package main

import (
    "fmt"
    "math"
)

type ErrNegativeSqrt float64

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

func Sqrt(x float64) (float64, error) {
    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    }
    var p float64
    for z := 1.0; z != p && math.Abs(z - p) > 0.000000000000001; z -= (z * z - x) / (2 * z) {
        p = z
    }
    return p, nil
}

func main() {
    fmt.Println(Sqrt(4))
    fmt.Println(Sqrt(-2))
}