From 65f4e93069c681ad01b4e44e86ccd9d46cb65782 Mon Sep 17 00:00:00 2001
From: Paul Fleischer <pf@qarmainspect.com>
Date: Thu, 27 Jun 2024 17:30:49 +0200
Subject: [PATCH] Handle undefined modules when patching and raise
 UndefinedFunctionError

---
 lib/patch.ex                            |  9 ++++++++-
 test/user/patch/unknown_module_test.exs | 16 ++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 test/user/patch/unknown_module_test.exs

diff --git a/lib/patch.ex b/lib/patch.ex
index e24c6b9..b530556 100644
--- a/lib/patch.ex
+++ b/lib/patch.ex
@@ -489,7 +489,14 @@ defmodule Patch do
   """
   @spec patch(module :: module(), function :: atom(), value :: Value.t()) :: Value.t()
   def patch(module, function, %value_module{} = value) when is_value(value_module) do
-    {:ok, _} = Patch.Mock.module(module)
+    case Patch.Mock.module(module) do
+      {:ok, _} ->
+        :ok
+
+      {:error, :nofile} ->
+        raise %UndefinedFunctionError{module: module, function: function, arity: 0}
+    end
+
     :ok = Patch.Mock.register(module, function, value)
     value
   end
diff --git a/test/user/patch/unknown_module_test.exs b/test/user/patch/unknown_module_test.exs
new file mode 100644
index 0000000..05bcde8
--- /dev/null
+++ b/test/user/patch/unknown_module_test.exs
@@ -0,0 +1,16 @@
+defmodule Patch.Test.User.Patch.UnknownModuleTest do
+  use ExUnit.Case
+  use Patch
+
+  describe "patching unknown module" do
+    test "results in an exception" do
+      assert_raise UndefinedFunctionError, fn ->
+        patch(NoSuchModule, :function_that_does_not_exist, :patched)
+      end
+
+      assert_raise UndefinedFunctionError, fn ->
+        patch(NoSuchModule, :function_that_does_not_exist, fn _x -> 42 end)
+      end
+    end
+  end
+end