Editing Introduction to QuickCheck1 (section)

== Testing with QuickCheck ==

Now we can test the 'guts' of the algorithm, the take5 function, in
isolation. Let's use QuickCheck. First we need an Arbitrary instance for
the Char type -- this takes care of generating random Chars for us to
test with. I'll restrict it to a range of nice chars just for
simplicity:

<haskell>
import Data.Char
import Test.QuickCheck

instance Arbitrary Char where
    arbitrary     = choose ('\32', '\128')
    coarbitrary c = variant (ord c `rem` 4)
</haskell>

Let's fire up GHCi (or Hugs) and try some generic properties (it's nice
that we can use the QuickCheck testing library directly from the
Haskell prompt). An easy one first, a [Char] is equal to itself:

<haskell>
*A> quickCheck ((\s -> s == s) :: [Char] -> Bool)
OK, passed 100 tests.
</haskell>

What just happened? QuickCheck generated 100 random [Char] values, and
applied our property, checking the result was True for all cases.
QuickCheck ''generated the test sets for us''!

A more interesting property now: reversing twice is the identity:

<haskell>
*A> quickCheck ((\s -> (reverse.reverse) s == s) :: [Char] -> Bool)
OK, passed 100 tests.
</haskell>

Great!