Convert a sorted list to a balanced binary search tree

sortedListToBST.hs

{-# LANGUAGE PackageImports #-}

import "mtl" Control.Monad.State

data BinTree a = Tip | Node (BinTree a) a (BinTree a) deriving Show

conv :: Int -> State [a] (BinTree a)
conv 0   = return Tip
conv len = liftM3 Node leftTree pop rightTree
              where leftTree   = conv mid
                    rightTree  = conv (len - mid - 1)
                    mid        = len `div` 2
                    pop        = get >>= \(l:ls) -> (put ls >> return l)

sortedListToBST :: [a] -> BinTree a
sortedListToBST ls = evalState (conv (length ls)) ls