Challenge 1 — Solution

Task: Declare var ErrEmptyName = errors.New("name cannot be empty").
Write a function greet(name string) error that returns this
sentinel (wrapped with fmt.Errorf and extra context "greet: %w") if
name is "". Use errors.Is to check the result and print an
appropriate message.

package main

import (
	"errors"
	"fmt"
)

var ErrEmptyName = errors.New("name cannot be empty")

func greet(name string) error {
	if name == "" {
		return fmt.Errorf("greet: %w", ErrEmptyName)
	}
	fmt.Println("Hello,", name)
	return nil
}

func main() {
	err := greet("")

	if errors.Is(err, ErrEmptyName) {
		fmt.Println("Validation failed: name was empty.")
	} else if err != nil {
		fmt.Println("Unexpected error:", err)
	}
}


Expected output:

Validation failed: name was empty.

Notes:
- err here is actually "greet: name cannot be empty", a wrapped
  version of ErrEmptyName — a plain err == ErrEmptyName comparison
  would incorrectly return false.
- errors.Is(err, ErrEmptyName) correctly unwraps err and finds
  ErrEmptyName underneath, returning true.
- greet("Philip") would instead print "Hello, Philip" and return
  nil, never reaching either error-handling branch.