The type  class protocol's  instance implementation  goes  into
      # `type_class.Proto.datatype` (in the case of `Monoid`, this will
      # be  `Witchcraft.Monoid.Proto.Function`),  as `defimpl/2`  below
      # defines its own context.
      instance = Module.concat([unquote(class), Proto, unquote(datatype)])

      defimpl unquote(class).Proto, for: unquote(datatype) do
        import TypeClass.Property.Generator.Custom

        Module.register_attribute(__MODULE__, :force_type_instance, [])
        @force_type_instance false

        @doc false
        def __custom_generator__, do: false
        defoverridable __custom_generator__: 0

        unquote(body)

        @doc false
        def __force_type_instance__, do: @force_type_instance
      end

      cond do
        unquote(class).__force_type_class__() ->
          IO.warn("...")

        instance.__force_type_instance__() ->
          IO.warn("...")

        true ->
          unquote(datatype) |> conforms(to: unquote(class))
      end                 #  /
    end                  #  /
  end                   #  /
                       #  /
  # (...)           _____/
                #  /
  defmacro conforms(datatype, opts) do
    class = Keyword.get(opts, :to)

    quote do
      # (Dependency check here, omitted for brevity.)

      for {prop_name, _one} <- unquote(class).Property.__info__(:functions) do
        TypeClass.Property.run!(unquote(datatype), unquote(class), prop_name)
      end
    end
  end
end