The thing about "the outside world" is that it is just one form of interpretation of a chunk of code. Inarguably, it is the most important one, but once you rec

The thing about "the outside world" is that it is just one form of interpretation of a chunk of code. Inarguably, it is the most important one, but once you recognize that it is not alone in that regard you get a very new appreciation for "internal consistency". Ultimately, this becomes an issue of "minimize your interfaces" which is widely known to be close to vital for writing successful code. If you can "purify" most of your code base then use types to ensure that it has internal consistency then the remaining task is to write (at least) one valid, trusty "interpreter" from your pure/trusted codebase into the real world. This is undoubtedly the most fragile piece of your application! But it's also eminently library-able and it places the risk-burden over a tiny fraction of your code. It's also the place where you want to property/unit/integration test the hell out of the world. So all of the standard CS adages apply again, but thanks to purity/type-checking you've reduced the risk exposure to a tiny fraction of what it is while using other tools.