Or, if duplicated computation offends you, replace (++) in the original version of powerset with an interleave operator:
powerset :: [a] -> [[a]] powerset [] = [[]] powerset (x:xs) = xss /\/ map (x:) xss where xss = powerset xs
(/\/) :: [a] -> [a] -> [a] [] /\/ ys = ys (x:xs) /\/ ys = x : (ys /\/ xs)
These two variants both run in constant space (assuming that your compiler isn't "smart" enough to do common subexpr elimination :-)
Interesting...
Picking up my theme or generating the powersets in increasing order of length, I tried a variation on that:
[[
powerset3 :: [a] -> [[a]]
powerset3 [] = [[]]
powerset3 (x:xs) = xss <<< map (x:) xss
where xss = powerset3 xs(<<<) :: [[a]] -> [[a]] -> [[a]]
[] <<< ys = ys
xs <<< [] = xs
(x:xs) <<< (y:ys) = if length x < length y
then x:(xs <<< (y:ys))
else y:((x:xs) <<< ys)testJ1 = powerset3 [1,2,3,4] testJ2 = powerset3 "abcdefgh" ]]
(The length-ordered interleave is a bit clumsy -- I think that could be improved by saving the length with each powerset as it's generated, or by other means.)
Empirically, I notice that this still seems to leak *some* space compared with your version, but not nearly as much as the simple version. I also notice, empirically, that these interleaving versions invoke garbage collection much more frequently than the naive version.
#g
------------------- Graham Klyne <[EMAIL PROTECTED]> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E
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