In functional languages you should write pure functions. However, sometimes we need functions to call external API’s that affect the state of the system. So these functions are impure. In non-functional languages you create mocks to test expectations. For example, you might create a mock of a repository and the test checks it calls the save function. You are testing a side effect. This is something you should avoid in functional languages.

Instead of mocks we should use stubs. Mocking frameworks tend to treat them as interchangeable and this makes it hard to tell them apart. So it is good to have a simple definition. Quoting Martin Fowler:

Monthly Downloads: 1,778
Programming language: Elixir
License: MIT License
Tags: Testing     Stubbing Framework     Stubbing     API    
Latest version: v1.4

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Stubr is a set of functions helping people to create stubs and spies in Elixir.


Elixir is a functional language, so you should aim to write pure functions. However, sometimes you need to call external API’s or check the current time. Since these actions can have side effects, they make it harder to unit test your system.

Stubr solves this problem by taking cues from mocks and explicit contracts. It provides a set of functions that help people create "mocks as nouns" and not "mocks as verbs":

iex> stub = Stubr.stub!([foo: fn _ -> :ok end], call_info: true)
iex> stub.foo(1)
iex> stub |> Stubr.called_once?(:foo)

iex> spy = Stubr.spy!(Float)
iex> spy.ceil(1.5)
iex> spy |> Stubr.called_with?(:ceil, [1.5])
iex> spy |> Stubr.called_twice?(:ceil)


Stubr is available in Hex, the package can be installed as:

Add stubr to your list of dependencies in mix.exs:

def deps do
  [{:stubr, "~> 1.5.1", only: :test}]

Developer documentation

Stubr documentation is available in hexdocs.


Random numbers

Use Stubr.stub! to set up a stub for the uniform/1 function in the :rand module. Note, there is no need to explicitly set the module option, however, it is useful to do so because it makes sure the uniform/1 function exists in the :rand module.

test "create a stub of the :rand.uniform/1 function" do
  rand_stub = Stubr.stub!([uniform: fn _ -> 1 end], module: :rand)

  assert rand_stub.uniform(1) == 1
  assert rand_stub.uniform(2) == 1
  assert rand_stub.uniform(3) == 1
  assert rand_stub.uniform(4) == 1
  assert rand_stub.uniform(5) == 1
  assert rand_stub.uniform(6) == 1


As above, use Stubr.stub! to stub the Timex.now/0 function in the Timex module. However, we also want the stub to act as a transparent proxy over the Timex module for all non-stubbed functions. To do this, we just set the module option to Timex and the auto_stub option to true.

test "create a stub of Timex.now/0 and defer on all other functions" do
  fixed_time = Timex.to_datetime({2999, 12, 30})

  timex_stub = Stubr.stub!([now: fn -> fixed_time end], module: Timex, auto_stub: true)

  assert timex_stub.now == fixed_time
  assert timex_stub.before?(fixed_time, timex_stub.shift(fixed_time, days: 1))


In this example, we create stubs of the functions get and post in the HTTPoison module and make them return different values based on their inputs:

setup_all do
  http_poison_stub = Stubr.stub!([
    get: fn("www.google.com") -> {:ok, %HTTPoison.Response{body: "search", status_code: 200}} end,
    get: fn("www.nasa.com") -> {:ok, %HTTPoison.Response{status_code: 401}} end,
    post: fn("www.nasa.com", _) -> {:error, %HTTPoison.Error{reason: :econnrefused}} end
  ], module: HTTPoison)

  [stub: http_poison_stub]

test "create a stub of HTTPoison.get/1", context do
  {:ok, google_response} = context[:stub].get("www.google.com")
  {:ok, nasa_response} = context[:stub].get("www.nasa.com")

  assert google_response.body == "search"
  assert google_response.status_code == 200
  assert nasa_response.status_code == 401

test "create a stub of HTTPoison.post/2", context do
  {:error, error} = context[:stub].post("www.nasa.com", "any content")

  assert error.reason == :econnrefused

TDD in functional languages using stubs: https://www.infoq.com/presentations/mock-fsharp-tdd

Mark Seemann's blog post about the difference between Mocks and Stubs in the context of commands and queries.