package main

import (
	"fmt"
	"time"
)

type ErrInfo struct {
	errTime time.Time
	errLine int
}

func (e ErrInfo) Error() string {
	return fmt.Sprintf("there was an error at %q, at line %d", e.errTime, e.errLine)
}

func Sqrt(x float64) (float64, error) {
	var z float64 = 1.0
	if x < 0 {
		return 0, ErrInfo{time.Now(), 20}
	} else {
		for i := 15; i > 0; i-- {
			z -= (z*z - x) / (2 * z)
		}
	}
	return z, nil
}

func main() {
	fmt.Println(Sqrt(2))
	fmt.Println(Sqrt(-2))
}