feat: add `watch command

lib/exlings/cli.ex

@@ -15,8 +15,8 @@ defmodule Exlings.CLI do
   Nexus.help()
 
   defcommand :list, type: :null, doc: "Print all available exercises"
-
   defcommand :verify, type: :null, doc: "Verifies all available exefcises"
+  defcommand :watch, type: :null, doc: "Waits for a change int oany exercise"
 
   @hint_help "Shows a hint for the specified exercise"
   defcommand :hint, type: :string, required: true, doc: @hint_help
@@ -48,6 +48,11 @@ defmodule Exlings.CLI do
     UI.write(:green, "You passed all the exercises")
   end
 
+  def handle_input(:watch) do
+    {:ok, _} = Exlings.FileWatcher.start_link(callback: &run_watched_exercise/1)
+    wait()
+  end
+
   @impl true
   def handle_input(:hint, %{value: exercise}) do
     if e = Exercises.find_by(name: exercise) do
@@ -73,6 +78,32 @@ defmodule Exlings.CLI do
     end
   end
 
+  @spec run_watched_exercise(Path.t()) :: :ok
+  defp run_watched_exercise(path) do
+    if e = Exercises.find_by(path: path) do
+      handle_exercise_result(e)
+    else
+      UI.write("Exercise with path #{path} not found!")
+    end
+  end
+
+  defp wait do
+    i = IO.read(:line)
+
+    cond do
+      String.match?(i, ~r"quit") ->
+        UI.write(:green, "Bye by exlings o/")
+        System.halt(0)
+
+      String.match?(i, ~r"exit") ->
+        UI.write(:green, "Bye by exlings o/")
+        System.halt(0)
+
+      true ->
+        wait()
+    end
+  end
+
   defp handle_exercise_result(%Exercise{} = e) do
     case Exercises.run(e) do
       {:stdout, out} -> success(e, out)

lib/exlings/exercises.ex

@@ -23,11 +23,15 @@ defmodule Exlings.Exercises do
     Enum.find(list(), &(Exercise.get_state(&1) == :pending))
   end
 
-  @spec find_by(name: String.t()) :: Exercise.t() | nil
+  @spec find_by([name: String.t()] | [path: Path.t()]) :: Exercise.t() | nil
   def find_by(name: name) do
     Enum.find(list(), &(&1.name == name))
   end
 
+  def find_by(path: path) do
+    Enum.find(list(), &(&1.path =~ path))
+  end
+
   @spec run(Exercise.t()) :: {:stdout, binary} | {:stderr, binary} | {:error, binary}
   def run(%Exercise{} = e) do
     if exec = System.find_executable("elixir") do

lib/exlings/file_watcher.ex

@@ -0,0 +1,32 @@
+defmodule Exlings.FileWatcher do
+  @moduledoc false
+
+  use GenServer
+
+  def start_link(args) do
+    GenServer.start_link(__MODULE__, args, name: __MODULE__)
+  end
+
+  @impl true
+  def init(args) do
+    priv = List.to_string(:code.priv_dir(:exlings))
+    path = Path.join(priv, "exercises")
+    {:ok, watcher} = FileSystem.start_link(dirs: [path], name: :exlings)
+    FileSystem.subscribe(:exlings)
+    {:ok, %{watcher: watcher, callback: args[:callback]}}
+  end
+
+  @impl true
+  def handle_info({:file_event, w, {path, e}}, %{watcher: w} = state) do
+    case e do
+      [_, :modified] -> state.callback.(Path.basename(path))
+      _ -> nil
+    end
+
+    {:noreply, state}
+  end
+
+  def handle_info({:file_event, w, :stop}, %{watcher: w} = state) do
+    {:stop, :normal, state}
+  end
+end