Hedgehog automatically generates a comprehensive array of test cases, exercising your software in ways human testers would never imagine.

Generate hundreds of test cases automatically, exposing even the most insidious of corner cases. Failures are automatically simplified, giving developers coherent, intelligible error messages.

Features

• Integrated shrinking, shrinks obey invariants by construction.
• Generators produce rose trees where the root is the generated value and children are shrunk alternatives.
• Range combinators for full control over the scope of generated numbers and collections.
• Equality and diff assertions with OCaml 5 algebraic effects.
• Monadic generator composition with let* / let+ / and+ binding operators.
• and+ enables parallel shrinking via Tree.mzip.
• State machine testing with sequential and parallel (linearizability) checking via Stm.
• Small footprint: the OCaml >= 5.1 stdlib plus domainslib for parallel checking.

Example

The main module, Hedgehog, includes everything you need to get started writing property tests.

open Hedgehog

Once you have your imports set up, you can write a simple property:

let prop_reverse =
  Property.property Gen.(
    let* xs = list (Range.linear 0 100) alpha in
    return (fun () ->
      Property.assert_ (List.rev (List.rev xs) = xs)))

You can then run it:

let () =
  if Property.check prop_reverse then
    print_endline "All tests passed."
  else
    exit 1

+++ OK, passed 100 tests.
All tests passed.

When a property fails, Hedgehog automatically finds a minimal counterexample:

let prop_bad =
  Property.property Gen.(
    let* n = int (Range.linear 0 1000) in
    return (fun () ->
      Property.annotate (Printf.sprintf "n = %d" n);
      Property.assert_ (n < 500)))

*** Failed! Falsifiable (after 17 tests):
  n = 500
  Assertion failed

State Machine Testing

The Stm module lets you test stateful systems by defining a model specification and checking that the real implementation matches:

open Hedgehog

module Counter_spec = struct
  type cmd = Incr | Decr | Get
  type state = int
  type sut = int ref
  type result = Unit | Int of int

  let show_cmd = function Incr -> "Incr" | Decr -> "Decr" | Get -> "Get"
  let show_result = function Unit -> "()" | Int n -> string_of_int n
  let gen_cmd _state = Gen.element [Incr; Decr; Get]
  let shrink_cmd _ = Seq.empty

  let init_state = 0
  let init_sut () = ref 0
  let cleanup _ = ()

  let next_state cmd state = match cmd with
    | Incr -> state + 1 | Decr -> state - 1 | Get -> state

  let precond _state _cmd = true

  let run cmd sut = match cmd with
    | Incr -> incr sut; Unit
    | Decr -> decr sut; Unit
    | Get -> Int !sut

  let postcond cmd state result = match cmd, result with
    | Get, Int n -> n = state
    | (Incr | Decr), Unit -> true
    | _ -> false
end

module Counter_stm = Stm.Make(Counter_spec)

Run a sequential test to check postconditions at each step:

let () =
  if Property.check (Counter_stm.sequential ()) then
    print_endline "Sequential: OK"

Run a parallel test to detect concurrency bugs via linearizability checking:

let () =
  if Property.check (Counter_stm.parallel ()) then
    print_endline "Parallel: OK"

Alcotest Integration

The hedgehog-alcotest package lets you run Hedgehog properties as Alcotest test cases:

opam install hedgehog-alcotest

Use Hedgehog_alcotest.to_alcotest to wrap a property:

let () =
  Alcotest.run "my-tests" [
    "properties", [
      Hedgehog_alcotest.to_alcotest "reverse involution"
        Hedgehog.(Property.property Gen.(
          let* xs = list (Range.linear 0 100) alpha in
          return (fun () ->
            Property.assert_ (List.rev (List.rev xs) = xs))));

      Hedgehog_alcotest.to_alcotest "small lists"
        Hedgehog.(Property.property Gen.(
          let* xs = list (Range.linear 0 100) (int (Range.linear 0 1000)) in
          return (fun () ->
            Property.annotate (Printf.sprintf "xs has %d elements" (List.length xs));
            Property.assert_ (List.length xs < 5))));
    ]
  ]

