Monads and custom traversal functions in Haskell
- by Bill
Given the following simple BST definition:
data Tree x = Empty | Leaf x | Node x (Tree x) (Tree x)
deriving (Show, Eq)
inOrder :: Tree x -> [x]
inOrder Empty = []
inOrder (Leaf x) = [x]
inOrder (Node root left right) = inOrder left ++ [root] ++ inOrder right
I'd like to write an in-order function that can have side effects. I achieved that with:
inOrderM :: (Show x, Monad m) => (x -> m a) -> Tree x -> m ()
inOrderM f (Empty) = return ()
inOrderM f (Leaf y) = f y >> return ()
inOrderM f (Node root left right) = inOrderM f left >> f root >> inOrderM f right
-- print tree in order to stdout
inOrderM print tree
This works fine, but it seems repetitive - the same logic is already present in inOrder and my experience with Haskell leads me to believe that I'm probably doing something wrong if I'm writing a similar thing twice.
Is there any way that I can write a single function inOrder that can take either pure or monadic functions?
