[go-nuts] Create a error value using reflect

On Sun, Sep 15, 2013 at 10:43 PM, Marcus Westin wrote:

Hello folks,

I'm trying to create an error value using reflect such that the value is
== nil. Like this:

package main

import (
"errors"
"fmt"
"reflect"
)

func main() {
err := reflect.Zero(reflect.TypeOf(errors.New("An
error"))).Interface().(error)
fmt.Println(err, err == nil)
}

Using Go 1.1, it outputs

<nil> false

(It's curious that the value prints as <nil>, yet is != nil).

Does anyone know if and how I can create a reflect.Value with its runtime
dynamic type to be equal to nil when type-asserted into an error? (or when
passed to a function expecting an error argument using reflect.Call?)

Any help would really be appreciated!

Cheers,
Marcus

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On Mon, Sep 16, 2013 at 8:59 AM, Ross Light wrote:

Slightly modified your snippet to show the problem:
http://play.golang.org/p/WyzaqCa2Y-
Explanation: http://golang.org/doc/faq#nil_error
Ross Light | Software Engineer | [email protected]

On Monday, September 16, 2013 9:12:07 AM UTC-7, Marcus Westin wrote:

Ross, thanks for the clarifying link (*http://golang.org/doc/faq#nil_error
*)

The problem still remains.

Jan: because I need to call functions with different argument signatures
using reflect.Call. All the functions take an error as their last argument,
and I wish to call be able to have the error argument == nil.

Anyone? Is it not doable?
-- while mobile

2013/9/16 Marcus Westin wrote:
Thanks for everyone's help. This is what I've ended up with:

package main

import (
"errors"
"fmt"
"reflect"
)

func main() {
nilError := reflect.Zero(reflect.TypeOf(&Error{}))
nonNilError := reflect.ValueOf(&Error{err: errors.New("A non-nil error")})
reflect.ValueOf(takesNilErrors).Call([]reflect.Value{nilError, nonNilError})
}

func takesNilErrors(nilError *Error, nonNilError *Error) {
fmt.Println("Wanted:", true, false)
fmt.Println("Actual:", nilError == nil, nonNilError == nil)
}

type Error struct {
err error
}

func (e *Error) Error() string {
return e.err.Error()
}

It's not ideal, but it gets the job done.

Thanks for the help everyone!

On Monday, September 16, 2013 10:46:57 AM UTC-7, Rémy Oudompheng wrote:

2013/9/16 Marcus Westin <[email protected] <javascript:>> wrote:
http://golang.org/doc/faq#nil_error)
signatures
argument,
I would have done this:
http://play.golang.org/p/GhxBiEIyfX

The program prints "hello".

package main

import (
"fmt"
"reflect"
)

func F(s string, err error) {
if err == nil {
fmt.Println(s)
}
}

var nilerror = error(nil)

func main() {
f := reflect.ValueOf(F)
h := reflect.ValueOf("hello")
n := reflect.ValueOf(&nilerror).Elem()
f.Call([]reflect.Value{h, n})
}

On Monday, September 16, 2013, Rémy Oudompheng wrote:
I would have done this:
http://play.golang.org/p/GhxBiEIyfX

The program prints "hello".

package main

import (
"fmt"
"reflect"
)

func F(s string, err error) {
if err == nil {
fmt.Println(s)
}
}

var nilerror = error(nil)

func main() {
f := reflect.ValueOf(F)
h := reflect.ValueOf("hello")
n := reflect.ValueOf(&nilerror).Elem()
f.Call([]reflect.Value{h, n})
}

On Mon, Sep 16, 2013 at 7:43 AM, Marcus Westin wrote:
Hello folks,

I'm trying to create an error value using reflect such that the value is ==
nil. Like this:

On 16 September 2013 06:43, Marcus Westin wrote:
Does anyone know if and how I can create a reflect.Value with its runtime
dynamic type to be equal to nil when type-asserted into an error? (or when
passed to a function expecting an error argument using reflect.Call?)