Hal Daume III wrote:

I have a function which behaves like map, except instead of applying the given function to, say, the element at position 5, it applies it to the entire list *without* the element at position 5. An implementation looks like:

...

mapWithout :: ([a] -> b) -> [a] -> [b] mapWithout f = mapWith' [] where mapWith' pre [] = [] mapWith' pre (x:xs) = f (pre ++ xs) : mapWith' (x:pre) xs

Unfortunately, this is very slow, due to the overhead of (++).

Any way I could speed this up would be great. Note that order doesn't matter to 'f', but I do need that the order in which the elements of the list are processed be the same as in map.

The following version is probably faster: mapWithout f [] = [] mapWithout f l = map_without l [] [] where map_without [e] t t2 = f t:t2 map_without [e1,e2] t t2 = f (e2:t):f (e1:t):t2 map_without l t t2 = map_without l1 (l2++t) (map_without l2 (l1++t) t2) where (l1,l2) = splitAt (length l `div` 2) l Regards, John van Groningen