another point: "deforested" treesort is slower. hartthoma@linuxpt:~/ProjectRepos/learning/quicksort>time ghc -e "test treeSort' 6" quicksort 1000000 real 4m3.615s user 1m59.525s sys 0m0.587s The commented indicated that -- If you deforest this algorithm (removing the intermediate tree structure) you are left with treeSort' [] = [] treeSort' (x:xs) = treeSort' (filter (<= x) xs) ++ [x] ++ treeSort' (filter (x <) xs) So.. my take home lesson is that deforestation isn't a performance neutral thing to do. Assuming the comment is correct. (I don't consider myself qualified to judge.) t. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.