[Haskell-cafe] Maintaining laziness: another example

Hello, While grading a Haskell student's work I ran into an example of a program not being lazy enough. Since it's such a basic and nice example I thought I'd share it with you: One part of the assignment was to define append :: [a] -> [a] -> [a], another to define cycle2 :: [a] -> [a]. This was her (the majority of the students in this course is female!) solution:

append :: [a] -> [a] -> [a] append [] ys = ys append xs [] = xs append (x:xs) ys = x : (append xs ys)

cycle2 :: [a] -> [a] cycle2 [] = error "empty list" cycle2 xs = append xs (cycle2 xs)

This definition of append works fine on any non-bottom input (empty, finite, infinite), but due to the unnecessary second append case, cycle2 never produces any result. Martijn.