For the purposes of learning, I am trying to optimize some variation of the following code for computing all perfect numbers less than 10000. divisors i = [j | j<-[1..i-1], i `mod` j == 0] main = print [i | i<-[1..10000], i == sum (divisors i)] I know this is mathematically stupid, but the point is to do a moderate nested-loops computation. On my 2.33GHz dual-core MacBookPro, the obvious C program takes about .3 seconds, and a compiled OCaML program (tail recursion, no lists) about .33 seconds. The above takes about 4 seconds. I've tried using foldl', and doing explicit tail recursion with strict accumulators, but I can't get the running time below 3 seconds. Is it possible to come within striking distance of the other languages? Thanks. --PR