What is Shapeless in Scala?

Shapeless is a type-level generic programming library for Scala, enabling polymorphic operations across arbitrary data structures. Core abstractions: HList: heterogeneous list with type-safe access — 1 :: "hello" :: true :: HNil has type Int :: String :: Boolean :: HNil. Generic: derive HList representation from case classes automatically. Poly: polymorphic functions that work across different types in an HList. Coproduct: type-safe discriminated union. Shapeless enables generic derivation of type class instances — for example, deriving Circe Encoder/Decoder for any case class automatically without writing boilerplate serialization code. Libraries like Circe, Magnolia (simpler alternative), and Monocle use Shapeless or similar techniques. In Scala 3, many Shapeless use cases are handled by the built-in Mirror mechanism and derives keyword, reducing Shapeless's necessity.