http://tour.golang.org/#55

sqrt_with_errors.go

package main
 
import (
    "fmt"
    "math"
)
 
type ErrNegativeSqrt float64
 
func (e ErrNegativeSqrt) Error() string {
    // fmt functions use Error() to get message, so it will be an infinite loop without conversion....
    return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}
 
func Sqrt(x float64) (float64, error) {
    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    }
    z := x
    delta := 0.001
    notDone := true
    fmt.Println("foo")
    for notDone {
        res := z - ((z*z) - x)/(2*z)
        if math.Abs(res - z) < delta {
            notDone = false
        }
        z = res
    }
    return z, nil
}
 
func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
    fmt.Printf("cannot Sqrt negative number: %v", ErrNegativeSqrt(-2))
}