Sentinel error pattern

The sentinel error pattern in Go uses a pre-declared error variable to represent a specific, known error condition that callers can check against using errors.Is or ==.

// Define the sentinel error once in the package
var ErrTooLong = errors.New("bufio.Scanner: token too long")

// Return it from a function when the condition occurs
func Scan() error {
    if len(token) > MaxScanTokenSize {
        return ErrTooLong
    }
    return nil
}

// Check for it in the caller
if err != nil && errors.Is(err, ErrTooLong) {
    // Handle the specific case
}

The Sentinel error pattern acts like a specific error code in a system that tells you exactly what went wrong. Instead of just saying "something failed," you return a unique error object that your code can recognize. This lets you write logic that handles specific problems differently from general failures.