QCheck vs Rapid

QCheck is a property-based testing library for OCaml, inspired by QuickCheck. Rather than relying only on hand-picked examples, it provides combinators to generate values such as lists, arrays, tuples, recursive types, and algebraic types for checking properties. When a check fails, generators can be paired with printers and shrinkers to show and reduce counterexamples. QCheck2 builds shrinking into its generators, though the project describes that API as less battle-tested than QCheck. The package family includes qcheck-core, qcheck-ounit, qcheck-alcotest, qcheck, and ppx_deriving_qcheck; the last can derive generators from type declarations. Runner options include QCheck_base_runner in qcheck-core and QCheck_runner with OUnit compatibility in qcheck, with integration layers for OUnit and Alcotest. The project also points to qcheck-stm for sequential and parallel model-based tests and qcheck-lin for sequential consistency testing. Installation is available through opam, and the source can be built with make. The minimal qcheck-core package depends only on unix and dune. QCheck is free and licensed under BSD-2-Clause.

Rapid has no write-up on Specifiction yet; its figures are above.