old parallel merge sort attempt

import Control.Parallel.Strategies

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Show)

listToTree [] = error "listToTree -- empty list"
listToTree [x] = Leaf x
listToTree xs = Node (listToTree $ take half xs) (listToTree $ drop half xs)
    where half = floor ((toRational $ length xs) / 2)

mergeSort' :: Ord a => Tree a -> Eval [a]
mergeSort' (Leaf v) = return [v]
mergeSort' (Node x y) = do
    xr <- mergeSort' x
    yr <- mergeSort' y
    rseq (merge xr yr)
    where
        merge [] y = y
        merge x [] = x
        merge (x:xs) (y:ys) | x < y = x : merge xs (y:ys)
                            | otherwise = y : merge (x:xs) ys
mergeSort = runEval . mergeSort'

main = (mergeSort $ listToTree [10000000,9999999..1]) `seq` putStrLn "done"