https://tour.golang.org/flowcontrol/8

exercise-loops-and-functions.go

package main

import (
	"fmt"
	"math"
)

const DELTA = 0.00000000001
const INITIAL_Z = 100.0

func Sqrt(x float64) (z float64) {
    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
}

func main() {
	fmt.Println(Sqrt(-5))
	fmt.Println(math.Sqrt(-500))
}