An answer of the exercise: Errors on a tour of go

errors.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 {
        e := ErrNegativeSqrt(f)
        return 0, e
    }

    z := 1.0
    var prev_z float64

    for math.Abs(prev_z - z) > 1e-10 {
        prev_z = z
        z = z - ((z*z - f) / (z * 2))
    }
    return z, nil
}

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