Designing Elixir Systems With OTP

Make Tests Repeatable

All of our tests are inside our functional core. In the core, calling a function with the same arguments will almost always result in the same output. That word “almost” is a killer because our whole strategy involves comparing our expectations to actual values. When we can’t have expectations from run to run, we must change our approach.

Sometimes, functions are not perfectly pure. Functions that create timestamps or random numbers are famously difficult to test. We have both types of functions in our codebase. For example, recall the response test:

test "a timestamp is added at build time", %{right: response} do
  assert %DateTime{ } = response.timestamp
  assert response.timestamp < DateTime.utc_now
end