Elixir's Pattern Matching and ExUnit

While writing tests for a Phoenix app, I found something very strange. Look at the code below (contrived tests for this example):

test "the solution to 2 + 2" do
  assert 2 + 2 == 4
end

test "the solution to 2 + 2" do
  assert 2 + 2 != 4
end

Running $ mix test in my terminal, I got 2 tests, 0 failures. Strange! Did I write my assertions correctly? Yes I did, but I should be getting 2 tests, 1 failure. Why am I getting them both to pass?

This is Elixir’s pattern matching at work! Looking at ExUnit.Case Module’s code, we define a test with a string. In the examples above, both test cases have the same string. Therefore, we will never match the second clause because the strings are the same. Because the first clause gets matched, and the first test passes, both tests will pass.

When I further examine the output of my tests, I can confirm this because I see that I get the warning test/models/user_group_test.exs:23: warning: this clause cannot match because a previous clause at line 19 always matches.

To get both tests to work as expected, I will need to make my test strings unique.

test "the solution to 2 + 2" do
  assert 2 + 2 == 4
end

test "the solution to 2 + 2 is not this" do
  assert 2 + 2 != 4
end

2 tests, 1 failure!