What are the use(s) for tags in Go?

Question

In the Go Language Specification, it mentions a brief overview of tags:

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. The tags are made visible through a reflection interface but are otherwise ignored.

// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
  microsec  uint64 "field 1"
  serverIP6 uint64 "field 2"
  process   string "field 3"
}

This is a very short explanation IMO, and I was wondering if anyone could provide me with what use these tags would be?

Answer 1

Here is a really simple example of tags being used with the encoding/json package to control how fields are interpreted during encoding and decoding:

Try live: http://play.golang.org/p/BMeR8p1cKf

package main

import (
    "fmt"
    "encoding/json"
)

type Person struct {
    FirstName  string `json:"first_name"`
    LastName   string `json:"last_name"`
    MiddleName string `json:"middle_name,omitempty"`
}

func main() {
    json_string := `
    {
        "first_name": "John",
        "last_name": "Smith"
    }`

    person := new(Person)
    json.Unmarshal([]byte(json_string), person)
    fmt.Println(person)

    new_json, _ := json.Marshal(person)
    fmt.Printf("%s\n", new_json)
}

// *Output*
// &{John Smith }
// {"first_name":"John","last_name":"Smith"}

The json package can look at the tags for the field and be told how to map json <=> struct field, and also extra options like whether it should ignore empty fields when serializing back to json.

Basically, any package can use reflection on the fields to look at tag values and act on those values. There is a little more info about them in the reflect package
http://golang.org/pkg/reflect/#StructTag :

By convention, tag strings are a concatenation of optionally space-separated key:"value" pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ' '), quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using U+0022 '"' characters and Go string literal syntax.

Answer 2

It's some sort of specifications that specifies how packages treat with a field that is tagged.

for example:

type User struct {
    FirstName string `json:"first_name"`
    LastName string `json:"last_name"`
}

json tag informs json package that marshalled output of following user

u := User{
        FirstName: "some first name",
        LastName:  "some last name",
    }

would be like this:

{"first_name":"some first name","last_name":"some last name"}

other example is gorm package tags declares how database migrations must be done:

type User struct {
  gorm.Model
  Name         string
  Age          sql.NullInt64
  Birthday     *time.Time
  Email        string  `gorm:"type:varchar(100);unique_index"`
  Role         string  `gorm:"size:255"` // set field size to 255
  MemberNumber *string `gorm:"unique;not null"` // set member number to unique and not null
  Num          int     `gorm:"AUTO_INCREMENT"` // set num to auto incrementable
  Address      string  `gorm:"index:addr"` // create index with name `addr` for address
  IgnoreMe     int     `gorm:"-"` // ignore this field
}

In this example for the field Email with gorm tag we declare that corresponding column in database for the field email must be of type varchar and 100 maximum length and it also must have unique index.

other example is binding tags that are used very mostly in gin package.

type Login struct {
    User     string `form:"user" json:"user" xml:"user"  binding:"required"`
    Password string `form:"password" json:"password" xml:"password" binding:"required"`
}


var json Login
if err := c.ShouldBindJSON(&json); err != nil {
     c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
     return
}

the binding tag in this example gives hint to gin package that the data sent to API must have user and password fields cause these fields are tagged as required.

So generraly tags are data that packages require to know how should they treat with data of type different structs and best way to get familiar with the tags a package needs is READING A PACKAGE DOCUMENTATION COMPLETELY.