Monthly Downloads: 56
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: Testing     Hedgehog    
Latest version: v0.2.5

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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](imgs/badlist.png "Here we can see a definition of foldl' that does not accumulate strictly")

[alt text](imgs/badsemigroup.png "Here we can see a semigroup instance which is not associative")

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.