ExCut defines an annotation construct that wraps a regular function and enable it to be decorated with a cross-cutting concern. This provide a clean mechanism to inject resusable behavior without cluttering your code base.

Let’s take a look at ExCut in action to add logging behavior to a set of functions.

defmodule Blee do
  use ExCut, marker: :log, pre: :pre_log, post: :post_log

  require Logger

  # Basic annotation setting a log level
  @log level: :info
  def elvis(a, b), do: a + b

  # You can also override module pre or post within an annotation
  @log level: :debug, pre: :cust_pre_log
  def elvis(a, b) when is_binary(a), do: a <> b

  @log level: :warm
  def elvis(a, b) when is_boolean(a), do: b

  defp pre_log(ctx) do
    ">>> #{ctx.target} with args #{ctx.args |> inspect}"
    |> log(ctx)
  end

  defp post_log(ctx, _pre, _res) do
    "<<< #{ctx.target} with args #{ctx.args |> inspect}"
    |> log(ctx)
  end

  defp cust_pre_log(ctx) do
    "[CUSTOM!] >>> #{ctx.target} with args #{ctx.args |> inspect}"
    |> log(ctx)
  end

  defp log(m, ctx) do
    ctx.meta[:level]
    |> case do
      :warn  -> m |> Logger.warn
      :debug -> m |> Logger.debug
      _      -> m |> Logger.info
    end
  end
end

ExCut provisions an ExCut.Context with call details and metadata that comes from the annotation. You can leverage this information in your cross-cutting functions.