The errors package has two methods, Is and As. What is the difference and when are they used?

In the Go errors package, there are two methods for working with errors: Is and As. They serve different purposes when checking and extracting errors.

• errors.Is(err, target error) bool — checks whether the error err or any error in its chain of wrappers matches the target error. It is used when you need to verify if an error is of a specific type or value.

• errors.As(err, target interface{}) bool — attempts to cast the error err or any error in its chain to the type specified in target (a pointer to an error variable). If successful, target is populated with this error. It is used when you need to access a specific error type and work with its fields.

Example:

if errors.Is(err, os.ErrNotExist) {
    // The error indicates that the file does not exist
}

var pathErr *os.PathError
if errors.As(err, &pathErr) {
    // You can access error details through pathErr
    fmt.Println("Path error:", pathErr.Path)
}

Thus, Is is for checking error matches, As is for extracting a specific error type from the chain.