frunk

frunk brings Haskell/Scala-style generic programming to Rust, inspired by Scala's shapeless library. It provides three major abstractions: heterogeneous lists, type-level sum types, and struct-to-struct conversion — all type-checked at compile time with zero runtime cost.

HList (heterogeneous lists)

HList!(i32, String, bool) creates a type-safe heterogeneous list with type-safe indexing, mapping, folding, and appending. Unlike tuples, HLists support generic operations over their structure — you can write code that works on any HList regardless of length.

Coproduct (type-level sum types)

Coprod!(i32, f32, bool) creates a type-level sum type with inject, get, and fold operations. This is the "advanced layer" alternative to either's simple Either<L, R> — supporting any number of types with compile-time type checking.

Generic / LabelledGeneric

The Generic trait converts structs to/from HLists, enabling struct-to-struct conversion between structurally identical types without manual field mapping. LabelledGeneric adds field-name awareness, so conversion works even when field order differs (as long as names and types match). This is frunk's killer feature for reducing boilerplate in DTO/model conversion layers.

Validated (error accumulation)

Validated collects all errors from a computation rather than short-circuiting on the first one — the Applicative-style pattern from functional programming. Useful for form validation, configuration parsing, and anywhere you want to report all problems at once.

When to use frunk

frunk is the right tool when you need type-level generic programming: struct conversion, heterogeneous collections, or type-safe coproducts. It's not a casual dependency — the learning curve mirrors Scala's shapeless. For simpler needs, either covers two-variant sum types, and derive-more handles basic trait derivation. See rust-enum-crates for the full landscape and typenum for the foundational type-level arithmetic.