Add credo to the project and:

- Run mix credo --all to identify possible code optimizations
 - Resolve most of the errors generated by credo such as:
   - Numbers larger than 9999 should be written with underscores: 58_127
   - Modules should have a @moduledoc tag
   - Comparison will always return true

lib/display/colours.ex

@@ -1,4 +1,5 @@
 defmodule Display.Paint do
+  @moduledoc false
   def red(str), do: painter().red(str)
   def cyan(str), do: painter().cyan(str)
   def green(str), do: painter().green(str)
@@ -13,6 +14,7 @@ defmodule Display.Paint do
 end
 
 defmodule Display.Colours do
+  @moduledoc false
   alias IO.ANSI
 
   def red(str), do: colourize(ANSI.red(), str)
@@ -26,6 +28,7 @@ defmodule Display.Colours do
 end
 
 defmodule Display.Uncoloured do
+  @moduledoc false
   def red(str), do: str
   def cyan(str), do: str
   def green(str), do: str

lib/display/failure.ex

@@ -1,4 +1,5 @@
 defmodule Display.Failure do
+  @moduledoc false
   alias Display.Paint
 
   @no_value :ex_unit_no_meaningful_value

lib/display/intro.ex

@@ -1,11 +1,12 @@
 defmodule Display.Intro do
+  @moduledoc false
   alias Display.Paint
 
   def intro(module, modules) do
-    if not (module in modules) do
-      show_intro(module.intro)
-    else
+    if module in modules do
       ""
+    else
+      show_intro(module.intro)
     end
   end
 

lib/display/notification.ex

@@ -1,4 +1,5 @@
 defmodule Display.Notifications do
+  @moduledoc false
   alias Display.Paint
 
   def congratulate do
@@ -13,8 +14,7 @@ defmodule Display.Notifications do
   defp module_names(modules) do
     modules
     |> Enum.map(&Atom.to_string/1)
-    |> Enum.map(&name/1)
-    |> Enum.join(", ")
+    |> Enum.map_join(", ", &name/1)
     |> Paint.red()
   end
 

lib/display/progress_bar.ex

@@ -1,4 +1,5 @@
 defmodule Display.ProgressBar do
+  @moduledoc false
   @progress_bar_length 30
 
   def progress_bar(%{current: current, total: total}) do