如何在运行时连接字符串和float64？

I am trying to solve this: http://tour.golang.org/#58

Here is what I have done;

#imports omitted
type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
    return "Cannot Sqrt negative number: " + string(e)
}

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, ErrNegativeSqrt(1)
    }
    # calculate z here...
    return z, nil
}
# main omitted

I have also tried e.String() and e.string() but those didn't work too...

Try using the fmt package

import "fmt"
...
return fmt.Sprint("Cannot Sqrt negative number ", float64(e))