Challenge 2 — Solution

Task: Define a custom error type RangeError with fields Min and Max
ints, and an Error() string method describing the valid range.
Write a function checkAge(age int) error returning a *RangeError if
age is outside 0-120. Use errors.As to extract it and print Min/Max.

package main

import (
	"errors"
	"fmt"
)

type RangeError struct {
	Min int
	Max int
}

func (e *RangeError) Error() string {
	return fmt.Sprintf("value must be between %d and %d", e.Min, e.Max)
}

func checkAge(age int) error {
	if age < 0 || age > 120 {
		return &RangeError{Min: 0, Max: 120}
	}
	return nil
}

func main() {
	err := checkAge(150)

	var rangeErr *RangeError
	if errors.As(err, &rangeErr) {
		fmt.Println("Out of range. Min:", rangeErr.Min, "Max:", rangeErr.Max)
	}
}


Expected output:

Out of range. Min: 0 Max: 120

Notes:
- *RangeError satisfies Go's built-in error interface purely because
  it has an Error() string method — there's no separate declaration
  needed connecting it to error.
- errors.As(err, &rangeErr) both checks the type AND assigns err
  into rangeErr if it matches, which is what makes rangeErr.Min and
  rangeErr.Max accessible afterward — a plain error variable alone
  could never expose those fields.
- var rangeErr *RangeError starts as nil; errors.As only succeeds
  (returns true) if err is genuinely a *RangeError somewhere in its
  chain.