Functional Core, Imperative Shell: Gary Bernhardt on Testing

HN recommends it as the canonical presentation of 'functional core, imperative shell' and a practical answer to unit versus integration testing.

Gary Bernhardt's RubyConf 2012 talk on testing in isolation without mocking everything. It goes through behavior versus data, mutation versus immutability, and how a functional core with an imperative shell yields natural isolation and parallelism.