Introduction

Tag is a lightweight Go library for parsing raw struct tag strings into strongly typed Go values.

Go's reflect.StructTag gives you tag values as strings. That is enough for simple tags, but custom tag formats often need more: default values, boolean flags, lists, maps, nested options, custom types, and consistent error handling.

Tag lets you define the shape of a tag with a Go struct. The parser reads the raw tag expression and fills that struct with typed values.

Why Use Tag?

Without a shared parser, custom tags usually produce repeated parsing code:

• splitting strings by commas
• detecting boolean flags
• converting strings into numbers and booleans
• handling quoted values
• mapping option names to fields
• repeating edge-case logic across packages

Tag moves that work into a reusable parser and keeps your application code close to the data it actually needs.

Good Fits

Tag is useful when building custom tag formats for:

• configuration
• validation
• metadata mapping
• code generation
• framework and library annotations