Testing b*tree in haskell

btree_test.hs

import Pipes
import qualified BTree as BT
import Control.Monad (unless, liftM)
import System.IO

-- probably don't need IO
listToProducer :: [BT.BLeaf String Int] -> Producer (BT.BLeaf String Int) IO ()
listToProducer [] = return ()
listToProducer a@(x:xs) =
    unless (null a) $ do
        yield x
        listToProducer xs

producer1 = listToProducer [BT.BLeaf x y | (x,y) <- zip ["ab", "bc", "cd"] [30..90] ]
producer2 = listToProducer [BT.BLeaf x y | (x,y) <- zip ["aa", "bc", "cc"] [10..20] ]

getTreeIO :: Producer (BT.BLeaf String Int) IO () -> IO (BT.LookupTree String Int)
getTreeIO producer = do
    let order = 10
    let size = 100
    hSetBuffering stdin LineBuffering
    tree <- liftM BT.fromByteString (BT.fromOrderedToByteString order size producer)
    return $ case tree of
        Left _  -> error "impossible"
        Right x -> x

tree1 = getTreeIO producer1
tree2 = getTreeIO producer2

merged :: IO (BT.LookupTree String Int)
merged = do
    t1 <- tree1
    t2 <- tree2
    BT.mergeTrees (\a b -> return a) 10 fname [t1, t2]
    --            ^^^^^^^^^^^^^^^^^ order to merge
    eitherTree <- BT.open fname
    return $ case eitherTree of
                Left _  -> error "impossible!!"
                Right x -> x
    where fname = "/tmp/tree.tmp"

-- to test, do the following in ghci
-- t <- merged
-- BT.lookup t "bc"