shapeless

Shapeless is a Scala library designed to support generic and type-level programming. It enables programmers to write code that operates on many different data types by converting values to and from generic representations and by manipulating types directly at compile time. The central ideas are heterogeneous lists (HLists), which are lists whose elements can have different types; records, which pair element types with labels; and coproducts, a type-level sum type for representing alternatives.

The library provides the Generic and LabelledGeneric type classes to convert between a concrete case class or sealed trait family and its generic representation. It also includes a system of polymorphic functions (Poly), lazy evaluation helpers (Lazy), and a set of type-level operators under shapeless.ops for operations such as mapping, folding, selecting fields, and deriving typeclass instances.

Shapeless is widely used to implement boilerplate-free typeclass derivation, to perform generic programming tasks such as transforming, encoding, or comparing data structures, and to build generic serializers or validators. It has been developed in the Scala community, notably by Miles Sabin, and is maintained as part of the broader Typelevel ecosystem, with contributions from many users.

Limitations include increased compile times and complexity, reliance on advanced features of the Scala type system, and version compatibility concerns with newer Scala releases.