Golang tutorial sqrt function with error

error.go

package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
	// Need to convert e to float64 before passing to Sprint
	// Or else Sprint will call Error() on e,
	// and send progrem into infinite loop
	return fmt.Sprintf("cannot Sqrt negative number: %g", float64(e))
}

func Sqrt(x float64) (float64, error) {
	if (x < 0) {
		return 0, ErrNegativeSqrt(x) 	
	}
	z := 1.0
	for {
		temp := z - (z*z-x)/(2*z)
		if math.Abs(temp-z)<0.0000000001 {
			return temp, nil
		}
		z = temp
	}
}

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