Add exercise 3 from chapter 21

chapter_21/linking_modules_behaviour_and_use_3.exs

@@ -0,0 +1,82 @@
+defmodule Tracer do
+  def dump_args(args) do
+    args
+    |> Enum.map(&inspect/1)
+    |> Enum.join(", ")
+  end
+
+  def dump_defn(name, args), do:
+    "#{name}(#{dump_args(args)})"
+
+  defmacro def(definition = {:when, [{name, _, args}, _guard]}, do: content), do:
+    traced_def(definition, name, args, content)
+  defmacro def(definition = {name, _, args}, do: content), do:
+    traced_def(definition, name, args, content)
+
+  defp traced_def(definition, name, args, content) do
+    quote do
+      Kernel.def(unquote(definition)) do
+        IO.puts """
+         #{IO.ANSI.blue}\
+         ==> method call: \
+         #{IO.ANSI.green}\
+         #{IO.ANSI.bright}\
+         #{Tracer.dump_defn(unquote(name), unquote(args))}\
+        """
+        result = unquote(content)
+        IO.puts "#{IO.ANSI.yellow}\ <== result: #{result}"
+        result
+      end
+    end
+  end
+
+  defmacro __using__(_opts) do
+    quote do
+      import Kernel, except: [def: 2]
+      import unquote(__MODULE__), only: [def: 2]
+    end
+  end
+end
+
+defmodule MathTracer do
+  use Tracer
+
+  def puts_sum_three(a, b, c), do:
+    IO.inspect a + b + c
+
+  def add_list(list), do:
+    Enum.reduce(list, 0, &(&1 + &2))
+
+  def is_one(x) when is_number(x) and x == 1, do:
+    "It is one!"
+end
+
+ExUnit.start
+
+defmodule MathTracerTest do
+  use ExUnit.Case
+  import ExUnit.CaptureIO
+
+  test "puts_sum_three" do
+    messages_string = " \e[34m ==> method call:  \e"
+      <> "[32m \e[1m puts_sum_three(1, 2, 3)\n6\n\e[33m <== result: 6\n"
+
+    expected = capture_io fn ->
+      MathTracer.puts_sum_three(1, 2, 3)
+    end
+
+    assert expected == messages_string
+  end
+
+  test "add_list" do
+
+    messages_string = " \e[34m ==> method call:  \e[32m \e"
+      <> "[1m add_list([1, 2, 3])\n\e[33m <== result: 6\n"
+
+    expected = capture_io fn ->
+      MathTracer.add_list([1, 2, 3])
+    end
+
+    assert expected == messages_string
+  end
+end