Golang Error Unwrap

During a code review, my colleague mentioned about %w format directive in fmt.Errorf() . Googled about that and reliazed this thing called error.Unwrap()

package main

import (
	"errors"
	"fmt"
	"log"
	"strings"
)

type parentError struct{}

func (e parentError) Error() string {
	return "Error parentError"
}

type child1Error struct{}

func (e child1Error) Error() string {
	return "Error major child1"
}

type child2Error struct{}

func (e child2Error) Error() string {
	return "Error minor child2"
}

func main() {
	child2 := child2Error{}
	parent := fmt.Errorf("parent: %w", child2)
	grandparent := fmt.Errorf("grandparent: %w", parent)
	if err := checkErrorLevel(grandparent); err != nil {
		log.Printf("Kid, please don't repeat it %v", err)
	}

	child1 := child1Error{}
	parent = fmt.Errorf("parent: %w", child1)
	grandparent = fmt.Errorf("grandparent: %w", parent)
	checkErrorLevel(grandparent)
	if err := checkErrorLevel(grandparent); err != nil {
		log.Printf("Kid, please don't repeat it %v", err)
	}
}

func checkErrorLevel(err error) error {
	if unWrapedErr := errors.Unwrap(err); unWrapedErr != nil {
		if strings.Contains(unWrapedErr.Error(), "minor") {
			return nil
		} else {
			return err
		}
	}
	return nil
}

parentError{} wraps child1Error{} and child2Error{} which in turn wrapped by grandparent error.

In checkErrorLevel() we Unwrap it to examine the error to check whether its minor or not. If its minor we return nil, if not the error itself. Unwrap is possible only if the error is wrapped in the first place using %w in fmt.Errorf().

vigneshravichandran@localhost go-til % go run error-wrapping/main.go
2021/04/15 20:47:22 Kid, please don't repeat it grandparent: parent: Error major child1

changing the format directive to %v and running the same returns no output. Because, unwrapping isn’t happening.

Source code: https://github.com/viggy28/go-til/blob/main/error-wrapping/main.go