Add Mox.deny/3 (#146)

Allow denying a call to a mock with clearer intentions.

Closes #145.

lib/mox.ex

@@ -507,7 +507,7 @@ defmodule Mox do
 
       expect(MockWeatherAPI, :get_temp, 5, fn _ -> {:ok, 30} end)
 
-  To expect `MockWeatherAPI.get_temp/1` not to be called:
+  To expect `MockWeatherAPI.get_temp/1` not to be called (see also `deny/3`):
 
       expect(MockWeatherAPI, :get_temp, 0, fn _ -> {:ok, 30} end)
 
@@ -538,7 +538,31 @@ defmodule Mox do
   def expect(mock, name, n \\ 1, code)
       when is_atom(mock) and is_atom(name) and is_integer(n) and n >= 0 and is_function(code) do
     calls = List.duplicate(code, n)
-    add_expectation!(mock, name, code, {n, calls, nil})
+    arity = arity(code)
+    add_expectation!(mock, name, arity, {n, calls, nil})
+    mock
+  end
+
+  @doc """
+  Ensures that `name`/`arity` in `mock` is not invoked.
+
+  When `deny/3` is invoked, any previously declared `stub` for the same `name` and arity will
+  be removed. This ensures that `deny` will fail if the function is called. If a `stub/3` is
+  invoked **after** `deny/3` for the same `name` and `arity`, the stub will be used instead, so
+  `deny` will have no effect.
+
+  ## Examples
+
+  To expect `MockWeatherAPI.get_temp/1` to never be called:
+
+      deny(MockWeatherAPI, :get_temp, 1)
+
+  """
+  @doc since: "1.2.0"
+  @spec deny(mock, atom(), non_neg_integer()) :: mock when mock: t()
+  def deny(mock, name, arity)
+      when is_atom(mock) and is_atom(name) and is_integer(arity) and arity >= 0 do
+    add_expectation!(mock, name, arity, {0, [], nil})
     mock
   end
 
@@ -563,7 +587,8 @@ defmodule Mox do
   @spec stub(mock, atom(), function()) :: mock when mock: t()
   def stub(mock, name, code)
       when is_atom(mock) and is_atom(name) and is_function(code) do
-    add_expectation!(mock, name, code, {0, [], code})
+    arity = arity(code)
+    add_expectation!(mock, name, arity, {0, [], code})
     mock
   end
 
@@ -631,9 +656,12 @@ defmodule Mox do
     |> List.flatten()
   end
 
-  defp add_expectation!(mock, name, code, value) do
+  defp arity(code) do
+    :erlang.fun_info(code)[:arity]
+  end
+
+  defp add_expectation!(mock, name, arity, value) do
     validate_mock!(mock)
-    arity = :erlang.fun_info(code)[:arity]
     key = {mock, name, arity}
 
     unless function_exported?(mock, name, arity) do

test/mox_test.exs

@@ -345,6 +345,61 @@ defmodule MoxTest do
     end
   end
 
+  describe "deny/3" do
+    test "allows asserting that function is not called" do
+      deny(CalcMock, :add, 2)
+
+      msg = ~r"expected CalcMock.add/2 to be called 0 times but it has been called once"
+
+      assert_raise Mox.UnexpectedCallError, msg, fn ->
+        CalcMock.add(2, 3) == 5
+      end
+    end
+
+    test "raises if a non-mock is given" do
+      assert_raise ArgumentError, ~r"could not load module Unknown", fn ->
+        deny(Unknown, :add, 2)
+      end
+
+      assert_raise ArgumentError, ~r"module String is not a mock", fn ->
+        deny(String, :add, 2)
+      end
+    end
+
+    test "raises if function is not in behaviour" do
+      assert_raise ArgumentError, ~r"unknown function oops/2 for mock CalcMock", fn ->
+        deny(CalcMock, :oops, 2)
+      end
+
+      assert_raise ArgumentError, ~r"unknown function add/3 for mock CalcMock", fn ->
+        deny(CalcMock, :add, 3)
+      end
+    end
+
+    test "raises even when a stub is defined" do
+      stub(CalcMock, :add, fn _, _ -> :stub end)
+      deny(CalcMock, :add, 2)
+
+      assert_raise Mox.UnexpectedCallError, fn ->
+        CalcMock.add(2, 3)
+      end
+    end
+
+    test "raises if you try to add expectations from non global process" do
+      set_mox_global()
+
+      Task.async(fn ->
+        msg =
+          ~r"Only the process that set Mox to global can set expectations/stubs in global mode"
+
+        assert_raise ArgumentError, msg, fn ->
+          deny(CalcMock, :add, 2)
+        end
+      end)
+      |> Task.await()
+    end
+  end
+
   describe "verify!/0" do
     test "verifies all mocks for the current process in private mode" do
       set_mox_private()