You’re making two invalid assumptions that break down when dealing with a functional paradigm.

1. You don’t have breakpoints, your code doesn’t run. Your code is the thing that configures the code that runs. Your code is descriptive.

2. Nothing unexpected can happen. That’s the point of pure functions and a strong type system. You don’t need to know the exact values that flow through at runtime to know that your code functions as written.

Granted, if you write code that doesn’t make sense then it will produce* results that don’t make sense. If you don’t fully understand the problem you’re trying to solve no programming paradigm can save you from creating bugs.