10 Debugging

Debugging: The act of being the detective in the crime movie where you are also the murderer.

Recall... testing reveals a falut in a program, and debugging reveals the cause of that fault. Debugging takes mroe time than programming, so try to get it right the first time.

Some advice: follow the scientific method

• formulate a falsifiable hypothesis
• create an experiment that can refute that hypothesis
  • find the simplest possible input that causes that fault
• run that experiment
• keep a notebook

If you get flummoxed

• The bug is probably not where you think it is
• Get someone to help you. Use duck debugging. Verbalize. Talk to someone outloud. Explain the problem to something as if it's a right computer.
• If all else fails, doubt your sanity: Do you have the right compiler? Is this the right source code?
• Don't debug when angry or tired. It makes things harder.
• Think through fixes carefully. "Fixing" a bug often introduces more bugs

Defensive programming

• proactive debugging: make it easier to detect faults by writing fault-detection code during implementation
• Assert your preconditions and invariants
• Write exhaustive conditionals (if/match)

Q. Isn't this expensive? A. No! It only seems like that.

• For the implementer: the defensive code you write tends to pay off in the faults it catches early
• For performance: the faults you catch in production might save more money then the run-time cost of the checks.