My answer of "a tour of Go #56( http://go-tour-jp.appspot.com/#56 )"

gotour_56.go

package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt float64

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

func Sqrt(x float64) (float64, error) {
	if x < 0 {
		return 0.0, ErrNegativeSqrt(x)
	}
	z, count := 0.5, 1
	for ; ; count++ {
		old := z
		z = z - (z*z-x)/2*z
		if math.Abs(float64(old-z)) < 0.0001 {
			break
		}
	}
	fmt.Printf("count=%d\n", count)
	return z, nil
}

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