Writing tests for Haskell

There are many libraries for testing Haskell code. Besides classic unit tests with HUnit and spec testing in Ruby-style with Hspec, we can verify properties using SmallCheck and QuickCheck with exhaustive and randomized test cases, respectively.

Property checks

With QuickCheck we can test properties in a randomized fashion. We don't need to generate the test cases ourselves, as QuickCheck takes care of that. Here's a quick example of testing a simple arithmetic property:

stack ghci --package QuickCheck
> import Test.QuickCheck as QC
> QC.quickCheck $ \x y -> x > 0 ==> x + y >= y

All testable properties in QuickCheck are given as instances of the Testable class. As a quick reference, the core interface looks like this:

quickCheck :: Testable prop => prop → IO ()

class Testable prop where […]

instance Testable Property
instance Testable Bool
instance (Arbitrary a, Show a, Testable prop) => Testable (a → prop)

The last instance is perhaps most...