Make a MCTS in Haskell

Question

I have to implement a Monte Carlo Tree Search in Haskell. I defined a data type like this:

data Tree s a = TreeNode { 
                           rGame :: s, -- state
                        rPlayed :: a, -- action
                        visit :: Int, -- number of visits
                        score :: Float, 
                        left :: Tree s a,
                        right :: Tree s a} | TreeNil deriving Show

I also implemented a Zipper with a random number generator(I heard that I need one):

data Crumb s a = LeftCrumb s a (Tree s a) | RightCrumb s a (Tree s a) deriving Show

type Treecrumbs s a = [Crumb s a]

data Zipper s a = Zipper (Tree s a) (Treecrumbs s a) (StdGen)

My question is how can I implement the expand function for this MCTS. Because is lazy evaluation I also think I can generate the whole tree. In my opinion the function should look like this:

expand :: (s -> [(a, s)])  -- The generator
       -> s                -- first state
       -> Tree s a         -- The result tree
expand gen t =