Thank you very much m0nastic, it's a great explanation. Now I can see the mathematical elegance of the haskell code. An explicit algorithm of the same (pseudo

Thank you very much m0nastic, it's a great explanation. Now I can see the mathematical elegance of the haskell code. An explicit algorithm of the same (pseudo)code (using a mutable list) could be: function qsort(list) if list is [], return [] let x = extract_first_element(list) // car let smaller = [a | a <- list, a <= x] let larger = [b | b <- list, b > x] return qsort(smaller) ++ x ++ qsort(larger) And by writing this pseudo-code, I can see that in the haskell code, "smaller" and "larger" are clearly parallelizable, while in the "imperative"-"explicit algorithm" pseudo-code, the compiler cannot do such optimization... And then, I'm enlightened. Thank you.