Model Support Function Extension

nectar-web-product.ex

## Library Code
## Defines DSL to be used by Service Code
## in order to get properly consumed by Consumer
defmodule Nectar.ModelExtension do
  defmacro __using__(_opts) do
    quote do
      Module.register_attribute(__MODULE__, :method_block, accumulate: true)
      import Nectar.ModelExtension, only: [include_method: 1]
      @before_compile Nectar.ModelExtension
      
      defmacro __using__(_opts) do
        quote do
          # can be generlaised by unquote(__MODULE__)
          import Nectar.ExtendProduct, only: [include_methods: 0]
          @before_compile Nectar.ExtendProduct
        end
      end

    end
  end
  
  # Part of Method Addition DSL
  defmacro include_method([do: block]) do
    support_fn = Macro.escape(block)
    quote bind_quoted: [support_fn: support_fn] do
      Module.put_attribute(__MODULE__, :method_block, support_fn)
    end
  end

  defmacro __before_compile__(_env) do
    quote do
      defmacro include_methods do
        @method_block
      end

      ## before_compile hook as needed in ExtendProduct for Product
      defmacro __before_compile__(_env) do
        quote do
          include_methods
        end
      end
    end
  end
end

## Service Code
defmodule Nectar.ExtendProduct do
  ## Makes DSL available defined in library code
  use Nectar.ModelExtension

  include_method do: (def fn_from_outside, do: "support function")
  include_method do: (def get_name(product), do: product.name)
end

defmodule Nectar.Product do
  ...
  # Add new support function here
  ## Make Service Code available to be consumed
  ## through Library Code
  use Nectar.ExtendProduct
  ## Service Consumer Code
  ## not a direct macro
  ## @before_compile Nectar.ExtendProduct plays that role :)
  ...
end