Go: What is the difference between panic and error?

panic — immediate runtime failure. error — expected failure.

Panic:

• Stops goroutine.
• Runs deferred functions.
• Use for programmer errors.

1func Must(s string) Config {
2    cfg, err := Parse(s)
3    if err != nil {
4        panic("invalid: " + err.Error())
5    }
6    return cfg
7}

Error:

• Returned as value.
• Caller handles.
• Use for expected failures.

1func read(path string) ([]byte, error) {
2    data, err := os.ReadFile(path)
3    if err != nil {
4        return nil, fmt.Errorf("reading %s: %w", path, err)
5    }
6    return data, nil
7}

When to use panic:

• init() failures.
• Unrecoverable conditions.

When to use error:

• Library functions.
• I/O operations.

Common mistakes:

• Using panic for normal errors.
• Ignoring errors.
• Panicking in library code.
• Not using errors.Is/errors.As.
• Using log.Fatal instead of error.