58.go

// http://tour.golang.org/#58
package main

import (
    "fmt"
)

type ErrNegativeSqrt float64

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

func Sqrt(x float64) (float64, error) {
    z := float64(1)
    if (x < 0) {
      return z, ErrNegativeSqrt(x)
    }
    for i := 1; i < 100; i++ {
        z = z - (z * z - x) / (2 * z)
    }
    return z, nil
}

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