yak(mutation-testing-with-muex-blogex-pure-app-code-1341): mutation testing with muex (blogex + pure app code)

blogex/mix.exs

@@ -48,7 +48,8 @@ defmodule Blogex.MixProject do
       {:plug, "~> 1.15"},
       {:ex_doc, "~> 0.34", only: :dev, runtime: false},
       {:rustler, "~> 0.38.0", optional: true},
-      {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false}
+      {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false},
+      {:muex, "~> 0.12", only: [:dev, :test], runtime: false}
     ]
   end
 
@@ -91,12 +92,74 @@ defmodule Blogex.MixProject do
   # Same `precommit` alias name and shape as app/mix.exs, so both apps share
   # one loop: compile (warnings fatal), format, test. Heavy sensors belong in
   # the Lens pass (`make verify`), not here.
+  #
+  # `mutate` / `mutate.fast` are a sensor, not a gate: they are deliberately
+  # absent from Makefile's VERIFY_SENSORS and always pass `--fail-at 0`, so a
+  # survivor is reported rather than fatal. The scope lives in the plain-text
+  # files beside this one; the aliases read them at run time, so the text file
+  # stays the single source of truth.
   defp aliases do
     [
-      precommit: ["compile --warning-as-errors", "format", "test"]
+      precommit: ["compile --warning-as-errors", "format", "test"],
+      mutate: &mutate/1,
+      "mutate.fast": &mutate_fast/1
     ]
   end
 
+  # No macro/`use` mutators: the scoped modules are plain functions, and the
+  # macro-heavy modules (blog.ex, router.ex, components.ex, layout.ex) are out
+  # of scope entirely. Spellings are muex 0.12's CLI names.
+  @mutation_mutators "arithmetic,boolean,comparison,conditional,function_call,literal,return_value"
+
+  defp mutate(args), do: run_mutation("mutation_scope.txt", args)
+
+  defp mutate_fast(args), do: run_mutation("mutation_scope_fast.txt", args)
+
+  defp run_mutation(scope_file, args) do
+    Mix.Task.run("muex", mutation_args(scope_file, args))
+  end
+
+  @doc false
+  def mutation_args(scope_file, extra_args \\ []) do
+    files = read_mutation_scope(scope_file)
+
+    # `--fail-at 0` comes first so an ad-hoc override cannot turn the sensor
+    # into a gate. Extra args come before our defaults, and OptionParser keeps
+    # the first value, so `mix mutate --files lib/blogex/seo.ex` targets one
+    # file while `mix mutate` covers the whole scope file.
+    ["--fail-at", "0"] ++
+      extra_args ++
+      ["--files", Enum.join(files, ","), "--mutators", @mutation_mutators]
+  end
+
+  @doc false
+  def read_mutation_scope(scope_file) do
+    path = Path.expand(scope_file, Path.dirname(Mix.Project.project_file()))
+
+    files =
+      case File.read(path) do
+        {:ok, contents} ->
+          contents
+          |> String.split("\n", trim: true)
+          |> Enum.map(&String.trim/1)
+          |> Enum.reject(&(&1 == ""))
+
+        {:error, reason} ->
+          Mix.raise(
+            "mutation scope #{scope_file} is unreadable (#{:file.format_error(reason)}); " <>
+              "refusing to run the mutation sensor over an unknown scope"
+          )
+      end
+
+    if files == [] do
+      Mix.raise(
+        "mutation scope #{scope_file} is empty; refusing to run the mutation sensor over nothing"
+      )
+    end
+
+    files
+  end
+
   defp package do
     [
       licenses: ["MIT"],