Why there are tags in Go?

A small tutorial with code examples about tags in Golang programming language
10 August 2017   1097

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"

GoLand 2018.3 to be Released by JetBrains

New version of IDE for Go programming language has a lot of new features
22 November 2018   381

Team JetBrains introduced a new version of IDE GoLand 2018.3. In this update, users will see:

  • refactoring by changing the signature;
  • memory dump;
  • Testify support;
  • new debugger tools;
  • Improved verification, prediction and code completion;
  • chart support;
  • updates for VCS, Docker, Kubernetes.

The new method of processing the code Change Signature will allow a single action to change the signature of functions and methods throughout the workspace. The function will show what will change in the code after application. The Inline tool will highlight the embedded code, and Rename will notify you of possible conflicts when renaming.

Added the ability to run and debug Google App Engine applications locally.

Go memory dumps are now available via Run / Open Core Dump directly in the IDE. To automatically create memory dumps, you need to enter GOTRACEBACK = crash in the Environment field.

Also, GoLand 2018.3 supports the Mozilla debugger rr, which allows you to search for a crash in the program by playing back its execution.

Get more info at official blog.

The previous version of GoLand 2018.2 was released in July 2018.