Hammox is a library for rigorous unit testing using mocks, explicit behaviours and contract tests. You can use it to ensure both your mocks and implementations fulfill the same contract.

It takes the excellent Mox library and pushes its philosophy to its limits, providing automatic contract tests based on behaviour typespecs while maintaining full compatibility with code already using Mox.

Hammox aims to catch as many contract bugs as possible while providing useful deep stacktraces so they can be easily tracked down and fixed.

If you are currently using Mox, delete it from your list of dependencies in mix.exs. Then add :hammox:

def deps do
  [
    {:hammox, "~> 0.7", only: :test}
  ]
end

Read "Mocks and explicit contracts" by José Valim. Then proceed to the Mox documentation. Once you are comfortable with Mox, switch to using Hammox.

Replace all occurrences of Mox with Hammox. Nothing more is required; all your mock calls in test are now ensured to conform to the behaviour typespec.

Let's say we have a database which can get us user data. We have a module, RealDatabase (not shown), which implements the following behaviour:

defmodule Database do
  @callback get_users() :: [binary()]
end

We use this client in a Stats module which can aggregate data about users:

defmodule Stats do
  def count_users(database \\ RealDatabase) do
    length(database.get_users())
  end
end

And we create a unit test for it:

defmodule StatsTest do
  use ExUnit.Case, async: true

  test "count_users/0 returns correct user count" do
    assert 2 == Stats.count_users()
  end
end

For this test to work, we would have to start a real instance of the database and provision it with two users. This is of course unnecessary brittleness — in a unit test, we only want to test that our Stats code provides correct results given specific inputs. To simplify, we will create a mocked Database using Mox and use it in the test:

defmodule StatsTest do
  use ExUnit.Case, async: true
  import Mox

  test "count_users/0 returns correct user count" do
    defmock(DatabaseMock, for: Database)
    expect(DatabaseMock, :get_users, fn ->
      ["joe", "jim"]
    end)

    assert 2 == Stats.count_users(DatabaseMock)
  end
end

The test now passes as expected.

Imagine that some time later we want to add error flagging for our database client. We change RealDatabase and the corresponding behaviour, Database, to return an ok/error tuple instead of a raw value:

defmodule Database do
  @callback get_users() :: {:ok, [binary()]} | {:error, term()}
end

However, The Stats.count_users/0 test will still pass, even though the function will break when the real database client is used! This is because the mock is now invalid — it no longer implements the given behaviour, and therefore breaks the contract. Even though Mox is supposed to create mocks following explicit contracts, it does not take typespecs into account.

This is where Hammox comes in. Simply replace all occurrences of Mox with Hammox (for example, import Mox becomes import Hammox, etc) and you will now get this when trying to run the test:

** (Hammox.TypeMatchError)
Returned value ["joe", "jim"] does not match type {:ok, [binary()]} | {:error, term()}.

Now the consistency between the mock and its behaviour is enforced.

Hammox automatically checks mocks with behaviours, but what about the real implementations? The real goal is to keep all units implementing a given behaviour in sync.

You can decorate any function with Hammox checks by using Hammox.protect/2. It will return an anonymous function which you can use in place of the original module function. An example test:

defmodule RealDatabaseTest do
  use ExUnit.Case, async: true

  test "get_users/0 returns list of users" do
    get_users_0 = Hammox.protect({RealDatabase, :get_users, 0}, Database)
    assert {:ok, ["real-jim", "real-joe"]} == get_users_0.()
  end
end

It's a good idea to put setup logic like this in a setup_all hook and then access the protected functions using the test context:

defmodule RealDatabaseTest do
  use ExUnit.Case, async: true

  setup_all do
    %{get_users_0: Hammox.protect({RealDatabase, :get_users, 0}, Database)}
  end

  test "get_users/0 returns list of users", %{get_users_0: get_users_0} do
    assert {:ok, ["real-jim", "real-joe"]} == get_users_0.()
  end
end

Hammox also provides a setup_all friendly version of Hammox.protect which leverages this pattern. Simply pass both the implementation module and the behaviour module and you will get a map of all callbacks defined by the behaviour as decorated implementation functions.

defmodule RealDatabaseTest do
  use ExUnit.Case, async: true

  setup_all do
    Hammox.protect(RealDatabase, Database)
  end

  test "get_users/0 returns list of users", %{get_users_0: get_users_0} do
    assert {:ok, ["real-jim", "real-joe"]} == get_users_0.()
  end
end

Alternatively, if you're up for trading explicitness for some macro magic, you can use use Hammox.Protect to locally define protected versions of functions you're testing, as if you imported the module:

defmodule RealDatabaseTest do
  use ExUnit.Case, async: true
  use Hammox.Protect, module: RealDatabase, behaviour: Database

  test "get_users/0 returns list of users" do
    assert {:ok, ["real-jim", "real-joe"]} == get_users()
  end
end

Dialyzer cannot detect Mox style mocks not conforming to typespec.

The main aim of Hammox is to enforce consistency between behaviours, mocks and implementations. This is best achieved when both mocks and implementations are subjected to the exact same checks.

Dialyzer is a static analysis tool; Hammox is a dynamic contract test provider. They operate differently and one can catch some bugs when the other doesn't. While it is true that Hammox would be redundant given a strong, strict, TypeScript-like type system for Elixir, Dialyzer is far from providing that sort of coverage.

A t() type defined on a protocol is taken by Hammox to mean "a struct implementing the given protocol". Therefore, trying to pass :atom for an Enumerable.t() will produce an error, even though the type is defined as term():

** (Hammox.TypeMatchError)
Returned value :atom does not match type Enumerable.t().
  Value :atom does not implement the Enumerable protocol.

Hammox also includes Mox as a dependency. This means that if you would like to disable Hammox protection for a specific mock, you can simply use vanilla Mox for that specific instance. They will interoperate without problems.

• For anonymous function types in typespecs, only the arity is checked. Parameter types and return types are not checked.

Hammox now includes telemetry events! See Telemetry Guide for more information.

Copyright 2019 Michał Szewczak

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.