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)
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