Hi, Is there a pattern for tree traversal with a state ? I am developing a small scenegraph represented by a tree. To draw a scenegraph one traverses over the graph starting with a global state. Inner Nodes can overwrite the inherited state for their subtree (e.g. Transformations are accumulated). The accumulated state is then either used immediately to draw the geometry in the leaf nodes, or a secondary data structure is build. This secondary data structure (a list or a tree) can then be sorted for optimal drawing performance. So i want to do the second and create a list of all leaves with the accumulated global state. To illustrate my problem i appended some code. The idea similar applies to a scenegraph. So my Question is: Is there allready a pattern for traversal with a state ?
module Main where
produces: Fork (0,"a") (Fork (1,"a") (Leaf (2,"a")) (Leaf (1,"a"))) (Leaf (0,"a"))
newTree :: BTree State newTree = traverse modState globalState sampleTree
produces: [(0,"a"),(1,"a"),(2,"a"),(1,"a"),(0,"a")]
stateList = flattenTree newTree
flattenTree (Leaf x) = [x] flattenTree (Fork x l r) = [x] ++ flattenTree l ++ flattenTree r
type State = (Int, String)
globalState :: State globalState = (0, "a")
State modifiers
data StateMod = ModInt | ModString | ModNop deriving Show
modState :: StateMod -> State -> State modState ModInt (x,w) = (x+1,w) modState ModNop s = s modState ModString (x,w) = (x,'b':w)
data BTree a = Fork a (BTree a) (BTree a) | Leaf a deriving Show
traverses the tree and executes a function which modifies the current state depending on the statemodifier
traverse :: (a -> b -> b) -> b -> BTree a -> BTree b traverse f state (Leaf x) = Leaf (f x state) traverse f state (Fork x l r) = Fork (f x state) newLeft newRight where newLeft = traverse f (f x state) l newRight = traverse f (f x state) r
an example tree
sampleTree :: BTree StateMod sampleTree = Fork ModNop (Fork ModInt (Leaf ModInt) (Leaf ModNop)) (Leaf ModNop)
creates a list from a tree
flattenTree (Leaf x) = [x] flattenTree (Fork x l r) = [x] ++ flattenTree l ++ flattenTree r
Thanks for any help and ideas Andreas