These QuickCheck properties don't really test your sort function. The `a' type variable will be instantiated to (). So you will test with lists of units, like so: ghci> quickCheck (\xs -> isSorted (reverse xs)) OK, passed 100 tests. This can be simply solved by added a more specific type signature: isSorted :: [Int] -> Bool Surly, if the sort function works for Ints, it'll work for any Ord a. - Tom

---------- Forwarded message ---------- From: Ryan Ingram To: Eugene Kirpichov Date: Mon, 18 May 2009 13:57:04 -0700 Subject: Re: [Haskell-cafe] Haskell in 3 Slides On Mon, May 18, 2009 at 11:33 AM, Eugene Kirpichov wrote:

...

The main bullet point is missing: Correctness.

How could we have forgotten quickcheck?

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quickCheck (\xs -> sort (sort xs) == sort xs) OK, 100 tests passed.

I like this, but given that you have a whole slide, I might write this:

isSorted :: Ord a => [a] -> Bool isSorted [] = True isSorted [x] = True isSorted (x:y:rest) = x <= y && isSorted (y:rest)

...

quickCheck (\xs -> isSorted (sort xs)) OK, 100 tests passed.

Or, if you want to lead into a talk about fusion and/or higher order functions:

isSorted [] = True isSorted (x:xs) = snd $ foldl' check (head x, True) xs where    check (prevElem, restSorted) thisElem = (thisElem, prevElem <= thisElem && restSorted)

 -- ryan