Is it possible to change a particular node of the doubly linked list? That is to say, that would like to have a function: update :: DList a -> a -> DList a where update node newValue returns a list where only the value at the node which is passed in is set to the new Value and all other values are the same.
What you need is for the nodes to keep track of the length of the list. Here's a different solution from that oleg posted, to me it's slightly more intuitive, since the updates work directly on the dlists instead of via (elegant) proxy functions. ---------------------------------------------------------------------- module DList where data DList a = DNode Int (DList a) a (DList a) | Empty mkDList :: [a] -> DList a mkDList [] = Empty mkDList xs = let len = length xs this = DNode len farLeft (head xs) nearRight (nearRight,farLeft) = mkRestDList len (tail xs) this this in this mkRestDList :: Int -> [a] -> DList a -> DList a -> (DList a, DList a) mkRestDList _ [] _ farRight = (farRight, farRight) -- will only happen if the initial list is singleton mkRestDList len [x] nearLeft farRight = let this = DNode len nearLeft x farRight in (this, this) mkRestDList len (x:xs) nearLeft farRight = let this = DNode len nearLeft x nearRight (nearRight,farLeft) = mkRestDList len xs this farRight in (this,farLeft) takeD :: Int -> DList a -> [a] takeD 0 _ = [] takeD _ Empty = [] takeD n (DNode _ _ x r) = x : takeD (n-1) r leftD, rightD :: DList a -> DList a leftD Empty = Empty leftD (DNode _ l _ _) = l rightD Empty = Empty rightD (DNode _ _ _ r) = r updateD :: a -> DList a -> DList a updateD _ Empty = Empty updateD x (DNode len _ _ r) = let this = DNode len farLeft x nearRight (nearRight,farLeft) = updateRestD (len-1) r this this in this updateRestD :: Int -> DList a -> DList a -> DList a -> (DList a, DList a) updateRestD 0 _ _ farRight = (farRight, farRight) -- will only happen if the initial list is singleton updateRestD 1 (DNode len _ x _) nearLeft farRight = let this = DNode len nearLeft x farRight in (this, this) updateRestD n (DNode len _ x r) nearLeft farRight = let this = DNode len nearLeft x nearRight (nearRight,farLeft) = updateRestD (n-1) r this farRight in (this,farLeft) updateRestD _ Empty _ _ = undefined -- can't happen ----------------------------------------------------- *DList> let dl = mkDList [1..5] *DList> takeD 11 dl [1,2,3,4,5,1,2,3,4,5,1] *DList> let dl' = updateD (-1) dl *DList> takeD 11 dl' [-1,2,3,4,5,-1,2,3,4,5,-1] Cheers, /Niklas