/ˈniːmiː/ - Snapshot testing utilities for Elixir

demo.mp4

Mneme augments ExUnit.Assertions with a set of assertions that know how to update themselves. This is sometimes called snapshot, approval, or golden master testing.

With Mneme, you write something like...

auto_assert my_function()

...and next time you run your tests, Mneme:

1. runs the code,
2. generates a pattern from the result,
3. prints a diff,
4. and asks if you'd like the test updated.

When you say yes, your test now looks like this:

auto_assert %MyAwesomeValue{so: :cool} <- my_function()

This lets you quickly write lots of tests, and like ordinary tests, you'll see when they fail. But, unlike ordinary tests, Mneme asks if you'd like the test updated for the new value.

Features:

• Auto-updating assertions: maintain correct tests as behavior changes during development.
• Alternatives to familiar assertions: auto_assert, auto_assert_raise, auto_assert_receive, auto_assert_received.
• Seamless integration with ExUnit: no need to change your workflow, just run mix test.
• Interactive prompts in your terminal when a new assertion is added or an existing one changes.
• Syntax-aware diffs highlight the meaningful changes in a value.
• Built-in test watcher: see changes immediately with mix mneme.watch.

Getting started

1. Add :mneme as a dependency in your mix.exs.

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

2. Fetch dependencies and run mix mneme.install with MIX_ENV=test. You're prompted with a diff before any files are saved.

   $ mix deps.get
   
   # If MIX_ENV=test is not set, you will see a "task could not be found" error
   $ MIX_ENV=test mix mneme.install

3. Add use Mneme wherever you use ExUnit.Case.

   defmodule MyTest do
     use ExUnit.Case, async: true
     use Mneme
   
     test "arithmetic" do
       auto_assert 2 + 2
     end
   end

4. Run mix test and type y<ENTER> when prompted. The auto_assert call should be updated to:

   auto_assert 4 <- 2 + 2

Interactive tour

If you'd like to see Mneme in action, you can download and run examples/tour_mneme.exs, a standalone tour that only requires that you have Elixir installed. Give it a try without installing Mneme into your own project.

$ curl -o tour_mneme.exs https://raw.githubusercontent.com/zachallaun/mneme/main/examples/tour_mneme.exs

$ elixir tour_mneme.exs

Requirements

Elixir

Mneme requires Elixir version 1.14 or later.

Formatter

If you do not use a formatter, the first auto-assertion will reformat the entire file, introducing unrelated formatting changes. Mneme rewrites your test scripts when updating an assertion using the formatter configuration for your project.

It's highly recommended to configure your editor to format Elixir files on-save.

Supported formatters:

• Mix.Tasks.Format (Elixir's default formatter)
• FreedomFormatter

Command line interface

Auto-assertions are run with your normal tests when you run mix test and a terminal prompt is used whenever one needs to be updated. Here's what that might look like:

Whenever that happens, you have a few options:

Key	Action	Description
y	Accept	Accept the proposed change. The assertion will be re-run and should pass.
n	Reject	Reject the proposed change and fail the test.
s	Skip	Skip this assertion. The test will not fail, but the mix test process will exit with 1.
k	Next	If multiple patterns have been generated, cycle to the next one.
K	Last	If multiple patterns have been generated, cycle to the last one.
j	Previous	If multiple patterns have been generated, cycle to the previous one.
J	First	If multiple patterns have been generated, cycle to the first one.

Note that the CLI is not available when tests are run in a CI environment.

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

Links and acknowledgements

Special thanks to:

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

• My Kind of REPL, an article by Ian Henry that shows how snapshot testing can change your workflow.

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

• Rewrite, which makes updating source code a breeze.

• 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.