How to convert an unmarshalled Golang object to the type of a specified variable

I'd like to marshal a variety of objects to file, then unmarshal them, and convert them back to their original type by getting the type of the variables which were marshalled.
The key point is that I'd like to convert the unmarshalled object to the type of a specified variable, without specifying the type.

The short pseudocode:

// Marshal this
item := Book{"The Myth of Sisyphus", "Albert Camus"}
// Then unmarshal and convert to the type of the item variable.
itemType := reflect.TypeOf(item)
newItem itemType = unmarshalledItem.(itemType)  // This is the problem.
fmt.Println("Unmarshalled is:", reflect.TypeOf(newItem)) // Should print *main.Book

The full code:

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"os"
	"reflect"
)

type Book struct {
	Title  string
	Author string
}

func main() {
	// Create objects to marshal.
	book := Book{"The Myth of Sisyphus", "Albert Camus"}
	box := make(map[string]interface{})
	box["The Myth of Sisyphus"] = &book
	itemType := reflect.TypeOf(box["The Myth of Sisyphus"])
	fmt.Println("Book is:", itemType)

	// Marshal objects to file.
	err := Write(&book)
	if err != nil {
		fmt.Println("Unable to save store.", err)
		return
	}

	// Unmarshal objects from file.
	untyped := make(map[string]interface{})
	bytes, err := ioutil.ReadFile("store.txt")
	if err != nil {
		fmt.Println("Unable to load store.", err)
		return
	}
	err = json.Unmarshal(bytes, &untyped)
	if err != nil {
		fmt.Println("Err in store unmarshal.", err)
		return
	}

	// Get Title property of unmarshalled object,
	// and use that to get variable type from box map.
	for k, v := range untyped {
		if k == "Title" {
			itemTitle := v.(string)
			fmt.Println("Cast item having title:", itemTitle)
			targetType := reflect.TypeOf(box[itemTitle])
			fmt.Println("Type to cast to is:", targetType)
			// Convert untyped to targetType.
			// This is the problem.
			typed targetType = untyped.(targetType)
			fmt.Println("Unmarshalled is:", reflect.TypeOf(typed)) // Should print *main.Book
		}
	}
}

func Write(b *Book) error {
	data, err := json.Marshal(b)
	if err != nil {
		return err
	}
	newFilename := "store.txt"
	f, err := os.OpenFile(newFilename, os.O_CREATE|os.O_TRUNC, 0660)
	if err != nil {
		return err
	}
	_, err = f.WriteString(string(data) + "\n")
	if err != nil {
		return err
	}
	return nil
}

This might work in a dynamically typed language, but it wont work here because Go is statically typed.

The book is not stored as a Book, its stored as a json string and the json unmarshaller has no idea that its a Book unless you tell it so. i.e. it doesn't know to map the fields to a Book object.

You can't cast the unmarshalled 'untyped' into a Book because it is not a Book it is a map[string]interface{} which happens to look exactly like a Book.

What you would need to do is

1. unmarshal the json into a map[string]interface{} then read the title.
2. use an if statement or a switch statement based on the type
3. unmarshal the json again into an object of that type (e.g. a Book)

Something like this I guess:

// check the type
if targetType.String() == "*main.Book" {
    // unmarshall it again as a Book
    var typedBook Book
    _ = json.Unmarshal(bytes, &typedBook)
    fmt.Println("Unmarshalled is:", reflect.TypeOf(typedBook)) // Should print *main.Book
} else if targetType.String() == "*main.Magazine" {
    // unmarshal a magazine or whatever
}