alt Hacker News> Then the complex pure part you test the heck out of, and the stateful/side effectful impure part becomes barely a wrapper over system APIs.
In practice the issues I see with this are that the "side effect" part is usually either: extensive enough to still justify mocking around testing it, and also intertwined enough with your logic to be hard to remove all the "pure" logic. I rarely see 90-95% of functions being pure logic vs side effects.
E.g. for the first, you could have an action that requires several sequenced side effects and then your "wrapper over APIs" still needs validation of calling the right APIs in the right oder with the right params, for various scenarios. Enter mocks or fakes. (And sometimes people will get clever and say use pubsub or events for this, but... you're usually just making the full-system-level testing there harder, as well as introducing less determinism around your consistency.)
For the second, something like "do steps I and J. If the API you call in step J fails, unwind the change in I." Now you've got some logic back in there. And it's not uncommon for the branching to get more complex. Were you building everything in the system from first principles, you could try to architect something where I and J can be combined or consolidated in a way to work around this; when I and J are third party dependencies, that gets harder.
You're right that it's not always easy to separate pure from effectful code. But you definitely can (model it as a state machine) and I think it's worth it, especially if those steps can take a long time, can fail, might be executed in parallel, etc.
For instance, I once worked on payment-related code at a large online retailer. The steps I and J from your example would have been calls to the payment gateway's API (payment initiation, actual payment request). There was also a step K (polling for payment confirmation) and even a step K' (a payment confirmation callback the gateway might or might not call before or after we get around polling for the payment status ourselves). And often there was even user interaction in between (the 3DS/3DS2 credit card payment scheme that's common here in the EU). Every single one of those steps could fail for a myriad of reasons, e.g. time out, be rejected, … and we had to make sure we always failed gracefully and, most importantly, didn't mess up our payment or order records.
Of course this was an old enterprise Java code base, created by people who had long left the company, and all this had been written just the way you imagine it. It was an absolute mess.
Every single time I worked on this code base I secretly wished one of the original authors had heard of state machines, pure vs. effectful code, and unit tests.