Can You Use Variables in Go Struct Tags?

Embedding Variables in Go Struct Tags

Go's struct tags, often used for annotation and metadata, generally involve straightforward string literals. However, users may encounter situations where dynamic or computed values are desired in these tags.

Consider the following structure with a "type" field annotated for JSON marshaling:

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

This approach works perfectly and directly embeds a string literal in the tag. However, one may attempt to inject a variable into the tag:

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

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

This more flexible approach fails with a syntax error due to the nature of Go struct tags. The language restricts struct tags to compile-time string literals and disallows dynamic or variable-based expressions.

Therefore, in Go, it is not possible to utilize variables in struct tags. This limitation stems from the fact that struct tags are essentially lexical annotations that must be known and processed at compile-time. Runtime evaluations are incompatible with this mechanism.

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