Hi Stuart,
If you're testing stand-alone functions, defaulting all type variables
to Integer can probably find a counterexample, provided it's not
something that depends on properties of an instance of Integer like:
prop_assoc a b c = (a + b) + c == a + (b + c)
-- fails for Double
But another type might find the counterexample faster. See this paper
(called Testing Polymorphic Properties):
http://publications.lib.chalmers.se/records/fulltext/local_99387.pdf
Unfortunately, I don't know of an implementation of the method they describe.
Regards,
Adam
On Wed, Feb 18, 2015 at 3:41 PM, Stuart Hungerford
Hi,
I'd like to use QuickCheck to test polymorphic properties and I'm wondering if there's any community consenus on how to do this while avoiding creating large amounts of boilerplate test code.
As an example, suppose I have this simple simplistic Monoid typeclass:
class Monoid a where zero :: a (|+|) :: a -> a -> a
And suppose I make these types Monoid instances:
instance Monoid Integer where zero = 0 (|+|) = (+)
instance Monoid Rational where zero = 0 (|+|) = (+)
instance (Monoid a, Monoid b) => Monoid (a, b) where zero = (zero, zero) (|+|) (x, y) (u, v) = (x |+| u, y |+| v)
Using QuickCheck and the Tasty test framework I'd like to test the Monoid laws across the relevant types (I'm only showing one part of one law here) as QuickCheck properties:
prop_left_add_id :: (Eq a, Monoid a) => a -> Bool prop_left_add_id x = zero |+| x == x
monoid_suite :: TestTree monoid_suite = testGroup "monoid" [ testProperty "left additive identity (Integer)" (prop_left_add_id :: Integer -> Bool),
testProperty "left additive identity (Rational)" (prop_left_add_id :: Rational -> Bool),
testProperty "left additive identity ((Integer, Integer))" (prop_left_add_id :: (Integer, Integer) -> Bool), -- ...]
And this is where I start creating a lot of essentially boilerplate code. I'm wondering if there's any consensus on the way to go about this kind of testing?
Researching this issue I've found ways of combining various GHC extensions and the type system to (I think) write the properties more at the type level. There are QuickCheck and Tasty modules that aim to find all properties in a module and test them, but polymorphic properties are automatically defaulted to Integer arguments. I'm also tempted by Template Haskell to write what's essentially code generation templates.
Is there a better way to go about this?
Thanks,
Stu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe