It sounds like you've got the monadic solution figured out, so that's good. Even though I would recommend using State (or StateT if necessary), I wanted to be sure this question was answered.
> ranq1List :: (Word64 -> a ) -> Word64 -> [a] > ranq1List converter state = converter newState : ranq1List converter > newState > where > newState = ranq1Increment state > > > One more question on this - the other concern I had with the recursive list > approach was that although lazy evaluation prevents me generating numbers > before I 'ask' for them, I figured that if I was going to be asking for say > 10 million over the course of one simulation, that although I request them > one by one, over hours or even days, at the end of the simulation I will > still have a list of 10 million word64s - each of which I could throw away > within minutes of asking for it. This seemed like huge memory bloat, and > thus probably I was taking the wrong approach. This "memory bloat" is a space leak. In general they are to be avoided. GHC's garbage collection is quite efficient, so if you structure your program properly the space will be reclaimed after the values are used. Values will only be collected if they cannot be referenced from live code (just like Java, Python, C#, or other languages with GC). If you are able to structure your code like this: doSomething :: [Word64] -> IO () doSomething (rand:rands) = do someIO rand otherIO rand ... if endFlag then return () else doSomething rands main = do let rands = ranq1List id 1 doSomething rands you won't have a space leak. The head of the list is used within doSomething, while only the tail is passed in the recursion. At the time of the recursive call to doSomething, the head 'rand' value is no longer accessible by any running code, so it can be GC'd. Contrast that with this version (somewhat contrived): doSomething2 :: [Word64] -> Int -> IO () -- the Int is an index into the list. doSomething2 rands i = do let rand = rands !! i someIO rand otherIO rand ... if endFlag then return () else doSomething2 rands (i + 1) This version is Really Bad. Firstly the entire list is passed on each recursion. Every element remains within scope, so nothing is GC'd. That's the space leak. There's another big problem: the current random value is computed by traversing from the head of the list. As the index value grows, it takes progressively longer to traverse the list to retrieve the current value. (N.B. I haven't tested these specific examples, but I think I'm right.) Cheers, John Lato _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
