errors.As and errors.Is

In Go, error handling is a fundamental aspect of robust code. Starting from Go 1.13, the errors package introduced two powerful functions, errors.As and errors.Is, which help in handling and analyzing errors more effectively.

Using errors.Is

The errors.Is function checks if an error is, or wraps, a specific target error. This is useful for comparing errors that are wrapped within other errors using fmt.Errorf with %w.

package main

import (
	"errors"
	"fmt"
)

func main() {
	originalErr := errors.New("network failure")
	wrappedErr := fmt.Errorf("operation failed: %w", originalErr)

	if errors.Is(wrappedErr, originalErr) {
		fmt.Println("The error is a network failure.")
	} else {
		fmt.Println("The error is of a different type.")
	}
}

Using errors.As

The errors.As function is used to test whether an error is or wraps a specific error type. It can be used especially when dealing with custom error types.

package main

import (
	"errors"
	"fmt"
)

type CustomError struct {
	Code int
	Msg  string
}

func (c CustomError) Error() string {
	return fmt.Sprintf("Error %d: %s", c.Code, c.Msg)
}

func main() {
	customErr := CustomError{Code: 404, Msg: "Not Found"}
	wrappedErr := fmt.Errorf("operation failed: %w", customErr)

	var err CustomError
	if errors.As(wrappedErr, &err) {
		fmt.Printf("Custom error found: %d, %s\n", err.Code, err.Msg)
	} else {
		fmt.Println("Error is not of type CustomError.")
	}
}

Best Practices

• Use errors.Is for comparing static error values and ensure that errors are wrapped with %w.
• Use errors.As for error types when you need to extract additional context or information specific to those custom error types.
• Always check and handle the 'nil' error case before using errors.Is or errors.As to avoid unnecessary operations.

Common Pitfalls

• Forgetting to use %w to wrap errors in fmt.Errorf, which will lead to errors.Is and errors.As not functioning as expected.
• Not checking for error types correctly, which could lead to misleading error handling.

Performance Tips

• Only perform deep error analysis (like type assertion with errors.As) when necessary to avoid performance hits in high-throughput or low-latency applications.
• Ensure that the error types you define or expect are used judiciously to avoid excessive complexity in error handling.