http://go-tour-jp.appspot.com/#56

answer_exercise_gotour_56.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(f float64) (float64, error) {
	if f < 0 {
		return 0, ErrNegativeSqrt(f)
	}
	z := 1.0
	for {
		zz := z - ((z*z - f) / (2 * f))
		if math.Abs(z-zz) < 0.0000001 {
			return z, nil
		}
		z = zz
	}
	return z, nil
}

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