Why Can\'t I Use Variables in Go Struct Tags?

Using Variables in Go Struct Tags

In Go, struct tags are used to specify metadata about the fields within a struct. While it is possible to define tags using string literals, attempts to use variables in their place result in errors.

Invalid Usage:

const (
    TYPE = "type"
)

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

This code will throw a syntax error, as it attempts to use a fmt.Sprintf statement to dynamically generate the tag value. Go requires struct tags to be compile-time string literals.

Valid Usage:

type Shape struct {
    Type string `json:"type"`
}

In this example, the tag is defined as a string literal. This is the correct way to specify a struct tag.

Explanation:

The key difference between the valid and invalid examples lies in the evaluation time. String literals are evaluated at compile time, while the fmt.Sprintf statement evaluates at runtime. Struct tags are used by various Go tools, such as JSON encoders and decoders, during compilation. These tools need the tag values to be known at compile time in order to properly generate the necessary code.

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