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

exercise-errors.go

package main

import (
	"fmt"
	"math"
)

const DELTA = 0.00000000001
const INITIAL_Z = 100.0

func Sqrt(x float64) (z float64, err error) {
	if x < 0 {
		return z, ErrNegativeSqrt(x)
	}
	
    z = INITIAL_Z
    
    step := func() float64 {
    	return z - (z*z - x) / (2 * z)
    }
    
    for zz := step(); math.Abs(zz - z) > DELTA
    {
    	z = zz
		zz = step()
    }
    return
}

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprint("cannot Sqrt negative number: ",
		float64(e))
}

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