You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: models_gen.go
    path of file: ./repos/gqlgen/_examples/type-system-extension
    
    the code of the file until where you have to start completion: // Code generated by github.com/99designs/gqlgen, DO NOT EDIT.

package type_system_extension

import (
	"fmt"
	"io"
	"strconv"
)

ty
    
func (e *State) UnmarshalGQL(v interface{}) error {
	str, ok := v.(string)
	if !ok {
		return fmt.Errorf("enums must be strings")
	}

	*e = State(str)
	if !e.IsValid() {
		return fmt.Errorf("%s is not a valid State", str)
	}
	return nil
}