[mattnewton]

> And because of that, functional code is often very tedious to test. Actually, in my experience, there is a clear tension between code that's referentially transparent and code that's easily testable. In practice, you have to pick one, you can't have both.

In all my years of software development, I've never encountered a referentially-transparent function that was even remotely hard to test, let alone harder than one with environmental baggage. In fact, being referentially transparent opens you up to new kinds of powerful testing strategies that are nearly impossible if the function isn't, like QuickCheck. (I can't highly recommend quick check enough, it's worth the little learning curve 100x over)