## 創建自定義錯誤 創建自定義錯誤最簡單的方法是使用 errors 包中的 New 函數 errors包New函數的具體實現： ~~~go package errors // New returns an error that formats as the given text. func New(text string) error { return &errorString{text} } // errorString is a trivial implementation of error. type errorString struct { s string } func (e *errorString) Error() string { return e.s } ~~~ ## 用法 1. 使用 New 函數創建自定義錯誤 `errors.New("Area calculation failed, radius is less than zero")` 2. 使用 Error 添加更多錯誤信息 `fmt.Errorf("Area calculation failed, radius %0.2f is less than zero", radius) ` 3. 自定義結構體實現error接口 ``` type areaError struct { err string radius float64 } func (e *areaError) Error() string { return fmt.Sprintf("radius %0.2f: %s", e.radius, e.err) } // 調用 &areaError{"radius is negative", radius} ``` 4. 使用結構體類型的方法來提供錯誤的更多信息 ~~~go type areaError struct { err string //error description length float64 //length which caused the error width float64 //width which caused the error } func (e *areaError) Error() string { return e.err } func (e *areaError) lengthNegative() bool { return e.length < 0 } func (e *areaError) widthNegative() bool { return e.width < 0 } func rectArea(length, width float64) (float64, error) { err := "" if length < 0 { err += "length is less than zero" } if width < 0 { if err == "" { err = "width is less than zero" } else { err += ", width is less than zero" } } if err != "" { return 0, &areaError{err, length, width} } return length * width, nil } func main() { length, width := -5.0, -9.0 area, err := rectArea(length, width) if err != nil { if err, ok := err.(*areaError); ok { if err.lengthNegative() { fmt.Printf("error: length %0.2f is less than zero\n", err.length) } if err.widthNegative() { fmt.Printf("error: width %0.2f is less than zero\n", err.width) } return } fmt.Println(err) return } fmt.Println("area of rect", area) } ~~~