Hi, OK so I had completed my (search) algorithm implementation. And the 'last' function was something like: "until p f x0". But now I wanted all iterations, I wanted to analyze the behavior of the algorithm. What could I do? I thought about using Trace... but it didn't seem adequate, I wanted to manipulate and use that info... maybe monads... that would cluter my algorithm so bad... This feeling lasted for a few seconds, then I thought, Lazy Evaluation! "iterate p f x0" There, done :) I had an infinite list, and I'd just take as much as I wanted. *Analysis* and the *algorithm* itself were *completely separated*, yet it was using the information as soon as it was calculated - or maybe "it was being calculated only when it was needed". I could not do this kind of stuff in imperative languages. Very nice :) ------------ Problem 1: ------------ I've used that many times since then, but lately it doesn't really seem to serve my needs. What if I need to get two different kinds of data, say (list1, list2), and now I want to write them: writeFile file1 (analyze list1) writeFile file2 (analyze list2) What if you need to evaluate list2 to evaluate list1? Not good! I'm evaluating list2 and not consuming it, therefore space leak. I could alternate between one list and another, but what if at a certain point, evaluating the next elem in "list1" caused the evaluation of a few thousand elements of "list2"? So how do you usually deal with this kind of stuff? Concurrency?? ------------ Problem 2: ------------ Yes "iterate p f x0" looks good. What if function "f" is not a pure one? If my monad is a Lazy one, streaming can be used(*), if it is not, that won't work. So streaming could work with monads like State, (lazy) ST. Not with IO, or (lazy) ST. (*) do list<-myalgorithm writeFile file1 (analyze list) So, how do you usually do this kind of thing - extracting lists of info from an algorithm? J.A.