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

In Go's errors package, the functions errors.Is and errors.As are designed for convenient working with error chains and their types.

• errors.Is(err, target error) bool checks whether the error err or any error in its chain is equal to target. This is useful when errors are wrapped and you need to check for a specific error type or value.

• errors.As(err, &target) 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, it returns true and assigns the error to target. This allows working with specific error types even if they are wrapped.

The problem these functions solve is the convenient and safe recognition and handling of errors that may be wrapped in multiple layers, without manually unwrapping the error chain.

Example:

if errors.Is(err, os.ErrNotExist) {
    // Handle "file does not exist" error
}

var pathErr *os.PathError
if errors.As(err, &pathErr) {
    fmt.Println("File path error:", pathErr.Path)
}