http://tour.golang.org/methods/9

methods-9.go

package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt float64

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

func Sqrt(x float64) (float64, error) {
	if x < 0 {
		return x, ErrNegativeSqrt(x)
	}
	return math.Sqrt(x), nil
}

func main() {

	if r, err := Sqrt(2); err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(r)

	}
	if r, err := Sqrt(-2); err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(r)

	}

}