From f4394413ab3cf2dfb94b1f024a047938cfd471e9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roman=20Smr=C5=BE?= <roman.smrz@seznam.cz>
Date: Wed, 30 Oct 2024 20:06:03 +0100
Subject: Rename Erebos.Storage module to Erebos.Object.Internal

---
 src/Erebos/Object/Internal.hs | 1081 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 1081 insertions(+)
 create mode 100644 src/Erebos/Object/Internal.hs

(limited to 'src/Erebos/Object')

diff --git a/src/Erebos/Object/Internal.hs b/src/Erebos/Object/Internal.hs
new file mode 100644
index 0000000..638689a
--- /dev/null
+++ b/src/Erebos/Object/Internal.hs
@@ -0,0 +1,1081 @@
+module Erebos.Object.Internal (
+    Storage, PartialStorage, StorageCompleteness,
+    openStorage, memoryStorage,
+    deriveEphemeralStorage, derivePartialStorage,
+
+    Ref, PartialRef, RefDigest,
+    refDigest,
+    readRef, showRef, showRefDigest,
+    refDigestFromByteString, hashToRefDigest,
+    copyRef, partialRef, partialRefFromDigest,
+
+    Object, PartialObject, Object'(..), RecItem, RecItem'(..),
+    serializeObject, deserializeObject, deserializeObjects,
+    ioLoadObject, ioLoadBytes,
+    storeRawBytes, lazyLoadBytes,
+    storeObject,
+    collectObjects, collectStoredObjects,
+
+    Head, HeadType(..),
+    HeadTypeID, mkHeadTypeID,
+    headId, headStorage, headRef, headObject, headStoredObject,
+    loadHeads, loadHead, reloadHead,
+    storeHead, replaceHead, updateHead, updateHead_,
+    loadHeadRaw, storeHeadRaw, replaceHeadRaw,
+
+    WatchedHead,
+    watchHead, watchHeadWith, unwatchHead,
+    watchHeadRaw,
+
+    MonadStorage(..),
+
+    Storable(..), ZeroStorable(..),
+    StorableText(..), StorableDate(..), StorableUUID(..),
+
+    Store, StoreRec,
+    evalStore, evalStoreObject,
+    storeBlob, storeRec, storeZero,
+    storeEmpty, storeInt, storeNum, storeText, storeBinary, storeDate, storeUUID, storeRef, storeRawRef,
+    storeMbEmpty, storeMbInt, storeMbNum, storeMbText, storeMbBinary, storeMbDate, storeMbUUID, storeMbRef, storeMbRawRef,
+    storeZRef,
+    storeRecItems,
+
+    Load, LoadRec,
+    evalLoad,
+    loadCurrentRef, loadCurrentObject,
+    loadRecCurrentRef, loadRecItems,
+
+    loadBlob, loadRec, loadZero,
+    loadEmpty, loadInt, loadNum, loadText, loadBinary, loadDate, loadUUID, loadRef, loadRawRef,
+    loadMbEmpty, loadMbInt, loadMbNum, loadMbText, loadMbBinary, loadMbDate, loadMbUUID, loadMbRef, loadMbRawRef,
+    loadTexts, loadBinaries, loadRefs, loadRawRefs,
+    loadZRef,
+
+    Stored,
+    fromStored, storedRef,
+    wrappedStore, wrappedLoad,
+    copyStored,
+    unsafeMapStored,
+
+    StoreInfo(..), makeStoreInfo,
+
+    StoredHistory,
+    fromHistory, fromHistoryAt, storedFromHistory, storedHistoryList,
+    beginHistory, modifyHistory,
+) where
+
+import Control.Applicative
+import Control.Concurrent
+import Control.Exception
+import Control.Monad
+import Control.Monad.Except
+import Control.Monad.Reader
+import Control.Monad.Writer
+
+import Crypto.Hash
+
+import Data.Bifunctor
+import Data.ByteString (ByteString)
+import qualified Data.ByteArray as BA
+import qualified Data.ByteString as B
+import qualified Data.ByteString.Char8 as BC
+import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString.Lazy.Char8 as BLC
+import Data.Char
+import Data.Function
+import qualified Data.HashTable.IO as HT
+import Data.List
+import qualified Data.Map as M
+import Data.Maybe
+import Data.Ratio
+import Data.Set (Set)
+import qualified Data.Set as S
+import Data.Text (Text)
+import qualified Data.Text as T
+import Data.Text.Encoding
+import Data.Text.Encoding.Error
+import Data.Time.Calendar
+import Data.Time.Clock
+import Data.Time.Format
+import Data.Time.LocalTime
+import Data.Typeable
+import Data.UUID (UUID)
+import qualified Data.UUID as U
+import qualified Data.UUID.V4 as U
+
+import System.Directory
+import System.FSNotify
+import System.FilePath
+import System.IO.Error
+import System.IO.Unsafe
+
+import Erebos.Storage.Internal
+
+
+type Storage = Storage' Complete
+type PartialStorage = Storage' Partial
+
+storageVersion :: String
+storageVersion = "0.1"
+
+openStorage :: FilePath -> IO Storage
+openStorage path = modifyIOError annotate $ do
+    let versionFileName = "erebos-storage"
+    let versionPath = path </> versionFileName
+    let writeVersionFile = writeFileOnce versionPath $ BLC.pack $ storageVersion <> "\n"
+
+    maybeVersion <- handleJust (guard . isDoesNotExistError) (const $ return Nothing) $
+        Just <$> readFile versionPath
+    version <- case maybeVersion of
+        Just versionContent -> do
+            return $ takeWhile (/= '\n') versionContent
+
+        Nothing -> do
+            files <- handleJust (guard . isDoesNotExistError) (const $ return []) $
