// New returns an error that formats as the given text.
func New(text string) error {
    return &errorString{text}
}

package main

import (
    "fmt"
    "math"
    "errors"
)

func main() {
    if _, err := Sqrt(-1); err != nil {
        fmt.Println(err)
    }
}

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, errors.New("math: square root of negative number")
    }

    // implementation
    return math.Sqrt(f), nil
}

package main

import (
    "fmt"
    "math"
    "errors"
)

func main() {
    if _, err := Sqrt(-1); err != nil {
        fmt.Println(err)
    }
}

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, fmt.Errorf("math: square root of negative number %g", f)
    }

    // implementation
    return math.Sqrt(f), nil
}

type SyntaxError struct {
    Offset int64  // error occurred after reading Offset bytes
}

func (e *SyntaxError) Error() string { return e.msg }

if err := dec.Decode(&val); err != nil {
    if serr, ok := err.(*json.SyntaxError); ok {
        line, col := findLine(f, serr.Offset)
        return fmt.Errorf("%s:%d:%d: %v", f.Name(), line, col, err)
    }
    return err
}

type Error interface {
	error
	Timeout() bool // Is the error a timeout?

	// Deprecated: Temporary errors are not well-defined.
	// Most "temporary" errors are timeouts, and the few exceptions are surprising.
	// Do not use this method.
	Temporary() bool
}

if nerr, ok := err.(net.Error); ok && nerr.Temporary() {
    time.Sleep(1e9)
    continue
}
if err != nil {
    log.Fatal(err)
}

package main

import (
    "fmt"
    "math"
)

// NegativeSqrtError reflects an error can be caused by calculating Sqrt of a negative number.
type NegativeSqrtError float64

func (f NegativeSqrtError) Error() string {
    return fmt.Sprintf("math: square root of negative number %g", float64(f))
}

func main() {
    if _, err := Sqrt(-1); err != nil {
        fmt.Println(err)
    }
}

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, NegativeSqrtError(f)
    }

    // implementation
    return math.Sqrt(f), nil
}