Dear All I'm learning Haskell from a background in Python, and I'm just looking at the sort and sortBy functions in Data.List. In Python, the decorate-sort-undecorate pattern is a popular alternative to using an explicit compare function. For example, to sort a list of lists by length: def sortByLength(xs): dec_xs = [(len(x), x) for x in xs] dec_xs.sort() undec_xs = [x[1] for x in dec_xs] return undec_xs This is often preferred to something like: xs.sort(lambda x,y: len(x) > len(y)) # just used lambda so it fits on one line I think the reasoning is that plain sort() is done at C speed, whereas sort(func) is done in Python, so, depending on the list I suppose, dsu can be worth the trouble. (Sorry for vagueness). Anyway, I was wondering if dsu is popular in Haskell, and whether or when it would make sense to use dsu rather than sortBy. Here's a verbose dsu I wrote myself: sortDSU decFunc a = undecorate (sort (decorate decFunc a)) decorate decFunc [] = [] decorate decFunc (x:xs) = ( ((decFunc x), x) : decorate decFunc xs ) undecorate [] = [] undecorate ( (_, y) :xs) = ( y : undecorate xs ) Here's a terser and perhaps more idiomatic (but I think equivalent) dsu which I then found in a comment at the Real World Haskell website: dsuSort decFunc a = map snd (sort (zip (map decFunc a) a)) I have tested both functions with length and sum in ghci. So, do Haskell programmers use the decorate-sort-undecorate pattern? If not, why not? If so, when? Thanks and best wishes Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan@llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================