Passing properties return normally. Failures call Alcotest.fail with the shrunk counterexample:

[FAIL]  properties  1  small lists.
*** Failed! Falsifiable (after 9 tests):
  xs has 5 elements
  Assertion failed

Building

opam install . --deps-only --with-test
dune build

Running Tests

dune runtest

How It Works

Hedgehog is built on integrated shrinking, the same foundation QCheck2 adopted in 2021:

• Integrated shrinking via rose trees — Gen.t = int -> Seed.t -> 'a Tree.t option. Every generator produces a rose tree where the root is the generated value and children are shrunk alternatives, so shrinking always respects generator invariants (filters, preconditions) and no manual shrink functions are needed. QCheck2 works the same way; QCheck's original API and JS Quickcheck keep generators and shrinkers separate.
• Recursive tree binding — Gen.bind recursively binds through the entire shrink tree, matching Haskell Hedgehog's TreeT monad semantics. Composed generators (let*) automatically compose their shrinking.
• Subterm combinators — Gen.subterm, subterm2, subterm3 shrink recursive structures (ASTs, expressions) by trying subterms directly before shrinking within the constructor. Neither QCheck nor JS Quickcheck has an equivalent.
• Range-controlled generation — Range.t is a reusable value pairing an origin with size-dependent bounds, so Range.linear 0 100 scales with the size parameter and shrinks toward the origin wherever it is used. QCheck2 has the pieces separately (Gen.int_range ?origin, Gen.sized) but no combined abstraction. Golden ratio scaling in Gen.recursive naturally controls recursion depth.
• Algebraic effects — assert_, annotate, cover, classify use Effect.perform, keeping test logic cleanly separated from generators. QCheck returns test outcomes as values; JS Quickcheck uses exceptions and integrates with Expect_test.
• Coverage enforcement — cover 50.0 "positive" (n > 0) fails the property if fewer than 50% of test cases satisfy the condition. QCheck reports distributions via ?collect and ?stats but cannot enforce a threshold; JS Quickcheck has no equivalent.
• State machine testing in the library — Stm supports sequential and parallel (linearizability) checking with Domain.spawn. QCheck offers this through the separate qcheck-stm and qcheck-lin packages, which pioneered the approach for OCaml 5.
• Parallel property runner — Property.check_parallel runs a group's properties concurrently across domains via domainslib. QCheck's runners are sequential.
• LCS diff on failure — === and diff assertions produce line-level diffs between expected and actual values.
• Small dependency footprint — the stdlib plus domainslib. qcheck-core needs only unix; base_quickcheck pulls in base, ppxlib and several ppx_* libraries.

What Hedgehog lacks:

1. ppx derivation (QCheck has ppx_deriving_qcheck, JS Quickcheck [%quickcheck.generator: int list]) and
2. function generation (QCheck's Observable/Fn, JS Quickcheck's Observer.t).

Both require writing generators explicitly.

See the Alternatives page for a detailed comparison.

Architecture

Documentation

Full documentation is available at https://tmcgilchrist.github.io/ocaml-hedgehog/.

To build and preview the docs site locally:

opam install odoc          # one-time: needs odoc >= 3.2.1
cd website && npm install  # one-time: install Astro/Starlight
make website-dev           # generate markdown, then start the dev server

The site will be available at http://localhost:4321/ocaml-hedgehog/.

The documentation source lives in doc/*.mld files. These are processed by odoc's markdown backend into Starlight-compatible markdown, then built into a static site with Astro. Generating the markdown needs odoc >= 3.2.1 and dune >= 3.22 (which added the @doc-markdown alias); building and testing the library itself has no such requirement.

Resources

• haskell-hedgehog — The original Haskell implementation.
• Fast Splittable Pseudorandom Number Generators — Steele et al., on the design of splittable PRNGs.