An experiment in self-modifying code, or, instant snapshot testing inside your unit tests.

The snap_assert macro inserts the result of evaluating its argument directly into your source code as its second argument. snap_assert with two arguments becomes a regular ex_unit assert.

If available in Hex, the package can be installed by adding snap_assert to your list of dependencies in mix.exs:

def deps do
  [
    {:snap_assert, "~> 0.1.0"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/snap_assert.

1. Import Snap in your test file
2. Use snap_assert <some expression> in one of the tests
3. Run the tests with mix test
4. Tests will pass and snap_assert <some expression> will turn into snap_assert <some expression>, <expression value>
5. Run the tests again; snap_assert with two arguments will now be equivalent to a regular assert so tests will pass.

More concrete example:

• Add snap_assert(String.upcase("hello")) to a test
• Run mix test (tests will pass)
• The test file is now updated so that you have snap_assert(String.upcase("hello"), "HELLO")
• Run mix test again - tests will pass
• Change the expression to snap_assert(String.upcase("hello"), "GOODBYE")
• Run mix test again - tests will fail, ie. now you have a regular assert.

When working with snap_assert, it's useful to have the tests run on every change, for example with:

fswatch lib test | mix test --listen-on-stdin --stale

I was inspired by this post by Ian Henry: https://ianthehenry.com/posts/my-kind-of-repl/

At first, the idea of inserting the value of an expression into an assert seems tautological. However, consider how we often write functions: start with a draft implementation, try it out in the REPL, realise it's incomplete, make tweaks, try again, repeat until done. Then write unit tests, possibly by copy-pasting from the REPL.

snap_assert moves some of that REPL into your test file. When you run tests which include a snap_assert, you'll see the results directly in your test file. If they are as expected - great, you now have a unit test for free. If not, delete the incorrect value, fix your function, run the tests again. Rinse and repeat.

I was skeptical at first, but I've been trying this macro while working on a package, and it feels good.

Of course, snap_assert isn't suitable for all tests: sometimes you want to check for a substring match, or a pattern match, or check that a message is received, and in those cases you still need to use regular ex_unit functionality.