Gary Bernhardt：边界（Ruby Conf 2012）

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.

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

“I highly recommend this video for a more philosophical take on how to do "impure FP" without OO”