I really like the idea of tests right beside the code. I don't like how it's code though. Muddles what the actual implementation is a bit. Maybe with more usage the where statements start to blur and you don't notice them as much.

I like Elixir's approach better where your test cases live as documentation examples in comment form, right above the function declaration. It's fantastic and free. Your mix test command runs tests, in addition to any examples you have as tests.

http://elixir-lang.org/getting-started/mix-otp/docs-tests-an...

You put your tests as comments below an ## Example header. Oh and these examples go into your auto generated documentation as well. Ridiculously effective and free. You feel bad not using them.