Why there are tags in Go?

A small tutorial with code examples about tags in Golang programming language
10 August 2017   956

Hype.Codes team continues to answer popular technical questions about programming languages. We made a research and found out what tags "do" in Go.

A tag for a field allows you to attach meta-information to the field which can be acquired using reflection. Usually it is used to provide transformation info on how a struct field is encoded to or decoded from another format (or stored/retrieved from a database), but you can use it to store whatever meta-info you want to, either intended for another package or for your own use.

As mentioned in the documentation of reflect.StructTag, by convention the value of a tag string is a space-separated key:"value" pairs, for example:

type User struct {
    Name string `json:"name" xml:"name"`
}

The key usually denotes the package that the subsequent "value" is for, for example jsonkeys are processed/used by the encoding/json package.

If multiple information is to be passed in the "value", usually it is specified by separating it with a comma (','), e.g.

Name string `json:"name,omitempty" xml:"name"`

Usually a dash value ('-') for the "value" means to exclude the field from the process (e.g. in case of json it means not to marshal or unmarshal that field).

Example of accessing your custom tags using reflection

We can use reflection (reflect package) to access the tag values of struct fields. Basically we need to acquire the Type of our struct, and then we can query fields e.g. with Type.Field(i int) or Type.FieldByName(name string). These methods return a value of StructFieldwhich describe / represent a struct field; and StructField.Tag is a value of type StructTagwhich describes / represents a tag value.

Previously we talked about "convention". This convention means that if you follow it, you may use the StructTag.Get(key string) method which parses the value of a tag and returns you the "value" of the key you specify. The convention is implemented / built into this Get() method. If you don't follow the convention, Get() will not be able to parse key:"value" pairs and find what you're looking for. That's also not a problem, but then you need to implement your own parsing logic.

Also there is StructTag.Lookup() (was added in Go 1.7) which is "like Get() but distinguishes the tag not containing the given key from the tag associating an empty string with the given key".

So let's see a simple example:

type User struct {
    Name  string `mytag:"MyName"`
    Email string `mytag:"MyEmail"`
}

u := User{"Bob", "bob@mycompany.com"}
t := reflect.TypeOf(u)

for _, fieldName := range []string{"Name", "Email"} {
    field, found := t.FieldByName(fieldName)
    if !found {
        continue
    }
    fmt.Printf("\nField: User.%s\n", fieldName)
    fmt.Printf("\tWhole tag value : %q\n", field.Tag)
    fmt.Printf("\tValue of 'mytag': %q\n", field.Tag.Get("mytag"))
}

Output :

Field: User.Name
    Whole tag value : "mytag:\"MyName\""
    Value of 'mytag': "MyName"

Field: User.Email
    Whole tag value : "mytag:\"MyEmail\""
    Value of 'mytag': "MyEmail"

Go 1.11 to be Released

According to the developers, the most significant changes in the release concern the support of WebAssembly and the new concept of modules
27 August 2018   395

The Go team announced a stable release of the version of the language under the number 1.11. According to the developers, the most significant changes in the release concern the support of WebAssembly and the new concept of modules. Go 1.11 requires versions of the OS no older than OpenBSD 6.2, macOS 10.10 Yosemite or Windows 7.

Go 1.11 New Features

  • Go 1.11 adds preliminary support for a new concept called “modules', an alternative to GOPATH with integrated support for versioning and package distribution. Using modules, developers are no longer confined to working inside GOPATH, version dependency information is explicit yet lightweight, and builds are more reliable and reproducible.
  • Go 1.11 adds an experimental port to WebAssembly (js/wasm).
  • A new package is presented that provides a simple API for searching and downloading packages with the original Go-code.
  • Improved presentation of information during debugging, including information on line numbers and placement of breakpoints.
  • Added support for more functions for embedding by default, including those that cause panic.
  • A new format for exporting these packages is presented. It is assumed that for end users it will be more transparent and understandable, besides, it speeds up the assembly of large projects. In case of problems, you can turn it off at compile time.

Learn more at Release Notes. Previous Go version was released in February.