How to change empty *strings to nil using mold package? - go

Question

During validation of user input I wanna set to nil some empty *strings in some structs.
E.g. I have this struct:
type User {
Username *string
}
If Username == "" I wanna set Username to nil before proceed.
But I wanna do this using tags, e.g.:
type User {
Username *string `mod:"nil_if_empty"`
}
because I'm already using this package: https://github.com/go-playground/mold in my project for sanitize.
So I'm creating a custom function to set empty strings to nil, but I'm stuck.
Reproduction: https://play.golang.org/p/_DlluqE2Y3k
package main
import (
"context"
"fmt"
"log"
"reflect"
"github.com/go-playground/mold/v3"
)
var tform *mold.Transformer
func main() {
tform = mold.New()
tform.Register("nilEmptyString", nilEmptyString)
type Test struct {
String *string `mold:"nilEmptyString"`
}
myEmptyString := ""
tt := Test{String: &myEmptyString}
err := tform.Struct(context.Background(), &tt)
if err != nil {
log.Fatal(err)
}
fmt.Printf("%+v\n", tt)
}
func nilEmptyString(_ context.Context, _ *mold.Transformer, v reflect.Value, _ string) error {
s, ok := v.Interface().(*string)
if !ok { // ok is always false, why?
return nil
}
if s != nil && *s != "" {
return nil
}
if s != nil && *s == "" {
// v.SetPointer(nil) // should I use this?
}
// v.Set(nil) // how to do this?
return nil
}
Why ok in s, ok := v.Interface().(*string) is always false?
How to set v to nil? Why is v.Set(nil) wrong?
I hope this is a good question to ask on StackOverflow. If not tell me how to improve.
Thanks.