Laziness

Could someone explain to me why this doesn't work test l = hs where hs = map (\x -> [x]) [0..abs(l `div` hLen)] hLen = length $ head hs whereas this does test l = hs where hs = map (\x -> [x]) (0:[1..abs(l `div` hLen)]) hLen = length $ head hs I would have thought laziness would allow the compiler to know that hs would contain at least one element and therefore calculate hLen. Dominic.