HCons

HCons is a constructor used for heterogeneous lists, commonly referred to as HLists, in typed functional programming languages such as Haskell. Unlike ordinary lists that hold elements of one uniform type, HLists permit elements of different types, with the type of each element encoded in the list’s type.

In a typical HList implementation, the type-level list describes the sequence of element types. The HCons constructor takes a head value of some type a and a tail that is itself an HList of types ts, producing an HList of types a ': ts. The empty list is represented by HNil. For example, the expression HCons 3 (HCons "four" HNil) yields an HList of types [Int, String], often written as HList '[Int, String] in Haskell with DataKinds.

HCons provides the primary means of constructing heterogeneously typed lists. Once built, elements are accessed in a type-safe way through pattern matching or type-driven operations, and the full type information is available at compile time. This enables generic programming techniques such as type-indexed lookups, type-level folds, and maps, while preserving precise element types.

Languages and libraries vary in naming and syntax. In Haskell, HCons is associated with libraries implementing HList, which rely on advanced features like GADTs and DataKinds. Other ecosystems offer analogous concepts under different names, such as the cons operator for heterogenous lists in Scala’s shapeless library.

Limitations include increased complexity and reliance on language extensions. HLists are most useful in generic programming, where maintaining strong type information across heterogeneous data structures provides compile-time guarantees at the cost of added verbosity and learning curve.