Negative sqrt error

exercise6.go

package main

import (
    "fmt"
    "math"
)

type ErrNegativeSqrt float64

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

func Sqrt(f float64) (float64, error) {
    if f < 0 {
   		return 0, ErrNegativeSqrt(f)
    }
    
    z := 1.0
    for {
        new_z := z - ((z * z - f) / (2 * f))
        if math.Abs(new_z - z) < 1e-10 {
            return new_z, nil
        }
        z = new_z
    }
}

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