Typetag

Typetag, or type tag, is a data construct used in programming languages to carry information about a type alongside values or within the type system itself. It enables runtime reflection and generic programming in environments where type information is typically erased at runtime. A typetag can be a runtime object, a language feature, or part of a reflection API that encodes the type's name, structure, and parameters, allowing code to inspect or manipulate values based on their type without static assumptions.

In Scala, the term TypeTag refers to a runtime representation of a type, provided by the scala.reflect API. It captures full type information, including type parameters, and contrasts with ClassTag, which preserves only the erased runtime class. TypeTag enables operations such as inspecting type arguments of a generic collection or creating values via reflection.

Other languages provide similar mechanisms: Java uses Class<T> objects as a runtime handle to a type; C++ offers RTTI through typeid and dynamic_cast. Languages with reified generics or specialization (like Kotlin, Swift) offer different tags or metadata for generics.

Common uses of typetags include generic deserialization, dynamic dispatch in generic containers, serialization frameworks, and testing tools that require knowledge of concrete types at runtime. Limitations can include runtime overhead, increased complexity, and partial or restricted support for reified types depending on the language and its type system.