smallcheck: Another lightweight testing library in Haskell.

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SmallCheck is similar to QuickCheck (Claessen and Hughes 2000-) but instead of testing for a sample of randomly generated values, SmallCheck tests properties for all the finitely many values up to some depth, progressively increasing the depth used.

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Dependencies base (==4.*) [details]
License BSD-3-Clause
Author Colin Runciman
Maintainer Roman Cheplyaka <>
Category Testing
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Uploaded by RomanCheplyaka at Sat Nov 19 08:47:41 UTC 2011
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Readme for smallcheck-0.5

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SmallCheck: another lightweight testing library in Haskell

If you are a Haskell programmer and a QuickCheck user do you ever wish you could:

  • write test generators for your own types more easily?
  • be sure that any counter-examples found are minimal?
  • write properties using existentials as well as universals?
  • establish complete coverage of a defined test-space?
  • display counter-examples of functional type?
  • always repeat tests and obtain the same results?

If so, try SmallCheck! This note should be enough to get you started, assuming some prior experience with QuickCheck.

Similarities and Differences

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.

QuickCheck's statistics-gathering operators have been omitted from SmallCheck's property language, as they seem more relevant to the random-testing approach.

Data Generators

SmallCheck itself defines data generators for all the data types used by the Prelude.

Writing SmallCheck generators for application-specific types is straightforward. Just as the QuickCheck user defines 'arbitrary' generators, so a SmallCheck user defines 'series' generators -- but it is a more straightforward task, using SmallCheck's cons<N> family of generic combinators where N is constructor arity. For example:

data Tree a = Null | Fork Tree a Tree

instance Serial a => Serial (Tree a) where
  series = cons0 Null \/ cons3 Fork

The default interpretation of depth for datatypes is the depth of nested construction: constructor functions, including those for newtypes, build results with depth one greater than their deepest argument. But this default can be over-ridden by composing a cons<N> application with an application of 'depth', like this:

newtype Light a = Light a

instance Serial a => Serial (Light a) where
  series = cons1 Light . depth 0

The depth of Light x is just the depth of x.

Function Generators

To generate functions of an application-specific argument type requires a second method 'coseries' -- cf. 'coarbitrary' in QuickCheck. Again there is a standard pattern, this time using the alts<N> combinators where again N is constructor arity. Here are Tree and Light instances:

coseries rs d = [ \t -> case t of
                        Null         -> z
                        Fork t1 x t2 -> f t1 x t2
                |  z <- alts0 rs d ,
                   f <- alts3 rs d ]

coseries rs d = [ \l -> case l of
                        Light x -> f x
                |  f <- (alts1 rs . depth 0) d ]

(NB changed from Version 0.2: 'coseries' and 'alts<N>' family now take a series argument -- here rs. In the coseries definitions we simply pass on rs as series argument in each 'alts<N>' application.)

Automated Derivation of Generators

For small examples, Series instances are easy enough to define by hand, following the above patterns. But for programs with many or large data type definitions, automatic derivation using a tool such as 'derive' is a better option. For example, the following command-line appends to Prog.hs the Series instances for all data types defined there.

$ derive Prog.hs -d Serial --append 


SmallCheck's testable properties are closely based on those of QuickCheck but with the introduction of existential quantifiers. Suppose we have defined a function

isPrefix :: Eq a => [a] -> [a] -> Bool

and wish to specify it by some suitable property. Using QuickCheck we might define

prop_isPrefix1 :: String -> String -> Bool
prop_isPrefix1 xs ys = isPrefix xs (xs++ys)

where xs and ys are universally quantified. This property is necessary but not sufficient for a correct isPrefix. For example, it is satisfied by the function that always returns True! We can test the same property using SmallCheck. But we can also test the following property, which involves an existentially quantified variable:

prop_isPrefix2 :: String -> String -> Property
prop_isPrefix2 xs ys = isPrefix xs ys ==>
                         exists $ \xs' -> ys == xs++xs'

