Not conceptually. From the perspective of the rest of the program, isPrime(x) has no side effect regardless of which method you use to implement it. If you are working in a purely functional language, then it makes sense to have introducing IO be a major change, as it has huge potential to make a function impure, and assuming IO is a pure function is roughly the equivalent of unsafePerformIO. However, in this case, we are not even working in a pure functional language. In concept, there is no reason that implementing a function with IO should require writing code outside of that function differently than implementing it with CPU.