Golang : Struct field tags and what is their purpose?

Got a curious question asked by a colleague today on what's the purpose on field tags inside a struct. For the uninitiated, field tags look like this :

 type Employee struct {
 Name string `json:"name" xml:"name"`
 Age int `json:"age,omitempty" xml:"age,omitempty"`
 }

the [json:"name" xml:"name"] is what a Golang programmer refer to field tags.

Basically, it is a form of meta-data that instruct your software on how to handle data fields inside the structure during compile-time or runtime. For the example code above, the instruction is to map the Name and Age data fields to their equivalent field names should the program generate JSON or XML files. The tag instruction omitempty instruct the JSON or XML code generated to ignore the empty data fields should the Age field is empty.

Apart from the most common field tags for JSON and XML. You may encounter the schema field tags use by http://godoc.org/github.com/gorilla/schema or the datastore field tags - used by Google App Engine.