solution for http://tour.golang.org/#58

errors.go

package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt struct {
	Num float64
}

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

func Sqrt(f float64) (float64, error) {
	result := float64(0)
	z := float64(2)
	if f < 0 {
		err := &ErrNegativeSqrt{f}
		return result, err
	}
	for {
		z = z - (z*z - f) / (2*z)
		if math.Abs(result - z) < 1e-15 {
			break
		}
		result = z
	}
	return result, nil
}

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