hi all, i'm new to haskell and i have a question on a general problem concerning monad binding and garbage collection. my question will be on how to approach this type of problem? consider the following simple program:

------ buildIOList :: [a] -> IO [a] buildIOList li = return li

------

------

main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); } <------ if i compile this program with ghc and run it it has constant memory usage because of the lazy nature of haskell. on the other hand if i use runhugs then i get: ERROR - Garbage collection fails to reclaim sufficient space i think this is because the compiler can examine the code and sees that the binding li is not used after putStr any more and therefore the garbage collector can safely collect all the list elements that were already put onto the screen, but hugs as a interpreter does not have this knowledge and therefore has to keep all the memory that the binding li refers to. if i modify this program a bit: main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); putStr (show (take 100000000 li)); } <------ i get the same memory problem with a ghc compiled file, because now all the cells in the list bound by li that are evaluated stay in the system, because the gc cannot collect them as the third line in the do expression uses li again. with a little modification: main = do {li <- buildIOList [1..]; putStr (show (take 100000000 li)); li <- buildIOList [1..]; putStr (show (take 100000000 li)); } <------ the program has again constant memory usage, because the name li is bound again later in the program so the gc can collect the elements of the first li. again a modification would be:

------ buildIOList :: [a] -> [IO a] buildIOList [] = [] buildIOList (hd : tl) = (return hd) : buildIOList tl

main = do {printString (buildIOList [1..]); printString (buildIOList [1..]); } printString :: Show a => [IO a] -> IO () printString [] = return () printString (hd : tl) = do {c <- hd; putStr (show c); printString tl} <------ this program has also constant memory usage, but here i have to modify a lot to propagate the "little bits of monads" in the program. my question is now how an experienced haskell programmer would attack the problem? i would also like to have a solution that works for hugs so that my code is independent of the environment it is run in. many thanks for any suggestions! -- Christian Schuhegger The most likely way for the world to be destroyed, most experts agree, is by accident. That's where we come in; we're computer professionals. We cause accidents. -- Nathaniel Borenstein