Your pointer to the struct should implement the Interface. In that way you can modify its fields.
Look at how I modified your code, to make it working as you expect:
package main
import (
"fmt"
)
type IFace interface {
SetSomeField(newValue string)
GetSomeField() string
}
type Implementation struct {
someField string
}
func (i *Implementation) GetSomeField() string {
return i.someField
}
func (i *Implementation) SetSomeField(newValue string) {
i.someField = newValue
}
func Create() *Implementation {
return &Implementation{someField: "Hello"}
}
func main() {
var a IFace
a = Create()
a.SetSomeField("World")
fmt.Println(a.GetSomeField())
}