BSD-3-Clause licensed by Colin Runciman, Roman Cheplyaka
Maintained by Andrew Lelechenko
This version can be pinned in stack with:smallcheck-,1862

Module documentation for

SmallCheck: a property-based testing library for Haskell

As of 2023, this library is largely obsolete: arbitrary test generators with shrinking such as falsify offer much better user experience.

SmallCheck is a testing library that allows to verify properties for all test cases up to some depth. The test cases are generated automatically by SmallCheck.

Usefulness of such an approach to testing is based on the following observation:

If a program fails to meet its specification in some cases, it almost always fails in some simple case.

In many ways SmallCheck is very similar to QuickCheck. It uses the idea of type-based generators for test data, and the way testable properties are expressed is closely based on the QuickCheck approach. Like QuickCheck, SmallCheck tests whether properties hold for finite completely defined values at specific types, and reports counter-examples.

The big difference is that instead of using a sample of randomly generated values, SmallCheck tests properties for all the finitely many values up to some depth, progressively increasing the depth used. For data values, depth means depth of construction. For functional values, it is a measure combining the depth to which arguments may be evaluated and the depth of possible results.

The package is based on the paper by Colin Runciman, Matthew Naylor and Fredrik Lindblad.




  • Annotate everything with @since pragmas.

Version 1.2.1

  • Add Serial and CoSerial instances for Ordering.

Version 1.2.0

  • Add Serial and CoSerial instances for (,,,,), (,,,,,), Compose, Foreign.C.Types, Data.List.NonEmpty, Void, Complex.
  • Add Bounded, Functor, Foldable and Traversable instances for Positive and NonNegative wrappers.
  • Add NonZero wrapper for non-zero integers.
  • Add cons5, cons6, alts5, alts6.

Version 1.1.7

  • Fix overlapping instances of GSerial.

Version 1.1.6

  • Mark modules as Safe, not just Trustworthy.

Version 1.1.5

  • Add limit :: Monad m => Int -> Series m a -> Series m a
  • Add genericSeries and genericCoseries, so that you can use the generic implementations more flexibly. Previously, the generic implementation was only available as the default method for series/coseries but not as standalone functions.

Version 1.1.4

  • Add instances for fixed-width Int and Word types (Int8, Word8 etc.)


  • Fix compatibility with GHC 7.8 and older

Version 1.1.3

  • Add Serial and CoSerial instances for Word and Natural

Version 1.1.2

  • Export the test function
  • Add a listSeries function

Version 1.1.1

Export some auxiliary functions from T.S.Series, and document how to express consN and altsN for N > 4.


Documentation fixes

Version 1.1

  • Add a Serial instance for Ratio
  • Add the NonEmpty wrapper for lists
  • Add listM (the monadic version of list)
  • Add optional explanation for test outcomes

Version 1.0.4

Fix compatibility with GHC 7.4.1

Version 1.0.3

Fix a bug where no test cases were generated for some functional types (#19).

Version 1.0.2

Fix a bug in the generic instance

Version 1.0.1

Make SmallCheck build with GHC 7.4

Version 1.0

This is a major incompatible release of SmallCheck. Virtually every function has changed its name, type, semantics or module. So please carefully read the docs when upgrading.

For some highlights, see this blog post.

Version 0.6.2

  • Derive Typeable Property instance
  • Add smallCheckPure

Version 0.6.1

  • Documentation improvements
  • Make the package build with GHC 7.4.1

Version 0.6

  • Default Generic implementation of Serial instance (by Bas van Dijk)
  • The code is split into modules
  • Convert much of README into haddock documentation
  • Many small API changes
  • Remove impure Testable (IO a) instance

Version 0.5

Make the package build with GHC 7.2. Some cosmetic changes.

Version 0.4

The module SmallCheck is now Test.SmallCheck. Packaged with Cabal.

Version 0.3

Existential quantifiers now have unique variants for which two witnesses are reported when uniqueness fails. The over-generating coseries method for functions of functional arguments has been replaced; now ‘coseries’ and the ‘alts’ family take a series argument. Test counters are now Integers, not Ints. Ord and Eq are now derived for the N types. Examples extended.

Version 0.2

The ‘smallCheck’ driver now takes an argument d and runs test series at depths 0..d without interaction, stopping if any test fails. The interactive variant is still available as ‘smallCheckI’. All Prelude numeric types now have Serial instances, including floating-point types. Serial types Nat and Natural are also defined. Examples extended.

Version 0.1

The differences from 0.0 are two fixes (space-fault, output buffering), an ‘unsafe’ but sometimes useful Testable (IO a) instance and additional examples.