What are errors.Is and errors.As used for, and what problem do they solve?

In the Go errors package, the functions errors.Is and errors.As are designed for convenient and safe working with error chains that can be wrapped or nested.

• errors.Is(err, target error) bool checks whether the error err or any error in its chain contains the error target. This is useful when you need to determine if an error matches a specific type or value, despite wrappers.

• 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 the desired type). If successful, it returns true and assigns the error to target.

The problem they solve is the need to work with errors that may be wrapped using fmt.Errorf("...: %w", err) or other mechanisms, while still being able to check the specific type or value of the original error.

Example:

if errors.Is(err, os.ErrNotExist) {
    fmt.Println("File not found")
}

var pathErr *os.PathError
if errors.As(err, &pathErr) {
    fmt.Printf("Path error: %s\n", pathErr.Path)
}