/ni:mi:/ - Snapshot testing for Elixir ExUnit

Early days

Mneme is in its infancy and has an intentionally minimal API consisting largely of a single macro. Please feel free to submit any feedback, bugs, or suggestions as issues on Github. Thanks!

Snapshot tests assert that some expression matches a reference value. It's like an ExUnit assert, except that the reference value is managed for you by Mneme.

Mneme follows in the footsteps of existing snapshot testing libraries like Insta (Rust), expect-test (OCaml), and assert_value (Elixir). Instead of simple value or string comparison, however, Mneme leans heavily into pattern matching.

example

Example

Let's say you've written a test for a function that removes even numbers from a list:

test "drop_evens/1 should remove all even numbers from an enum" do
  auto_assert drop_evens(1..10)

  auto_assert drop_evens([])

  auto_assert drop_evens([:a, :b, 2, :c])
end

The first time you run this test, you'll see interactive prompts for each call to auto_assert showing a diff and asking if you'd like to accept the generated pattern. After accepting them, your test is updated:

test "drop_evens/1 should remove all even numbers from an enum" do
  auto_assert [1, 3, 5, 7, 9] <- drop_evens(1..10)

  auto_assert [] <- drop_evens([])

  auto_assert [:a, :b, :c] <- drop_evens([:a, :b, 2, :c])
end

The next time you run this test, you won't receive a prompt and these will act (almost) like any other assertion. If the result of the call ever changes, you'll be prompted again and can choose to update the test or reject it and let it fail.

With a few exceptions, auto_assert/1 acts very similarly to a normal assert. See the macro docs for a list of differences.

quick-start

Quick start

1. Add :mneme do your deps in mix.exs:

   defp deps do
     [
       {:mneme, ">= 0.0.0", only: :test}
     ]
   end

2. Add :mneme to your :import_deps in .formatter.exs:

   [
     import_deps: [:mneme],
     inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
   ]

3. Start Mneme right after you start ExUnit in test/test_helper.exs:

   ExUnit.start()
   Mneme.start()

4. Add use Mneme wherever you use ExUnit.Case:

   defmodule MyTest do
     use ExUnit.Case, async: true
     use Mneme
   
     test "arithmetic" do
       # use auto_assert instead of ExUnit's assert - run this test
       # and delight in all the typing you don't have to do
       auto_assert 2 + 2
     end
   end

match-patterns

Match patterns

Mneme tries to generate match patterns that are equivalent to what a human (or at least a nice LLM) would write. Basic data types like strings, numbers, lists, tuples, etc. will be as you would expect.

Some values, however, do not have a literal representation that can be used in a pattern match. Pids are such an example. For those, guards are used:

auto_assert self()

# after running the test and accepting the change
auto_assert pid when is_pid(pid) <- self()

Additionally, local variables can be found and pinned as a part of the pattern. This keeps the number of hard-coded values down, reducing the likelihood that tests have to be updated in the future.

test "create_post/1 creates a new post with valid attrs", %{user: user} do
  valid_attrs = %{title: "my_post", author: user}

  auto_assert create_post(valid_attrs)
end

# after running the test
test "create_post/1 creates a new post with valid attrs", %{user: user} do
  valid_attrs = %{title: "my_post", author: user}

  auto_assert {:ok, %Post{title: "my_post", author: ^user}} <- create_post(valid_attrs)
end

In many cases, multiple valid patterns will be possible. Usually, the "simplest" pattern will be selected by default when you are prompted, but you can cycle through the options as well.

non-exhaustive-list-of-special-cases

Non-exhaustive list of special cases

• Pinned variables are generated by default if a value is equal to a variable in scope.

• Date and time values are written using their sigil representation.

• Struct patterns only include fields that are different from the struct defaults.

• Structs defined by Ecto schemas exclude primary keys, association foreign keys, and auto generated fields like :inserted_at and :updated_at. This is because these fields are often randomly generated and would fail on subsequent tests.

formatting

Formatting

Mneme uses Rewrite to update source code, formatting that code before saving the file. Currently, the Elixir formatter and FreedomFormatter are supported. If you do not use a formatter, the first auto-assertion will reformat the entire file.

continuous-integration

Continuous Integration

In a CI environment, Mneme will not attempt to prompt and update any assertions, but will instead fail any tests that would update. This behavior is enabled by the CI environment variable, which is set by convention by many continuous integration providers.

export CI=true

editor-support

Editor support

Guides for optional editor integration can be found here:

• VS Code

acknowledgements

Acknowledgements

Special thanks to:

• What if writing tests was a joyful experience?, from the Jane Street Tech Blog, for inspiring this library.

• Sourceror, a library that makes complex code modifications simple.

• Rewrite, which provides the diff functionality present in Mneme.

• Owl, which makes it much easier to build a pretty CLI.

• Insta, a snapshot testing tool for Rust, whose great documentation provided an excellent reference for snapshot testing.

• assert_value, an existing Elixir project that provides similar functionality. Thank you for paving the way!

configuration

Configuration

Certain behavior can be configured globally using application config or locally in test modules either at the module, describe-block, or test level.

To configure Mneme globally, you can set :defaults for the :mneme application:

config :mneme,
  defaults: [
    diff: :semantic
  ]

These defaults can be overriden in test modules at various levels either as options to use Mneme or as module attributes.

defmodule MyTest do
  use ExUnit.Case

  # reject all changes to auto-assertions by default
  use Mneme, action: :reject

  test "this test will fail" do
    auto_assert 1 + 1
  end

  describe "some describe block" do
    # accept all changes to auto-assertions in this describe block
    @mneme_describe action: :accept

    test "this will update without prompting" do
      auto_assert 2 + 2
    end

    # prompt for any changes in this test
    @mneme action: :prompt
    test "this will prompt before updating" do
      auto_assert 3 + 3
    end
  end
end

Configuration that is "closer to the test" will override more general configuration:

@mneme > @mneme_describe > opts to use Mneme > :mneme app config

The exception to this is the CI environment variable, which causes all updates to be rejected. See the "Continuous Integration" section for more info.

options

Options

• :action - The action to be taken when an auto-assertion updates. Actions are one of :prompt, :accept, or :reject. If CI=true is set in environment variables, the action will always be :reject. The default value is :prompt.

• :default_pattern - The default pattern to be selected if prompted to update an assertion. Can be one of :infer, :first, or :last. The default value is :infer.

• :diff - Controls the diff engine used to display changes when an auto-assertion updates. If :semantic, uses a custom diff engine to highlight only meaningful changes in the value. If :text, uses the Myers Difference algorithm to highlight all changes in text. The default value is :semantic.

• :target - The target output for auto-assertions. If :mneme, the expression will remain an auto-assertion. If :ex_unit, the expression will be rewritten as an ExUnit assertion. The default value is :mneme.