+                listDirectory path
+            when (not $ or
+                    [ null files
+                    , versionFileName `elem` files
+                    , (versionFileName ++ ".lock") `elem` files
+                    , "objects" `elem` files && "heads" `elem` files
+                    ]) $ do
+                fail "directory is neither empty, nor an existing erebos storage"
+
+            createDirectoryIfMissing True $ path
+            writeVersionFile
+            takeWhile (/= '\n') <$> readFile versionPath
+
+    when (version /= storageVersion) $ do
+        fail $ "unsupported storage version " <> version
+
+    createDirectoryIfMissing True $ path </> "objects"
+    createDirectoryIfMissing True $ path </> "heads"
+    watchers <- newMVar (Nothing, [], WatchList 1 [])
+    refgen <- newMVar =<< HT.new
+    refroots <- newMVar =<< HT.new
+    return $ Storage
+        { stBacking = StorageDir path watchers
+        , stParent = Nothing
+        , stRefGeneration = refgen
+        , stRefRoots = refroots
+        }
+  where
+    annotate e = annotateIOError e "failed to open storage" Nothing (Just path)
+
+memoryStorage' :: IO (Storage' c')
+memoryStorage' = do
+    backing <- StorageMemory <$> newMVar [] <*> newMVar M.empty <*> newMVar M.empty <*> newMVar (WatchList 1 [])
+    refgen <- newMVar =<< HT.new
+    refroots <- newMVar =<< HT.new
+    return $ Storage
+        { stBacking = backing
+        , stParent = Nothing
+        , stRefGeneration = refgen
+        , stRefRoots = refroots
+        }
+
+memoryStorage :: IO Storage
+memoryStorage = memoryStorage'
+
+deriveEphemeralStorage :: Storage -> IO Storage
+deriveEphemeralStorage parent = do
+    st <- memoryStorage
+    return $ st { stParent = Just parent }
+
+derivePartialStorage :: Storage -> IO PartialStorage
+derivePartialStorage parent = do
+    st <- memoryStorage'
+    return $ st { stParent = Just parent }
+
+type Ref = Ref' Complete
+type PartialRef = Ref' Partial
+
+zeroRef :: Storage' c -> Ref' c
+zeroRef s = Ref s (RefDigest h)
+    where h = case digestFromByteString $ B.replicate (hashDigestSize $ digestAlgo h) 0 of
+                   Nothing -> error $ "Failed to create zero hash"
+                   Just h' -> h'
+          digestAlgo :: Digest a -> a
+          digestAlgo = undefined
+
+isZeroRef :: Ref' c -> Bool
+isZeroRef (Ref _ h) = all (==0) $ BA.unpack h
+
+
+refFromDigest :: Storage' c -> RefDigest -> IO (Maybe (Ref' c))
+refFromDigest st dgst = fmap (const $ Ref st dgst) <$> ioLoadBytesFromStorage st dgst
+
+readRef :: Storage -> ByteString -> IO (Maybe Ref)
+readRef s b =
+    case readRefDigest b of
+         Nothing -> return Nothing
+         Just dgst -> refFromDigest s dgst
+
+copyRef' :: forall c c'. (StorageCompleteness c, StorageCompleteness c') => Storage' c' -> Ref' c -> IO (c (Ref' c'))
+copyRef' st ref'@(Ref _ dgst) = refFromDigest st dgst >>= \case Just ref -> return $ return ref
+                                                                Nothing  -> doCopy
+    where doCopy = do mbobj' <- ioLoadObject ref'
+                      mbobj <- sequence $ copyObject' st <$> mbobj'
+                      sequence $ unsafeStoreObject st <$> join mbobj
+
+copyRecItem' :: forall c c'. (StorageCompleteness c, StorageCompleteness c') => Storage' c' -> RecItem' c -> IO (c (RecItem' c'))
+copyRecItem' st = \case
+    RecEmpty -> return $ return $ RecEmpty
+    RecInt x -> return $ return $ RecInt x
+    RecNum x -> return $ return $ RecNum x
+    RecText x -> return $ return $ RecText x
+    RecBinary x -> return $ return $ RecBinary x
+    RecDate x -> return $ return $ RecDate x
+    RecUUID x -> return $ return $ RecUUID x
+    RecRef x -> fmap RecRef <$> copyRef' st x
+    RecUnknown t x -> return $ return $ RecUnknown t x
+
+copyObject' :: forall c c'. (StorageCompleteness c, StorageCompleteness c') => Storage' c' -> Object' c -> IO (c (Object' c'))
+copyObject' _ (Blob bs) = return $ return $ Blob bs
+copyObject' st (Rec rs) = fmap Rec . sequence <$> mapM (\( n, item ) -> fmap ( n, ) <$> copyRecItem' st item) rs
+copyObject' _ ZeroObject = return $ return ZeroObject
+copyObject' _ (UnknownObject otype content) = return $ return $ UnknownObject otype content
+
+copyRef :: forall c c' m. (StorageCompleteness c, StorageCompleteness c', MonadIO m) => Storage' c' -> Ref' c -> m (LoadResult c (Ref' c'))
+copyRef st ref' = liftIO $ returnLoadResult <$> copyRef' st ref'
+
+copyRecItem :: forall c c' m. (StorageCompleteness c, StorageCompleteness c', MonadIO m) => Storage' c' -> RecItem' c -> m (LoadResult c (RecItem' c'))
+copyRecItem st item' = liftIO $ returnLoadResult <$> copyRecItem' st item'
+
