For lazy I/O, using shows in Haskell is a good analogue of using #printOn: in Smalltalk. The basic form is "include this as PART of a stream", with "convert this to a whole string" as a derived form.

What the equivalent of this would be for Iteratees I don't yet understand.

Why not to try simple generators first, which are simpler, truly. It seems generators reproduce the Smalltalk printing patterns pretty well -- even simpler since we don't have to specify any stream. The printing takes linear time in input size. The same `printing' generator can be used even if we don't actually want to see any output -- rather, we only want the statistics (e.g., number of characters printed, or number of lines, etc). Likewise, the same printing generator print_yield can be used if we are to encode the output somehow (e.g., compress it). The entire pipeline can run in constant space (if encoding is in constant space). Here is the code module PrintYield where -- http://okmij.org/ftp/continuations/PPYield/ import GenT import Data.Set as S import Data.Foldable import Control.Monad.State.Strict type Producer m e = GenT e m () class PrintYield a where print_yield :: Monad m => a -> Producer m String instance PrintYield Int where print_yield = yield . show instance (PrintYield a, PrintYield b) => PrintYield (Either a b) where print_yield (Left x) = yield "Left " >> print_yield x print_yield (Right x) = yield "Right " >> print_yield x instance (PrintYield a) => PrintYield (Set a) where print_yield x = do yield "{" let f True x = print_yield x >> return False f False x = yield ", " >> print_yield x >> return False foldlM f True x yield "}" instance PrintYield ISet where print_yield (ISet x) = print_yield x newtype ISet = ISet (Either Int (Set ISet)) deriving (Eq, Ord) set1 :: Set ISet set1 = Prelude.foldr (\e s -> S.fromList [ISet (Left e), ISet (Right s)]) S.empty [1..200000] -- Real printing print_set :: Set ISet -> IO () print_set s = print_yield s `runGenT` putStr t1 = print_set set1 -- Counting the number of characters -- Could use Writer but the Writer is too lazy, may leak memory count_printed :: Set ISet -> Integer count_printed s = (print_yield s `runGenT` counter) `execState` 0 where counter _ = get >>= put . succ_strict succ_strict x = x `seq` succ x -- According to GHCi statistics, counting is linear in time -- (space is harder to estimate: it is not clear what GHCi prints -- for memory statistics; we need max bytes allocated rather than -- total bytes allocated) t2 = count_printed set1 -- Doesn't do anything but ensures the set is constructed t3 :: IO () t3 = print_yield set1 `runGenT` (\x -> return ())