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)
if space is the main issue, you could try to avoid the sharing (so that each part of the computation unfolds and throws away its own copy of the input list): initlast :: (()->[a]) -> ([a], a) initlast xs = (init (xs ()), last (xs ())) main = print $ case initlast (\()->[0..1000000000]) of (init, last) -> (length init, last) hth, claus ps. it would occasionally be nice to be able to undo sharing instead of having to avoid it, by asking for an independent, equally unevaluated, copy of some expression. that would make it easy to write initlast with your original type. pps. compiling with -O recreates the space problem? (ghc-6.5)