Hedgehog

Property-based testing with integrated shrinking for OCaml.

Hedgehog is a property-based testing library in the spirit of Haskell Hedgehog. It provides:

• Integrated shrinking — generators produce a rose tree of outcomes, so every generator shrinks for free. No manual shrink functions needed.
• Range-controlled generation — numeric ranges separate the concept of bounds from the origin, giving you precise control over value distribution and shrink direction.
• Algebraic effects for assertions — test bodies use OCaml 5 effects for assert_, annotate, cover, and more, keeping generator and test logic cleanly separated.
• State machine testing — the Hedgehog.Stm module lets you test stateful systems against abstract models, with sequential and parallel linearizability checking.

Quick example

  open Hedgehog

  let () =
    Property.check
      Property.(property Gen.(
        let* xs = list (Range.linear 0 100) (int (Range.linear 0 1000)) in
        return (fun () ->
          assert_ (List.rev (List.rev xs) = xs))))
    |> ignore

Guides

• getting-started — Installation and first property
• motivation — Why integrated shrinking matters
• tutorial — Generators, properties, assertions, coverage
• state-testing — State machine testing with Stm
• alternatives — Comparison with QCheck and Jane Street Quickcheck
• resources — Talks, papers, and related projects

API reference

• Hedgehog.Gen — Generator combinators
• Hedgehog.Property — Property runner and assertion effects
• Hedgehog.Range — Numeric ranges for generation
• Hedgehog.Tree — Rose tree implementation
• Hedgehog.Shrink — Pure shrinking strategies
• Hedgehog.Seed — Splittable PRNG
• Hedgehog.Stm — Abstract state machine testing
• Hedgehog.Diff — Line-level diff algorithm