How to convert interface to another interface which it implements? - go

Question

In short - I would like to be able to cast an interface type whose underlying type implements a specific interface to that specific interface.
I am using the plugin package to lookup a New function which looks like so (I have many others the same):
func NewDomainPrimaryKey() any { return DomainPrimaryKey{} }
(This is generated at run-time so I can't just reference it as DomainPrimaryKey)
My lookup and call is like so:
plugin, err := plugin.Open("my-plugin")
if err != nil {
return err
}
symGet, err := plugin.Lookup("New" + pluginName)
if err != nil {
return err
}
newGenModel, ok := symGet.(func() any)
if !ok {
return errors.New("unexpected type from module symbol")
}
anyGenModel := newGenModel()
genModel, ok := anyGenModel.(GenModel) // **this is where the problem is
if !ok {
return errors.New("unexpected type from module symbol")
}
genModelInstance := genModel.Get()
In the above I am trying to cast 'anyGenModel' (an interface) to the 'GenModel' interface which it implements, however, this doesn't work.
I am certain it implements this interface because when I do the following, I get no errors.
type GenModel interface {
Get() any
TableName() string
}
var _ GenModel = (*DomainPrimaryKey)(nil) // this doesn't complain
How can I do this? I found this article which I don't think is what I am looking for but seems similar.
Thanks in advance for any help on this - this has become a real blocker for me.

Answer 1

If the underlying type implements both interfaces, this is very straightforward:
package main
import "fmt"
type IFace1 interface {
DoThis()
}
type IFace2 interface {
DoThat()
}
type impl struct{}
func (i *impl) DoThis() {
fmt.Println("I implement IFace1")
}
func (i *impl) DoThat() {
fmt.Println("I implement IFace2")
}
func GetIFace1() IFace1 {
return &impl{}
}
func main() {
i1 := GetIFace1()
i1.DoThis()
i2 := i1.(IFace2)
i2.DoThat()
}
playground
If your code is not working, then I would begin by questioning your assertion that the underlying type of anyGenModel actually implements GenModel and work from there.