calculus

defmodule Calculus do
  @moduledoc """
  Real smart constructors, real private and immutable fields for Elixir data types. Inspired by Alonzo Church.
  """

  @doc """
  Imports `Calculus.defcalculus/2` macro
  """
  defmacro __using__(_) do
    quote location: :keep do
      import Calculus, only: [defcalculus: 2]
    end
  end

  defp add_security_key({:-> = exp, ctx, [left, right]}) do
    key = {
      :@,
      [context: Elixir, import: Kernel],
      [{:security_key, [context: Elixir], Elixir}]
    }

    new_left =
      case left do
        [{:when, ctx0, [e | es]}] -> [{:when, ctx0, [e, key | es]}]
        [e] -> [e, key]
      end

    {exp, ctx, [new_left, right]}
  end

  @doc """
  Macro to define λ-type
  """
  defmacro defcalculus(quoted_state, do: raw_eval_clauses) do
    first_defined_eval_clauses =
      quote location: :keep do
        :return, @security_key ->
          calculus(state: state, return: return)

        :is?, @security_key ->
          calculus(state: state, return: true)
      end

    middle_eval_clauses =
      raw_eval_clauses
      |> case do
        {:__block__, _, []} -> []
        [_ | _] -> raw_eval_clauses
      end
      |> Enum.map(&add_security_key/1)

    last_eval_clauses =
      quote location: :keep do
        method, security_key ->
          raise(
            "For value of the type #{inspect(__MODULE__)} got unsupported METHOD=#{inspect(method)} with SECURITY_KEY=#{
              inspect(security_key)
            }"
          )
      end

    eval_fn = {
      :fn,
      [],
      first_defined_eval_clauses ++ middle_eval_clauses ++ last_eval_clauses
    }

    quote location: :keep do
      @security_key 64 |> :crypto.strong_rand_bytes() |> Base.encode64() |> String.to_atom()

      @opaque t :: __MODULE__.t()

      defmacrop calculus(state: state, return: return) do
        quote location: :keep do
          {unquote(state), unquote(return)}
        end
      end

      defmacrop calculus(return: return, state: state) do
        quote location: :keep do
          {unquote(state), unquote(return)}
        end
      end

      defmacrop calculus(some) do
        "Calculus expression expect keyword list, example: calculus(state: foo, return: bar), but got term #{inspect(some)}"
        |> raise
      end

      defp eval(it, method) do
        case :erlang.fun_info(it, :module) do
          {:module, __MODULE__} ->
            #
            # TODO : test that "state" and "return" can not be overriden in "quoted_state" expression
            #
            calculus(state: state, return: return) = it.(method, @security_key)
            unquote(quoted_state) = state

            case method do
              :return -> return
              :is? -> return
              _ -> unquote(eval_fn)
            end

          {:module, module} ->
            "Value of the type #{inspect(__MODULE__)} can't be created in other module #{inspect(module)}"
            |> raise
        end
      end

      @doc """
      - Accepts value of `#{inspect(__MODULE__)}` λ-type
      - Returns result of the latest called method of this value
      """
      @spec return(__MODULE__.t()) :: term
      def return(it) do
        eval(it, :return)
      end

      @doc """
      - Accepts any term
      - Returns `true` if term is value of `#{inspect(__MODULE__)}` λ-type, otherwise returns `false`
      """
      @spec is?(term) :: boolean
      def is?(it) do
        try do
          eval(it, :is?)
        rescue
          _ -> false
        end
      end

      defmacrop construct(state) do
        quote location: :keep do
          fn :new, @security_key ->
            calculus(state: unquote(state), return: :ok)
          end
          |> eval(:new)
        end
      end
    end
  end
end