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hedgehog-classes Hackage

Hedgehog will eat your typeclass bugs.


hedgehog-classes is a wrapper around Hedgehog that aims to provide a simple, straightforward API for testing common typeclass laws quickly, while providing good error messages to help debug any failing tests. It is inspired by the quickcheck-classes library.

API Overview

The API of hedgehog-classes is dead simple. There are three parts.

The first part is a datatype, called ‘Laws’, which looks like this:

data Laws = Laws
  { lawsTypeclass :: String
  , lawsProperties :: [(String,Property)]

It is a typeclass name along with a list of named property tests.

The second part of hedgehog-classes are the functions, which follow a simple structure. All functions in hedgehog-classes have one of the following three type signatures, based on the kind of the type which the corresponding typeclass parameterises (Nullary, Unary, or Binary). Note that they all return a ‘Laws’, only the inputs are different. Below, ‘Ctx’ refers to the typeclass in question:

-- Typeclasses that have kind 'Type -> Constraint', e.g. 'Eq'
tcLaw :: (Ctx a, Eq a, Show a) => Gen a -> Laws

-- Typeclasses that have kind '(Type -> Type) -> Constraint', e.g. 'Functor'
tcLaw1 ::
  ( Ctx f
  , forall x. Eq x => Eq (f x)
  , forall x. Show x => Show (f x)
  ) => (forall x. Gen x -> Gen (f x)) -> Laws

-- Typeclasses that have kind '(Type -> Type -> Type) -> Constraint', e.g. 'Bifunctor'
tcLaw2 ::
  ( Ctx f
  , forall x y. (Eq x, Eq y) => Eq (f x y)
  , forall x y. (Show x, Show y) => Show (f x y)
  ) => (forall x y. Gen x -> Gen y -> Gen (f x y)) -> Laws

The third and last part of hedgehog-classes are the three convenience functions used to run your tests. They all return an IO Bool, where True is returned if all the tests pass, and False otherwise. They are as following:

-- Test a single typeclasses' laws.
lawsCheck :: Laws -> IO Bool

-- Test multiple typeclass laws for a single type.
lawsCheckOne :: Gen a -> [Gen a -> Laws] -> IO Bool

-- Test mutliple typeclass laws for multiple types.
-- The argument is pairs of type names and their associated laws to test.
lawsCheckMany :: [(String, [Laws])] -> IO Bool

That is all there is to using hedgehog-classes in your test suite. For usage examples, see the haddocks.

Distributing your own Laws

hedgehog-classes also exports some functions which you may find useful for writing functions that allow users to test the laws of typeclasses you define in your own libraries, along with utilities for providing custom error messages. They can be found here.

Example error messages

Below is an example of an error message one might get from a failed test from hedgehog-classes:

alt text

alt text

Similar libraries

There are a number of libraries that have similar goals to hedgehog-classes:

Supported Typeclasses

  • base
    • Alternative
    • Applicative
    • Arrow
    • Bifoldable
    • Bifunctor
    • Bitraversable
    • Bits/FiniteBits
    • Category
    • Contravariant
    • Enum
    • Eq
    • Foldable
    • Functor
    • Generic
    • Integral
    • Monad
    • MonadIO
    • MonadPlus
    • MonadZip
    • Ord
    • Semigroup
    • Show
    • ShowRead
    • Storable
    • Traversable
  • aeson
    • ToJSON
    • ToJSON/FromJSON
  • comonad
    • Comonad
  • semirings
    • Semiring
    • Ring
  • primitive
    • Prim

Some typeclasses can have additional laws, which are not part of their sufficient definition. A common example is commutativity of a monoid. In such cases where this is sensible, hedgehog-classes provides functions such commutativeMonoidLaws, commutativeSemigroupLaws, etc. hedgehog-classes also tests that foldl'/foldr' actually accumulate strictly. There are other such cases that are documented on Hackage.

Support will be added for the typeclasses from semigroupoids.

Support will be added for the Semiring/Ring typeclasses from semirings.


Currently, you need GHC >= 8.5 to build this (because of -XQuantifiedConstraints). Some CPP can be used to make this buildable with older GHCs, I just have not done so yet. I would gladly take a PR that does so, but only for GHC 8.2.2 and newer.

To use this library for testing, just add it to a test stanza of your cabal file.

To use this library to export your own Laws functions which you wish to distribute, add it to the library stanza of your cabal file.


There are a number of improvements that can be made to the API of hedgehog-classes:

  • Traversable needs better error messages, without exposing library internals.
  • Arrow Laws 5/6/7 need names.
  • Some laws could use better names, as some of them I had to make up.
  • ixLaws can accidentally be extremely inefficient and I’m not sure how to fix that.
  • The test suite is incomplete.
  • There is no ‘bad’ test suite, for testing error messages.
  • There could be spelling mistakes/grammatical errors/inconsistencies in the custom error messages.

You can help fix any of the above by opening an issue/PR! Thanks.



hedgehog-classes uses PVP Versioning. The changelog is available on GitHub.

  • Bump upper bound on pretty-show from <1.10 to <1.11


  • Add MUVector laws
  • Update upper bounds on dependencies

  • Fix error introduced by change of hedgehog’s internal API between hedgehog-1.0.1 and hedgehog-1.0.2.
  • Re-add GHC 8.8.1 to cabal’s tested-with field.


  • Semirings upper bound increased to 0.6. [0.2, 0.5) -> [0.2, 0.6)
  • Add primLaws.
  • Remove GHC 8.8.1 from cabal’s tested-with field.
  • Add documentation to comonadLaws.


  • Semirings upper bound increased to 0.5. Lower bound not touched. [0.2, 0.4) -> [0.2, 0.5)
  • Add comonadLaws.


  • fix problem in storable set-get that caused attempt to index into 0-element malloc’d array
  • Test suite now tests almost all laws sans arrow/category (thanks @ag-eitilt!)
  • Correct tcName of MonadPlus. Was Monad, now it’s MonadPlus.


  • fix problem where ordLaws failed for everything. there was some messed up logic used to check that transitivity held. Thanks very much to @ocharles for reporting this.

  • improve reliability of hedgehog output filtering.


  • switch to hedgehog-1.0
  • add binaryLaws
  • relax cabal-version to 2.2
  • use randomly generated, not hard-coded functions, in bifoldable tests
  • significantly simplify pretty printing using silently package, and bad hack.
  • make several haddock improvements.


  • add semiringLaws, ringLaws, starLaws
  • fix bug in foldableLaws that could cause implementations of foldMap and fold that evaluate in weird orders to pass (rather than fail).


  • Initial (stable) hackage release.


  • Initially created.