+copyObject :: forall c c'. (StorageCompleteness c, StorageCompleteness c') => Storage' c' -> Object' c -> IO (LoadResult c (Object' c'))
+copyObject st obj' = returnLoadResult <$> copyObject' st obj'
+
+partialRef :: PartialStorage -> Ref -> PartialRef
+partialRef st (Ref _ dgst) = Ref st dgst
+
+partialRefFromDigest :: PartialStorage -> RefDigest -> PartialRef
+partialRefFromDigest st dgst = Ref st dgst
+
+
+data Object' c
+    = Blob ByteString
+    | Rec [(ByteString, RecItem' c)]
+    | ZeroObject
+    | UnknownObject ByteString ByteString
+    deriving (Show)
+
+type Object = Object' Complete
+type PartialObject = Object' Partial
+
+data RecItem' c
+    = RecEmpty
+    | RecInt Integer
+    | RecNum Rational
+    | RecText Text
+    | RecBinary ByteString
+    | RecDate ZonedTime
+    | RecUUID UUID
+    | RecRef (Ref' c)
+    | RecUnknown ByteString ByteString
+    deriving (Show)
+
+type RecItem = RecItem' Complete
+
+serializeObject :: Object' c -> BL.ByteString
+serializeObject = \case
+    Blob cnt -> BL.fromChunks [BC.pack "blob ", BC.pack (show $ B.length cnt), BC.singleton '\n', cnt]
+    Rec rec -> let cnt = BL.fromChunks $ concatMap (uncurry serializeRecItem) rec
+                in BL.fromChunks [BC.pack "rec ", BC.pack (show $ BL.length cnt), BC.singleton '\n'] `BL.append` cnt
+    ZeroObject -> BL.empty
+    UnknownObject otype cnt -> BL.fromChunks [ otype, BC.singleton ' ', BC.pack (show $ B.length cnt), BC.singleton '\n', cnt ]
+
+-- |Serializes and stores object data without ony dependencies, so is safe only
+-- if all the referenced objects are already stored or reference is partial.
+unsafeStoreObject :: Storage' c -> Object' c -> IO (Ref' c)
+unsafeStoreObject storage = \case
+    ZeroObject -> return $ zeroRef storage
+    obj -> unsafeStoreRawBytes storage $ serializeObject obj
+
+storeObject :: PartialStorage -> PartialObject -> IO PartialRef
+storeObject = unsafeStoreObject
+
+storeRawBytes :: PartialStorage -> BL.ByteString -> IO PartialRef
+storeRawBytes = unsafeStoreRawBytes
+
+serializeRecItem :: ByteString -> RecItem' c -> [ByteString]
+serializeRecItem name (RecEmpty) = [name, BC.pack ":e", BC.singleton ' ', BC.singleton '\n']
+serializeRecItem name (RecInt x) = [name, BC.pack ":i", BC.singleton ' ', BC.pack (show x), BC.singleton '\n']
+serializeRecItem name (RecNum x) = [name, BC.pack ":n", BC.singleton ' ', BC.pack (showRatio x), BC.singleton '\n']
+serializeRecItem name (RecText x) = [name, BC.pack ":t", BC.singleton ' ', escaped, BC.singleton '\n']
+    where escaped = BC.concatMap escape $ encodeUtf8 x
+          escape '\n' = BC.pack "\n\t"
+          escape c    = BC.singleton c
+serializeRecItem name (RecBinary x) = [name, BC.pack ":b ", showHex x, BC.singleton '\n']
+serializeRecItem name (RecDate x) = [name, BC.pack ":d", BC.singleton ' ', BC.pack (formatTime defaultTimeLocale "%s %z" x), BC.singleton '\n']
+serializeRecItem name (RecUUID x) = [name, BC.pack ":u", BC.singleton ' ', U.toASCIIBytes x, BC.singleton '\n']
+serializeRecItem name (RecRef x) = [name, BC.pack ":r ", showRef x, BC.singleton '\n']
+serializeRecItem name (RecUnknown t x) = [ name, BC.singleton ':', t, BC.singleton ' ', x, BC.singleton '\n' ]
+
+lazyLoadObject :: forall c. StorageCompleteness c => Ref' c -> LoadResult c (Object' c)
+lazyLoadObject = returnLoadResult . unsafePerformIO . ioLoadObject
+
+ioLoadObject :: forall c. StorageCompleteness c => Ref' c -> IO (c (Object' c))
+ioLoadObject ref | isZeroRef ref = return $ return ZeroObject
+ioLoadObject ref@(Ref st rhash) = do
+    file' <- ioLoadBytes ref
+    return $ do
+        file <- file'
+        let chash = hashToRefDigest file
+        when (chash /= rhash) $ error $ "Hash mismatch on object " ++ BC.unpack (showRef ref) {- TODO throw -}
+        return $ case runExcept $ unsafeDeserializeObject st file of
+                      Left err -> error $ err ++ ", ref " ++ BC.unpack (showRef ref) {- TODO throw -}
+                      Right (x, rest) | BL.null rest -> x
+                                      | otherwise -> error $ "Superfluous content after " ++ BC.unpack (showRef ref) {- TODO throw -}
+
+lazyLoadBytes :: forall c. StorageCompleteness c => Ref' c -> LoadResult c BL.ByteString
+lazyLoadBytes ref | isZeroRef ref = returnLoadResult (return BL.empty :: c BL.ByteString)
+lazyLoadBytes ref = returnLoadResult $ unsafePerformIO $ ioLoadBytes ref
+
+unsafeDeserializeObject :: Storage' c -> BL.ByteString -> Except String (Object' c, BL.ByteString)
+unsafeDeserializeObject _  bytes | BL.null bytes = return (ZeroObject, bytes)
+unsafeDeserializeObject st bytes =
+    case BLC.break (=='\n') bytes of
+        (line, rest) | Just (otype, len) <- splitObjPrefix line -> do
+            let (content, next) = first BL.toStrict $ BL.splitAt (fromIntegral len) $ BL.drop 1 rest
