Hey Guys! I was writing a small implementation of the earliest-end-first algorithm for the Interval Scheduling problem just now. When I was done, I thought it would be a nice thing to have a QuickCheck property for my code. This is what I came up with: -- Intervals are just triplets type Interval a t = (a,t,t) end :: Interval a t -> t end (_,_,t) = t begin :: Interval a t -> t begin (_,t,_) = t And so the property: prop_schedule :: Ord t => [Interval a t] -> Bool prop_schedule [] = True prop_schedule [a] = True prop_schedule (x:y:ys) = end x <= begin y && prop_schedule (y:ys) Essentially, it looks up wheather the given list is "sorted" (given the constraints of the problem). However, in this property I forgot to add that the lists should have been run through my algorithm, which I noticed only after this strange problem appeared: *Interval> quickCheck prop_schedule +++ OK, passed 100 tests. How come QuickCheck passes 100 tests of random lists? One would think that at least one of the generated lists would be unsorted. It also passes 1000 and even 10000 tests. It also seems that changing the type helps: prop_schedule :: [Interval a Int] -> Bool ... *Interval> quickCheck prop_schedule *** Failed! Falsifiable (after 5 tests and 1 shrink): [((),0,0),((),-1,0)] -- Tobias Olausson tobsan@gmail.com