Why can\'t I use variables in struct tags in Go?

Using Variables in Struct Tags

In Go, you can use a variable in a struct tag for metaprogramming purposes. This allows you to dynamically generate struct tags at compile time.

Working Example

Consider the following code:

<code class="go">type Shape struct {
    Type string `json:"type"`
}</code>

In this example, a literal string is used in the json struct tag to specify the field's JSON representation.

Using fmt.Sprintf

However, using fmt.Sprintf to build the struct tag dynamically does not work. The following code will result in a syntax error:

<code class="go">const (
    TYPE = "type"
)

type Shape struct {
    Type string fmt.Sprintf("json:\"%s\"", TYPE)
}</code>

The reason for this is that Go does not allow statements that evaluate at runtime in struct tags. Only compile-time string literals are permitted.

Why It's Not Allowed

Using variables in struct tags is not allowed in Go because it could lead to performance issues. If the struct tag values change dynamically, it could affect the program's behavior in unexpected ways. Therefore, Go restricts the use of struct tags to static string literals for performance and consistency reasons.

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