Debugging the blank identifier problems

Direct Answer

When a valid-looking zero appears unexpectedly, search for discarded errors or values near parsing, I/O, type assertions, and database calls. Restore the named value and handle it at the source. Check every multi-value call that uses _, range loops that may need the missing index, and blank imports that change program startup. Test invalid input as well as valid input, and assert that the original error is returned or wrapped instead of silently becoming a zero result.

Deep Dive & Architecture

When a valid-looking zero appears unexpectedly, search for discarded errors or values near parsing, I/O, type assertions, and database calls.

Restore the named value and handle it at the source.

Use _ for an unneeded range index or value, for an intentionally ignored return value only when failure is impossible or irrelevant, and for compile-time interface assertions such as var _ io.Reader = (*File)(nil). A named variable preserves information and should be used when the value affects correctness, logging, or debugging. _ is better only when retaining that value would add no meaning and the discard is deliberate.

The dangerous mistake is using _ to silence an error. The code may continue with a zero or partial result and fail later, far from the real cause. Name and handle errors, keep _ for truly irrelevant values, and add a comment when a side-effect import or deliberate discard is not obvious.

Code Example

n, err := strconv.Atoi(input)
if err != nil {
    return fmt.Errorf("invalid number %q: %w", input, err)
}
return use(n)

Common Mistakes & Pitfalls

Changing several lines before proving which operation first produced the wrong state.