The default testing of existentials is bounded by the same depth as their context, here the depth-bound for xs and ys. This rule has important consequences. Just as a universal property may be satisfied when the depth bound is shallow but fail when it is deeper, so the reverse may be true for an existential property. So when testing properties involving existentials it may be appropriate to try deeper testing after a shallow failure. However, sometimes the default same-depth-bound interpretation of existential properties can make testing of a valid property fail at all depths. Here is a contrived but illustrative example:

prop_append1 :: [Bool] -> [Bool] -> Property
prop_append1 xs ys = exists $ \zs -> zs == xs++ys

Customised variants of 'exists' are handy in such circumstances. For example, 'existsDeeperBy' transforms the depth bound by a given Int->Int function:

prop_append2 :: [Bool] -> [Bool] -> Property
prop_append2 xs ys = existsDeeperBy (*2) $ \zs -> zs == xs++ys

There are also quantifiers for unique existence. Their names include a 1 immediately after 'exists': eg. exists1, exists1DeeperBy.

Pragmatics of ==>

As in QuickCheck, the ==> operator can be used to express a restricting condition under which a property should hold. For example, testing a propositional-logic module (see examples/logical), we might define:

prop_tautEval :: Proposition -> Environment -> Property
prop_tautEval p e =
  tautology p ==> eval p e

But here is an alternative definition:

prop_tautEval :: Proposition -> Property
prop_taut p =
  tautology p ==> \e -> eval p e

The first definition generates p and e for each test, whereas the second only generates e if the tautology p holds. This difference is not great in QuickCheck where single random values are generated, but in SmallCheck the second definition is far better as the test-space is reduced from PE to T'+TE where P, T, T' and E are the numbers of propositions, tautologies, non-tautologies and environments.


Just as QuickCheck has a top-level function 'quickCheck' so SmallCheck has 'smallCheck d'.

smallCheck  :: Testable a => Int -> a -> IO ()

It runs series of tests using depth bounds 0..d, stopping if any test fails, and prints a summary report or a counter-example. The variant:

smallCheckI :: Testable a =>        a -> IO ()

is interactive. Instead of requiring a maximum-depth argument, it invites the user to decide whether to do deeper tests and whether to continue after a failure. The interface is low-tech: y<return> (or just <return>) means "yes", anything else means "no". For example:

haskell> smallCheckI prop_append1
Depth 0:
  Completed 1 test(s) without failure.
  Deeper? y
Depth 1:
  Failed test no. 5. Test values follow.
  Continue? n
  Deeper? n

Having methods to generate series of all (depth-bounded) values of an argument type, SmallCheck can give at least partial information about the extension of a function. For example, if we test the property

prop_assoc op =
  \x y z -> (x `op` y) `op` z == x `op` (y `op` z)
  typeInfo = op :: Bool -> Bool -> Bool

the result is shown as follows.

haskell> smallCheckI prop_assoc
Depth 0:
  Failed test no. 22. Test values follow.

When (unique) existential properties are tested, any failure reports conclude with "non-existence" (or "non-uniqueness" followed by two witnesses).

Large Test Spaces

Using the standard generic scheme to define series of test value, it often turns out that at some small depth d the 10,000-100,000 tests are quickly checked, but at depth d+1 it is infeasible to complete the billions of tests. There are ways to reduce some dimensions of the search space so that other dimensions can be tested more deeply: for example, cut the scope of quantifiers to a small fixed domain (forAllElem, thereExistsElem), use newtypes to define restricted series for some data types (see the 'examples' directory) or assign depth >1 to some constructors.

Function spaces grow exponentially in relation to their result and argument spaces. Even with a depth bound, testing all functional arguments is a challenge. Keep base-types as small as possible. For example, try testing higher-order polymorphic functions over their () or Bool instances.

Final Notes

The name is intended to acknowledge QuickCheck, not to suggest that SmallCheck replaces it. See also Lazy SmallCheck. Each tool has its advantages and disadvantages when compared with the others.

SmallCheck is a Haskell 98 package (aside from using unsafePerformIO to test IO computations). It can be obtained from hackage.

Comments and suggestions are welcome.