+            guard $ B.length content == len
+            (,next) <$> case otype of
+                 _ | otype == BC.pack "blob" -> return $ Blob content
+                   | otype == BC.pack "rec" -> maybe (throwError $ "Malformed record item ")
+                                                   (return . Rec) $ sequence $ map parseRecLine $ mergeCont [] $ BC.lines content
+                   | otherwise -> return $ UnknownObject otype content
+        _ -> throwError $ "Malformed object"
+    where splitObjPrefix line = do
+              [otype, tlen] <- return $ BLC.words line
+              (len, rest) <- BLC.readInt tlen
+              guard $ BL.null rest
+              return (BL.toStrict otype, len)
+
+          mergeCont cs (a:b:rest) | Just ('\t', b') <- BC.uncons b = mergeCont (b':BC.pack "\n":cs) (a:rest)
+          mergeCont cs (a:rest) = B.concat (a : reverse cs) : mergeCont [] rest
+          mergeCont _ [] = []
+
+          parseRecLine line = do
+              colon <- BC.elemIndex ':' line
+              space <- BC.elemIndex ' ' line
+              guard $ colon < space
+              let name = B.take colon line
+                  itype = B.take (space-colon-1) $ B.drop (colon+1) line
+                  content = B.drop (space+1) line
+
+              let val = fromMaybe (RecUnknown itype content) $
+                      case BC.unpack itype of
+                          "e" -> do guard $ B.null content
+                                    return RecEmpty
+                          "i" -> do (num, rest) <- BC.readInteger content
+                                    guard $ B.null rest
+                                    return $ RecInt num
+                          "n" -> RecNum <$> parseRatio content
+                          "t" -> return $ RecText $ decodeUtf8With lenientDecode content
+                          "b" -> RecBinary <$> readHex content
+                          "d" -> RecDate <$> parseTimeM False defaultTimeLocale "%s %z" (BC.unpack content)
+                          "u" -> RecUUID <$> U.fromASCIIBytes content
+                          "r" -> RecRef . Ref st <$> readRefDigest content
+                          _   -> Nothing
+              return (name, val)
+
+deserializeObject :: PartialStorage -> BL.ByteString -> Except String (PartialObject, BL.ByteString)
+deserializeObject = unsafeDeserializeObject
+
+deserializeObjects :: PartialStorage -> BL.ByteString -> Except String [PartialObject]
+deserializeObjects _  bytes | BL.null bytes = return []
+deserializeObjects st bytes = do (obj, rest) <- deserializeObject st bytes
+                                 (obj:) <$> deserializeObjects st rest
+
+
+collectObjects :: Object -> [Object]
+collectObjects obj = obj : map fromStored (fst $ collectOtherStored S.empty obj)
+
+collectStoredObjects :: Stored Object -> [Stored Object]
+collectStoredObjects obj = obj : (fst $ collectOtherStored S.empty $ fromStored obj)
+
+collectOtherStored :: Set RefDigest -> Object -> ([Stored Object], Set RefDigest)
+collectOtherStored seen (Rec items) = foldr helper ([], seen) $ map snd items
+    where helper (RecRef ref) (xs, s) | r <- refDigest ref
+                                      , r `S.notMember` s
+                                      = let o = wrappedLoad ref
+                                            (xs', s') = collectOtherStored (S.insert r s) $ fromStored o
+                                         in ((o : xs') ++ xs, s')
+          helper _          (xs, s) = (xs, s)
+collectOtherStored seen _ = ([], seen)
+
+
+type Head = Head' Complete
+
+headId :: Head a -> HeadID
+headId (Head uuid _) = uuid
+
+headStorage :: Head a -> Storage
+headStorage = refStorage . headRef
+
+headRef :: Head a -> Ref
+headRef (Head _ sx) = storedRef sx
+
+headObject :: Head a -> a
+headObject (Head _ sx) = fromStored sx
+
+headStoredObject :: Head a -> Stored a
+headStoredObject (Head _ sx) = sx
+
+deriving instance StorableUUID HeadID
+deriving instance StorableUUID HeadTypeID
+
+mkHeadTypeID :: String -> HeadTypeID
+mkHeadTypeID = maybe (error "Invalid head type ID") HeadTypeID . U.fromString
+
+class Storable a => HeadType a where
+    headTypeID :: proxy a -> HeadTypeID
+
+
+headTypePath :: FilePath -> HeadTypeID -> FilePath
+headTypePath spath (HeadTypeID tid) = spath </> "heads" </> U.toString tid
+
+headPath :: FilePath -> HeadTypeID -> HeadID -> FilePath
+headPath spath tid (HeadID hid) = headTypePath spath tid </> U.toString hid
+
+loadHeads :: forall a m. MonadIO m => HeadType a => Storage -> m [Head a]
+loadHeads s@(Storage { stBacking = StorageDir { dirPath = spath }}) = liftIO $ do
+    let hpath = headTypePath spath $ headTypeID @a Proxy
+
+    files <- filterM (doesFileExist . (hpath </>)) =<<
