Correct. FCIS is about testing. Testing pure code is a lot easier than testing impure code, but most of what you want to implement can be done with pure code,

Correct. FCIS is about testing. Testing pure code is a lot easier than testing impure code, but most of what you want to implement can be done with pure code, and you can isolate the impure code to a shell that can be tested apart from your pure business logic.