Snapshot/approval testing integrated into ExUnit

elixir, testing


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

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.


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])

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])

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

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

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

  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

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)

# 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)

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

  • 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.


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

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

Guides for optional editor integration can be found here:


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!


See the full module documentation for configuration options.