On 23 December 2010 22:01, Henning Thielemann
This could be seen as "type Step st a = (Maybe a, st)". I have thought about mapping from [Int] to [Maybe (Int, Int)] by mapAccumL, then compressing the result with catMaybes. However we need to append a final pair when the end of the list is reached, which risks a memory leak.
Hi Henning Thanks - that's an extra nesting of constructors though. Note the flush in the original was always doomed to be productive - this revision might be better, the flush now has the option of producing nothing or many: lessproductive :: (st -> a -> Step st b) -> (st -> [b]) -> st -> [a] -> [b] lessproductive phi flush = step where step st [] = flush st step st (x:xs) = case phi st x of Next st' -> step st' xs Yield b st' -> b : step st' xs ranges2 :: (Num a, Eq a) => [a] -> [(a,a)] ranges2 [] = [] ranges2 (x:xs) = lessproductive phi (\a -> [a]) (x,x) xs where phi (i,j) n | j+1 == n = Next (i,n) phi ans n = Yield ans (n,n)