Why there are tags in Go?

A small tutorial with code examples about tags in Golang programming language
10 August 2017   1204

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"

Microsoft to Develop Golua

Golua is Lua 5.3 engine, implemented in Go programming language
19 November 2018   368

Microsoft has released to the public golua engine designed to execute scripts in the Lua language. The development is distributed under the open MIT license and is available in the GitHub repository. The project is implemented in the language of Go.

There are already several implementations of Lua VM in Go, for example, DCLua, GoLua or glua. However, the developers state that they needed support for the syntax of version 5.3, and none of the existing tools could offer this either now or in the near future. In addition, simple and clear software interfaces were required to integrate the engine with Go.

The new development is based on the architecture, focused on convenient debugging, search and error handling. Although the creators of golua recognize that they have not yet managed to achieve full compliance with specification 5.3, they intend to further develop the project.

Lua is an open source scripting programming language interpreter. It is distinguished by simple integration into other languages ​​and the possibility of implementing a large number of software entities with a minimum of syntax tools.

The official Lua interpreter is written in C. Go was developed by Google as a replacement for C and C ++ and has the potential to provide greater speed. The company maintains and regularly updates its development; at the end of the summer of 2018, Go 1.11 was released.