+        handleJust (\e -> guard (isDoesNotExistError e)) (const $ return [])
+        (getDirectoryContents hpath)
+    fmap catMaybes $ forM files $ \hname -> do
+        case U.fromString hname of
+             Just hid -> do
+                 (h:_) <- BC.lines <$> B.readFile (hpath </> hname)
+                 Just ref <- readRef s h
+                 return $ Just $ Head (HeadID hid) $ wrappedLoad ref
+             Nothing -> return Nothing
+loadHeads Storage { stBacking = StorageMemory { memHeads = theads } } = liftIO $ do
+    let toHead ((tid, hid), ref) | tid == headTypeID @a Proxy = Just $ Head hid $ wrappedLoad ref
+                                 | otherwise                  = Nothing
+    catMaybes . map toHead <$> readMVar theads
+
+loadHead :: forall a m. (HeadType a, MonadIO m) => Storage -> HeadID -> m (Maybe (Head a))
+loadHead st hid = fmap (Head hid . wrappedLoad) <$> loadHeadRaw st (headTypeID @a Proxy) hid
+
+loadHeadRaw :: forall m. MonadIO m => Storage -> HeadTypeID -> HeadID -> m (Maybe Ref)
+loadHeadRaw s@(Storage { stBacking = StorageDir { dirPath = spath }}) tid hid = liftIO $ do
+    handleJust (guard . isDoesNotExistError) (const $ return Nothing) $ do
+        (h:_) <- BC.lines <$> B.readFile (headPath spath tid hid)
+        Just ref <- readRef s h
+        return $ Just ref
+loadHeadRaw Storage { stBacking = StorageMemory { memHeads = theads } } tid hid = liftIO $ do
+    lookup (tid, hid) <$> readMVar theads
+
+reloadHead :: (HeadType a, MonadIO m) => Head a -> m (Maybe (Head a))
+reloadHead (Head hid (Stored (Ref st _) _)) = loadHead st hid
+
+storeHead :: forall a m. MonadIO m => HeadType a => Storage -> a -> m (Head a)
+storeHead st obj = do
+    let tid = headTypeID @a Proxy
+    stored <- wrappedStore st obj
+    hid <- storeHeadRaw st tid (storedRef stored)
+    return $ Head hid stored
+
+storeHeadRaw :: forall m. MonadIO m => Storage -> HeadTypeID -> Ref -> m HeadID
+storeHeadRaw st tid ref = liftIO $ do
+    hid <- HeadID <$> U.nextRandom
+    case stBacking st of
+         StorageDir { dirPath = spath } -> do
+             Right () <- writeFileChecked (headPath spath tid hid) Nothing $
+                 showRef ref `B.append` BC.singleton '\n'
+             return ()
+         StorageMemory { memHeads = theads } -> do
+             modifyMVar_ theads $ return . (((tid, hid), ref) :)
+    return hid
+
+replaceHead :: forall a m. (HeadType a, MonadIO m) => Head a -> Stored a -> m (Either (Maybe (Head a)) (Head a))
+replaceHead prev@(Head hid pobj) stored' = liftIO $ do
+    let st = headStorage prev
+        tid = headTypeID @a Proxy
+    stored <- copyStored st stored'
+    bimap (fmap $ Head hid . wrappedLoad) (const $ Head hid stored) <$>
+        replaceHeadRaw st tid hid (storedRef pobj) (storedRef stored)
+
+replaceHeadRaw :: forall m. MonadIO m => Storage -> HeadTypeID -> HeadID -> Ref -> Ref -> m (Either (Maybe Ref) Ref)
+replaceHeadRaw st tid hid prev new = liftIO $ do
+    case stBacking st of
+         StorageDir { dirPath = spath } -> do
+             let filename = headPath spath tid hid
+                 showRefL r = showRef r `B.append` BC.singleton '\n'
+
+             writeFileChecked filename (Just $ showRefL prev) (showRefL new) >>= \case
+                 Left Nothing -> return $ Left Nothing
+                 Left (Just bs) -> do Just oref <- readRef st $ BC.takeWhile (/='\n') bs
+                                      return $ Left $ Just oref
+                 Right () -> return $ Right new
+
+         StorageMemory { memHeads = theads, memWatchers = twatch } -> do
+             res <- modifyMVar theads $ \hs -> do
+                 ws <- map wlFun . filter ((==(tid, hid)) . wlHead) . wlList <$> readMVar twatch
+                 return $ case partition ((==(tid, hid)) . fst) hs of
+                     ([] , _  ) -> (hs, Left Nothing)
+                     ((_, r):_, hs') | r == prev -> (((tid, hid), new) : hs',
+                                                                  Right (new, ws))
+                                     | otherwise -> (hs, Left $ Just r)
+             case res of
+                  Right (r, ws) -> mapM_ ($ r) ws >> return (Right r)
+                  Left x -> return $ Left x
+
+updateHead :: (HeadType a, MonadIO m) => Head a -> (Stored a -> m (Stored a, b)) -> m (Maybe (Head a), b)
+updateHead h f = do
+    (o, x) <- f $ headStoredObject h
+    replaceHead h o >>= \case
+        Right h' -> return (Just h', x)
+        Left Nothing -> return (Nothing, x)
+        Left (Just h') -> updateHead h' f
+
+updateHead_ :: (HeadType a, MonadIO m) => Head a -> (Stored a -> m (Stored a)) -> m (Maybe (Head a))
+updateHead_ h = fmap fst . updateHead h . (fmap (,()) .)
