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: stash_modifier.go
    path of file: ./repos/martian/stash
    
    the code of the file until where you have to start completion: // Copyright 2015 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file exc
    
func modifierFromJSON(b []byte) (*parse.Result, error) {
	// If you would like the saved state of the URL to be written in the response you must specify
	// this modifier's scope as both request and response.
	msg := &modifierJSON{}
	if err := json.Unmarshal(b, msg); err != nil {
		return nil, err
	}

	mod := NewModifier(msg.HeaderName)
	r, err := parse.NewResult(mod, msg.Scope)
	if err != nil {
		return nil, err
	}

	if r.ResponseModifier() != nil && r.RequestModifier() == nil {
		return nil, fmt.Errorf("to write header on a response, specify scope as both request and response")
	}

	return r, nil
}