https://tour.golang.org/methods/20

exercise_methods_20.go

package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("cannot Sqrt negative number: %f", e)
}

func Sqrt(x float64) (float64, error) {
	if x < 0{
		return 0, ErrNegativeSqrt(x)
	}
	z := 1.0
	z_pre := 0.0
	i := 1
	for math.Abs(z - z_pre) > 1e-10 {
		z_pre = z
		z -= (z*z - x) / (2*z)
		i++
	}
	return z, nil
}

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