+
+
+data WatchedHead = forall a. WatchedHead Storage WatchID (MVar a)
+
+watchHead :: forall a. HeadType a => Head a -> (Head a -> IO ()) -> IO WatchedHead
+watchHead h = watchHeadWith h id
+
+watchHeadWith :: forall a b. (HeadType a, Eq b) => Head a -> (Head a -> b) -> (b -> IO ()) -> IO WatchedHead
+watchHeadWith (Head hid (Stored (Ref st _) _)) sel cb = do
+    watchHeadRaw st (headTypeID @a Proxy) hid (sel . Head hid . wrappedLoad) cb
+
+watchHeadRaw :: forall b. Eq b => Storage -> HeadTypeID -> HeadID -> (Ref -> b) -> (b -> IO ()) -> IO WatchedHead
+watchHeadRaw st tid hid sel cb = do
+    memo <- newEmptyMVar
+    let addWatcher wl = (wl', WatchedHead st (wlNext wl) memo)
+            where wl' = wl { wlNext = wlNext wl + 1
+                           , wlList = WatchListItem
+                               { wlID = wlNext wl
+                               , wlHead = (tid, hid)
+                               , wlFun = \r -> do
+                                   let x = sel r
+                                   modifyMVar_ memo $ \prev -> do
+                                       when (Just x /= prev) $ cb x
+                                       return $ Just x
+                               } : wlList wl
+                           }
+
+    watched <- case stBacking st of
+         StorageDir { dirPath = spath, dirWatchers = mvar } -> modifyMVar mvar $ \(mbmanager, ilist, wl) -> do
+             manager <- maybe startManager return mbmanager
+             ilist' <- case tid `elem` ilist of
+                 True -> return ilist
+                 False -> do
+                     void $ watchDir manager (headTypePath spath tid) (const True) $ \case
+                         Added { eventPath = fpath } | Just ihid <- HeadID <$> U.fromString (takeFileName fpath) -> do
+                             loadHeadRaw st tid ihid >>= \case
+                                 Just ref -> do
+                                     (_, _, iwl) <- readMVar mvar
+                                     mapM_ ($ ref) . map wlFun . filter ((== (tid, ihid)) . wlHead) . wlList $ iwl
+                                 Nothing -> return ()
+                         _ -> return ()
+                     return $ tid : ilist
+             return $ first ( Just manager, ilist', ) $ addWatcher wl
+
+         StorageMemory { memWatchers = mvar } -> modifyMVar mvar $ return . addWatcher
+
+    cur <- fmap sel <$> loadHeadRaw st tid hid
+    maybe (return ()) cb cur
+    putMVar memo cur
+
+    return watched
+
+unwatchHead :: WatchedHead -> IO ()
+unwatchHead (WatchedHead st wid _) = do
+    let delWatcher wl = wl { wlList = filter ((/=wid) . wlID) $ wlList wl }
+    case stBacking st of
+        StorageDir { dirWatchers = mvar } -> modifyMVar_ mvar $ return . second delWatcher
+        StorageMemory { memWatchers = mvar } -> modifyMVar_ mvar $ return . delWatcher
+
+
+class Monad m => MonadStorage m where
+    getStorage :: m Storage
+    mstore :: Storable a => a -> m (Stored a)
+
+    default mstore :: MonadIO m => Storable a => a -> m (Stored a)
+    mstore x = do
+        st <- getStorage
+        wrappedStore st x
+
+instance MonadIO m => MonadStorage (ReaderT Storage m) where
+    getStorage = ask
+
+instance MonadIO m => MonadStorage (ReaderT (Head a) m) where
+    getStorage = asks $ headStorage
+
+
+class Storable a where
+    store' :: a -> Store
+    load' :: Load a
+
+    store :: StorageCompleteness c => Storage' c -> a -> IO (Ref' c)
+    store st = evalStore st . store'
+    load :: Ref -> a
+    load = evalLoad load'
+
+class Storable a => ZeroStorable a where
+    fromZero :: Storage -> a
+
+data Store = StoreBlob ByteString
+           | StoreRec (forall c. StorageCompleteness c => Storage' c -> [IO [(ByteString, RecItem' c)]])
+           | StoreZero
+           | StoreUnknown ByteString ByteString
+
+evalStore :: StorageCompleteness c => Storage' c -> Store -> IO (Ref' c)
+evalStore st = unsafeStoreObject st <=< evalStoreObject st
+
+evalStoreObject :: StorageCompleteness c => Storage' c -> Store -> IO (Object' c)
