This is a follow-up to a thread from June-July[1]. The question was how to write the function initlast :: [a] -> ([a], a) initlast xs = (init xs, last xs) so that it can be consumed in fixed space: main = print $ case initlast [0..1000000000] of (init, last) -> (length init, last) Attempts were along the lines of initlast :: [a] -> ([a], a) initlast [x] = ([], x) initlast (x:xs) = let (init, last) = initlast xs in (x:init, last) I seemed obvious to me at first (and for a long while) that ghc should force both computations in parallel; but finally at the hackathon (thanks to Simon Marlow) I realized I was expecting magic: The elements of the pair are simply independent thunks, and there's no way to "partly force" the second (ie, last) without forcing it all the way. Simon Peyton Jones graciously offered that it is "embarrassing" that we can't write this in Haskell, so to make him less embarrassed (and despite my adamance on the mailing list that the implementation be functional), I wrote an imperative version with the desired space behavior. Borrowing the insight that unsafePerform and unsafeInterleave can be thought of as hooks into the evaluator, this shows more or less what I would wish for ghc to do automatically. initlastST :: [a] -> ([a], a) initlastST xs = runST (m xs) where m xs = do r <- newSTRef undefined init <- init' r xs last <- unsafeInterleaveST (readSTRef r) return (init, last) init' r [x] = do writeSTRef r x return [] init' r (x:xs) = do writeSTRef r (last xs) liftM (x:) (unsafeInterleaveST (init' r xs)) Andrew [1] http://haskell.org/pipermail/haskell-cafe/2006-June/016171.html http://haskell.org/pipermail/haskell-cafe/2006-July/016709.html