Ran mix format & fixed syntax warnings, updated dependencies & specified an elixir version.

Also added :ecto to the application list for :dev to resolve a strange compilation issue:
 - `mix compile` fails complaining it can't find ecto.
 - `mix test` runs fine.
 - compiling a downstream project also breaks.

Author: montebrown

.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.tool-versions

@@ -0,0 +1,2 @@
+elixir 1.17.3
+erlang 26.2.5.6

config/config.exs

@@ -1,6 +1,6 @@
 # This file is responsible for configuring your application
 # and its dependencies with the aid of the Mix.Config module.
-use Mix.Config
+import Config
 
 # This configuration is loaded before any dependency and is restricted
 # to this project. If another project depends on this project, this
@@ -28,6 +28,6 @@ use Mix.Config
 # here (which is why it is important to import them last).
 #
 
-if Mix.env == :test do
+if Mix.env() == :test do
   import_config "test.exs"
 end

config/test.exs

@@ -1,4 +1,6 @@
-use Mix.Config
+import Config
+
+config :as_nested_set, ecto_repos: [AsNestedSet.TestRepo]
 
 config :as_nested_set, AsNestedSet.TestRepo,
   hostname: "localhost",

config/test.exs.travis

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 config :as_nested_set, AsNestedSet.TestRepo,
   hostname: "localhost",

lib/as_nested_set.ex

@@ -1,6 +1,7 @@
 defmodule AsNestedSet do
   defmacro __using__(args) do
     scope = Keyword.get(args, :scope, [])
+
     quote do
       use AsNestedSet.Model
       use AsNestedSet.Scoped, scope: unquote(scope)
@@ -9,7 +10,7 @@ defmodule AsNestedSet do
 
   @type t :: struct
 
-  @type executable :: (Ecto.Repo.t -> any)
+  @type executable :: (Ecto.Repo.t() -> any)
 
   @spec defined?(struct) :: boolean
   def defined?(struct) when is_atom(struct) do
@@ -21,12 +22,14 @@ defmodule AsNestedSet do
         false
     end
   end
+
   def defined?(%{__struct__: struct}) do
     defined?(struct)
   end
+
   def defined?(_), do: false
 
-  @spec execute(executable, Ecto.Repo.t) :: any
+  @spec execute(executable, Ecto.Repo.t()) :: any
   def execute(call, repo) do
     call.(repo)
   end

lib/as_nested_set/helper.ex

@@ -1,5 +1,4 @@
 defmodule AsNestedSet.Helper do
-
   def get_column_name(%{__struct__: struct}, field) do
     get_column_name(struct, field)
   end
@@ -19,6 +18,4 @@ defmodule AsNestedSet.Helper do
   def fields(module) when is_atom(module) do
     module.__as_nested_set_fields__()
   end
-
-
-end
+end

lib/as_nested_set/model.ex

@@ -1,5 +1,4 @@
 defmodule AsNestedSet.Model do
-
   defmacro __using__(_) do
     quote do
       @node_id_column :id
@@ -18,36 +17,43 @@ defmodule AsNestedSet.Model do
   end
 
   defp define_accessors(names, env) do
-    fields = Enum.map(names, fn
-      name ->
-        attribute_name = String.to_atom("#{name}_column")
-        column_name = Module.get_attribute(env.module, attribute_name)
-        {name, column_name}
-    end) |> Enum.into(%{})
+    fields =
+      Enum.map(names, fn
+        name ->
+          attribute_name = String.to_atom("#{name}_column")
+          column_name = Module.get_attribute(env.module, attribute_name)
+          {name, column_name}
+      end)
+      |> Enum.into(%{})
 
     Enum.map(fields, fn
-       {name, column_name}->
+      {name, column_name} ->
         quote do
           def __as_nested_set_column_name__(unquote(name)) do
             unquote(column_name)
           end
+
           def __as_nested_set_get_field__(model, unquote(name)) do
             Map.get(model, unquote(column_name))
           end
+
           def __as_nested_set_set_field__(model, unquote(name), value) do
             Map.put(model, unquote(column_name), value)
           end
         end
-    end) ++ [
-      quote do
-        def __as_nested_set_field__(field) do
-          raise ArgumentError, "Unknown AsNestedSet field #{inspect field} for #{inspect __MODULE__}"
+    end) ++
+      [
+        quote do
+          def __as_nested_set_field__(field) do
+            raise ArgumentError,
+                  "Unknown AsNestedSet field #{inspect(field)} for #{inspect(__MODULE__)}"
+          end
+
+          def __as_nested_set_get_field__(model, _), do: nil
+          def __as_nested_set_set_field__(model, _, _), do: model
+          def __as_nested_set_fields__(), do: unquote(fields |> Macro.escape())
         end
-        def __as_nested_set_get_field__(model, _), do: nil
-        def __as_nested_set_set_field__(model, _, _), do: model
-        def __as_nested_set_fields__(), do: unquote(fields |> Macro.escape)
-      end
-    ]
+      ]
   end
 
   defp define_queriers(_env) do