I created a custom error type to wrap errors around for easier debugging in Golang. It works when there are errors to print, but now it is causing a panic.
type Error struct {
ErrString string
}
func (e *Error) Error() string {
return e.ErrString
}
func Wrap(err error, str string) *Error {
if err == nil {
return nil
}
e := &Error{
ErrString: str + err.Error(),
}
return e
}
When I call a function an it doesn't return an error, I should still be able to wrap the error.
The expected behavior is that if the error is nil, it should simply ignore it, unfortunately it does the opposite.
func foo() error {
err := bar()
return Wrap(err, "bar called")
}
func bar() error {
return nil
}
func main() {
err := foo()
if err != nil {
fmt.Printf("Found error %v
",err)
return
}
fmt.Println("No Errors")
}
I expect it to print No errors. Instead it prints Found error <nil> even though the error is nil.
if err != nil
Is comparing a the err variable to a nil error , but its actually a nil *Error
Changing the code to
err:=foo()
var nilerror *Error = nil
if err != nilerror {
fmt.Printf("Found error %v
",err)
return
}
fmt.Println("No Errors")
Yields the predicted result.
The error built-in interface type is the conventional interface for representing an error condition, with the nil value representing no error.
type error interface { Error() string }
The value of err of interface type error is not nil. It is the value nil of type *main.Error. In fact, err != nil && err.(*Error) == nil is true
For example,
package main
import (
"fmt"
)
func error1() {
err := foo()
fmt.Printf("%T %v %v %v
", err, err, err == nil, err.(*Error) == nil)
if err != nil {
fmt.Printf("Found error %v
", err)
return
}
fmt.Println("No Errors")
}
func error2() {
err := foo()
fmt.Printf("%T %v %v %v
", err, err, err == nil, err.(*Error) == nil)
if err != nil && err.(*Error) != nil {
fmt.Printf("Found error %v
", err)
return
}
fmt.Println("No Errors")
}
type Error struct {
ErrString string
}
func (e *Error) Error() string {
return e.ErrString
}
func Wrap(err error, str string) *Error {
if err == nil {
return nil
}
e := &Error{
ErrString: str + err.Error(),
}
return e
}
func foo() error {
err := bar()
return Wrap(err, "bar called")
}
func bar() error {
return nil
}
func main() {
error1()
fmt.Println()
error2()
}
Playground: https://play.golang.org/p/nwNRa2sNwj0
Output:
*main.Error <nil> false true
Found error <nil>
*main.Error <nil> false true
No Errors
Since your Error type implements the error interface, the easiest solution, is to return an error in Wrap():
func Wrap(err error, str string) error {
if err == nil {
return nil
}
e := &Error{
ErrString: str + err.Error(),
}
return e
}