+evalStoreObject _ (StoreBlob x) = return $ Blob x
+evalStoreObject s (StoreRec f) = Rec . concat <$> sequence (f s)
+evalStoreObject _ StoreZero = return ZeroObject
+evalStoreObject _ (StoreUnknown otype content) = return $ UnknownObject otype content
+
+newtype StoreRecM c a = StoreRecM (ReaderT (Storage' c) (Writer [IO [(ByteString, RecItem' c)]]) a)
+    deriving (Functor, Applicative, Monad)
+
+type StoreRec c = StoreRecM c ()
+
+newtype Load a = Load (ReaderT (Ref, Object) (Except String) a)
+    deriving (Functor, Applicative, Alternative, Monad, MonadPlus, MonadError String)
+
+evalLoad :: Load a -> Ref -> a
+evalLoad (Load f) ref = either (error {- TODO throw -} . ((BC.unpack (showRef ref) ++ ": ")++)) id $ runExcept $ runReaderT f (ref, lazyLoadObject ref)
+
+loadCurrentRef :: Load Ref
+loadCurrentRef = Load $ asks fst
+
+loadCurrentObject :: Load Object
+loadCurrentObject = Load $ asks snd
+
+newtype LoadRec a = LoadRec (ReaderT (Ref, [(ByteString, RecItem)]) (Except String) a)
+    deriving (Functor, Applicative, Alternative, Monad, MonadPlus, MonadError String)
+
+loadRecCurrentRef :: LoadRec Ref
+loadRecCurrentRef = LoadRec $ asks fst
+
+loadRecItems :: LoadRec [(ByteString, RecItem)]
+loadRecItems = LoadRec $ asks snd
+
+
+instance Storable Object where
+    store' (Blob bs) = StoreBlob bs
+    store' (Rec xs) = StoreRec $ \st -> return $ do
+        Rec xs' <- copyObject st (Rec xs)
+        return xs'
+    store' ZeroObject = StoreZero
+    store' (UnknownObject otype content) = StoreUnknown otype content
+
+    load' = loadCurrentObject
+
+    store st = unsafeStoreObject st <=< copyObject st
+    load = lazyLoadObject
+
+instance Storable ByteString where
+    store' = storeBlob
+    load' = loadBlob id
+
+instance Storable a => Storable [a] where
+    store' []     = storeZero
+    store' (x:xs) = storeRec $ do
+        storeRef "i" x
+        storeRef "n" xs
+
+    load' = loadCurrentObject >>= \case
+                ZeroObject -> return []
+                _          -> loadRec $ (:)
+                                  <$> loadRef "i"
+                                  <*> loadRef "n"
+
+instance Storable a => ZeroStorable [a] where
+    fromZero _ = []
+
+
+storeBlob :: ByteString -> Store
+storeBlob = StoreBlob
+
+storeRec :: (forall c. StorageCompleteness c => StoreRec c) -> Store
+storeRec sr = StoreRec $ do
+    let StoreRecM r = sr
+    execWriter . runReaderT r
+
+storeZero :: Store
+storeZero = StoreZero
+
+
+class StorableText a where
+    toText :: a -> Text
+    fromText :: MonadError String m => Text -> m a
+
+instance StorableText Text where
+    toText = id; fromText = return
+
+instance StorableText [Char] where
+    toText = T.pack; fromText = return . T.unpack
+
+
+class StorableDate a where
+    toDate :: a -> ZonedTime
+    fromDate :: ZonedTime -> a
+
+instance StorableDate ZonedTime where
+    toDate = id; fromDate = id
+
+instance StorableDate UTCTime where
+    toDate = utcToZonedTime utc
+    fromDate = zonedTimeToUTC
+
+instance StorableDate Day where
+    toDate day = toDate $ UTCTime day 0
+    fromDate = utctDay . fromDate
+
+
+class StorableUUID a where
+    toUUID :: a -> UUID
+    fromUUID :: UUID -> a
+
+instance StorableUUID UUID where
+    toUUID = id; fromUUID = id
+
+
+storeEmpty :: String -> StoreRec c
+storeEmpty name = StoreRecM $ tell [return [(BC.pack name, RecEmpty)]]
+
+storeMbEmpty :: String -> Maybe () -> StoreRec c
+storeMbEmpty name = maybe (return ()) (const $ storeEmpty name)
+
+storeInt :: Integral a => String -> a -> StoreRec c
+storeInt name x = StoreRecM $ tell [return [(BC.pack name, RecInt $ toInteger x)]]
+
+storeMbInt :: Integral a => String -> Maybe a -> StoreRec c
+storeMbInt name = maybe (return ()) (storeInt name)
+
+storeNum :: (Real a, Fractional a) => String -> a -> StoreRec c
+storeNum name x = StoreRecM $ tell [return [(BC.pack name, RecNum $ toRational x)]]
+
+storeMbNum :: (Real a, Fractional a) => String -> Maybe a -> StoreRec c
+storeMbNum name = maybe (return ()) (storeNum name)
+
+storeText :: StorableText a => String -> a -> StoreRec c
