[Haskell-cafe] ANN: quickcheck-properties

[Alexey Khudyakov]

Hello

It always puzzled me why there are no packages for for testing general
type classes laws. (Monoid laws, monad laws etc). It looks like ideal
case for quickcheck and smallcheck.

I'm glad to present package quickcheck-properties[1]. It containt set
of generic properties for semiroups, monoids, groups and
unsatisfactory imlementatation of functor's properties. Despite its name
package do not depend on quickcheck and could be used with smallcheck as 
well.

   ====================
     Examples
   ====================

Here are few examples:

 >>> quickCheck $ eq $ prop_Monoid (T :: T [Int])
+++ OK, passed 100 tests.
 >>> quickCheck $ eq $ prop_Monoid (T :: T (Maybe [Int]))
+++ OK, passed 100 tests.


prop_Monoid tests all monoid properties: associativity, left and right
identity. T is used to fix type of monoid to test. It could be done
with type signature for prop_Monoid but writing signature for three
parameter function is just too cumbersome.

eq compares expression in properties using == operator. Many (most?)
of properties have form
 > expression = another expression

Easiest solution is to compare them with ==. But is's not
satisfactory. Data type may not have Eq instance. Eq instance may
provide structural equality but some form of equivalince is
needed. etc. It's left to library user to decide how to compare
values.

In this contrived example lists are compared by length.
 > eqOn length $ prop_Monoid (T :: T [Int])

   ====================
     Problems
   ====================

There are unsolved problems. So it's announcement with
question. Functor/Applicative/Monads laws involve arbitrary functions.
It would be natural to generate them using quickcheck but I don't know
how to generate random functions. Any suggestions?

[1] http://hackage.haskell.org/package/quickcheck-properties


--
   Aleksey Khudyakov