Behaviours with Defaults for Elixir

default_behaviour.ex

defmodule Default.Behaviour do
  @moduledoc """
  Creates a behaviour that carries its own default implementation.

  When used into a behaviour module, when that module in turn is used, all functions
  defined on it are given to the using module.
  
  This allows you to have concrete implementations of the behaviour's default functionality
  for testing, unlike cramming them all into a __using__ macro.
  
  When the behaviour is used all provided functions are correctly annotated with their
  `@impl` referring to it, and all made overridable.

  You can provide two options when using the resulting behaviour module:
  
  - `:docs` (default: true) will copy over the documentation for each function
  
  - `:inline` (default: false) will inline the default function's implementation,
  instead of just proxying to the implementation on the behaviour module, which is the
  standard behaviour.
  
  Example:
  
  ```elixir
  defmodule Custom.Behaviour do
    use Default.Behaviour
    
    @callback foo :: atom
    @doc "Computes foo, returns bar."
    def foo, do: :bar
  end
  
  defmodule Fizz do
    use Custom.Behaviour
  end
  
  defmodule Buzz do
    use Custom.Behaviour
    @doc "Computes foo, returns baz."
    def foo, do: :baz
  end
  
  Fizz.foo #=> :bar
  Buzz.foo #=> :baz
  ```
  """

  def __on_definition__(env, kind, name, params, guards, body) do
    doc = Module.get_attribute(env.module, :doc)
    Module.put_attribute(env.module, :__functions__, {doc, kind, name, params, guards, body})
  end

  defmacro __before_compile__(_env) do
    quote do
      @doc false
      def __functions__, do: @__functions__
    end
  end

  defmacro __using__(opts \\ []) do
    code = Keyword.get(opts, :do)
    quote do

      Module.register_attribute(__MODULE__, :__functions__, accumulate: true)
      @on_definition Default.Behaviour
      @before_compile Default.Behaviour

      defmacro __using__(opts \\ []) do
        docs = Keyword.get(opts, :docs, true)
        inline = Keyword.get(opts, :inline, false)

        defaults = for {doc, kind, name, params, guards, body} <- __MODULE__.__functions__ do
          info = %{module: __MODULE__, kind: kind, docs: docs, inline: inline}
          Default.Behaviour.compose_default(info, doc, name, params, guards, body)
        end

        [
          quote(do: @behaviour __MODULE__),
          defaults,
          quote(do: defoverridable __MODULE__),
          unquote(code),
        ] |> List.flatten |> Enum.filter(&(&1))

      end
    end
  end

  # Ignore macros
  def compose_default(%{kind: kind}, _doc, _name, _params, _guards, _body)
    when not kind in ~w[def defp]a, do: nil

  # If we are inlining, we may need any and all functions, private ones included
  def compose_default(%{inline: true} = info, doc, name, params, guards, body) do
    compose_definition(info, doc, name, params, guards, body)
  end

  # Otherwise we are only interested in public functions
  def compose_default(%{kind: :def, module: module} = info, doc, name, params, guards, _body) do
    delegate = compose_delegate(module, name, params)
    [
      compose_module_attribute(:impl, module),
      compose_definition(info, doc, name, params, guards, delegate),
    ]
  end

  # Throw away anything else
  def compose_default(_info, _doc, _name, _params, _guards, _body), do: nil

  defp compose_delegate(module, name, params) do
    args = Enum.map(params, fn
      {:\\, _, [arg, _default]} -> arg
      arg -> arg
    end)
    compose_application(module, name, args)
  end

  defp compose_definition(info = %{kind: :def}, doc, name, params, guards, body) do
    [
      compose_docs(info, doc),
      compose_function(:def, name, params, guards, body),
    ]
  end

  defp compose_definition(%{kind: :defp}, _doc, name, params, guards, body) do
    compose_function(:defp, name, params, guards, body)
  end

  defp compose_docs(%{docs: false} = info, _doc), do: compose_docs(info, false)
  defp compose_docs(_info, {_, doc}) when is_binary(doc) do
    compose_module_attribute(:doc, doc)
  end
  defp compose_docs(_info, _doc) do
    compose_module_attribute(:doc, false)
  end

  defp compose_function(:def, name, params, guards, body) do
    quote do
      def unquote(compose_definition(name, params, guards)) do
        unquote(body)
      end
    end
  end

  defp compose_function(:defp, name, params, guards, body) do
    quote do
      defp unquote(compose_definition(name, params, guards)) do
        unquote(body)
      end
    end
  end

  defp compose_definition(name, params, []) do
    compose_call(name, params)
  end
  defp compose_definition(name, params, guards) do
    Enum.reduce(guards, compose_call(name, params), fn guard, node ->
      {:when, [], [node, guard]}
    end)
  end

  defp compose_call(name, params) do
    {name, [], params}
  end

  defp compose_module_attribute(attribute, value) do
    {:@, [], [
      {attribute, [], [value]}
    ]}
  end

  defp compose_application(module, function, args) do
    {:apply, [], [module, function, args]}
  end

end