This is a "have you seen this monad?" post. I was trying to construct a search tree, and decided I wanted to do it in a monad (so I could apply StateT and keep state as I explored the space). I discovered that a tree with labeled leaves is a monad, but I wanted to label internal nodes, and such a tree (eg, Data.Tree.Tree) is not a monad (because it can only have one result type). Finally, I realized I could get a similar effect by labeling the edges and the leaves with different types: data Tree l a = Leaf a | Branch [(l, Tree l a)] instance Monad (Tree l) where return = Leaf Leaf a >>= f = f a Branch c >>= f = Branch [(l, t >>= f) | (l, t) <- c] You might use it as turn = do board <- getBoard move <- lift (Branch [(move, return move) | move <- findMoves board]) applyMove move turn I found this quite pleasing, though I hadn't run across trees as monads before. Has anyone else found this useful? Is it in a library somewhere? Andrew