+storeText name x = StoreRecM $ tell [return [(BC.pack name, RecText $ toText x)]]
+
+storeMbText :: StorableText a => String -> Maybe a -> StoreRec c
+storeMbText name = maybe (return ()) (storeText name)
+
+storeBinary :: BA.ByteArrayAccess a => String -> a -> StoreRec c
+storeBinary name x = StoreRecM $ tell [return [(BC.pack name, RecBinary $ BA.convert x)]]
+
+storeMbBinary :: BA.ByteArrayAccess a => String -> Maybe a -> StoreRec c
+storeMbBinary name = maybe (return ()) (storeBinary name)
+
+storeDate :: StorableDate a => String -> a -> StoreRec c
+storeDate name x = StoreRecM $ tell [return [(BC.pack name, RecDate $ toDate x)]]
+
+storeMbDate :: StorableDate a => String -> Maybe a -> StoreRec c
+storeMbDate name = maybe (return ()) (storeDate name)
+
+storeUUID :: StorableUUID a => String -> a -> StoreRec c
+storeUUID name x = StoreRecM $ tell [return [(BC.pack name, RecUUID $ toUUID x)]]
+
+storeMbUUID :: StorableUUID a => String -> Maybe a -> StoreRec c
+storeMbUUID name = maybe (return ()) (storeUUID name)
+
+storeRef :: Storable a => StorageCompleteness c => String -> a -> StoreRec c
+storeRef name x = StoreRecM $ do
+    s <- ask
+    tell $ (:[]) $ do
+        ref <- store s x
+        return [(BC.pack name, RecRef ref)]
+
+storeMbRef :: Storable a => StorageCompleteness c => String -> Maybe a -> StoreRec c
+storeMbRef name = maybe (return ()) (storeRef name)
+
+storeRawRef :: StorageCompleteness c => String -> Ref -> StoreRec c
+storeRawRef name ref = StoreRecM $ do
+    st <- ask
+    tell $ (:[]) $ do
+        ref' <- copyRef st ref
+        return [(BC.pack name, RecRef ref')]
+
+storeMbRawRef :: StorageCompleteness c => String -> Maybe Ref -> StoreRec c
+storeMbRawRef name = maybe (return ()) (storeRawRef name)
+
+storeZRef :: (ZeroStorable a, StorageCompleteness c) => String -> a -> StoreRec c
+storeZRef name x = StoreRecM $ do
+    s <- ask
+    tell $ (:[]) $ do
+        ref <- store s x
+        return $ if isZeroRef ref then []
+                                  else [(BC.pack name, RecRef ref)]
+
+storeRecItems :: StorageCompleteness c => [ ( ByteString, RecItem ) ] -> StoreRec c
+storeRecItems items = StoreRecM $ do
+    st <- ask
+    tell $ flip map items $ \( name, value ) -> do
+        value' <- copyRecItem st value
+        return [ ( name, value' ) ]
+
+loadBlob :: (ByteString -> a) -> Load a
+loadBlob f = loadCurrentObject >>= \case
+    Blob x -> return $ f x
+    _      -> throwError "Expecting blob"
+
+loadRec :: LoadRec a -> Load a
+loadRec (LoadRec lrec) = loadCurrentObject >>= \case
+    Rec rs -> do
+        ref <- loadCurrentRef
+        either throwError return $ runExcept $ runReaderT lrec (ref, rs)
+    _ -> throwError "Expecting record"
+
+loadZero :: a -> Load a
+loadZero x = loadCurrentObject >>= \case
+    ZeroObject -> return x
+    _          -> throwError "Expecting zero"
+
+
+loadEmpty :: String -> LoadRec ()
+loadEmpty name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbEmpty name
+
+loadMbEmpty :: String -> LoadRec (Maybe ())
+loadMbEmpty name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecEmpty) -> return (Just ())
+    Just _ -> throwError $ "Expecting type int of record item '"++name++"'"
+
+loadInt :: Num a => String -> LoadRec a
+loadInt name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbInt name
+
+loadMbInt :: Num a => String -> LoadRec (Maybe a)
+loadMbInt name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecInt x) -> return (Just $ fromInteger x)
+    Just _ -> throwError $ "Expecting type int of record item '"++name++"'"
+
+loadNum :: (Real a, Fractional a) => String -> LoadRec a
+loadNum name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbNum name
+
+loadMbNum :: (Real a, Fractional a) => String -> LoadRec (Maybe a)
+loadMbNum name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecNum x) -> return (Just $ fromRational x)
+    Just _ -> throwError $ "Expecting type number of record item '"++name++"'"
+
+loadText :: StorableText a => String -> LoadRec a
+loadText name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbText name
+
+loadMbText :: StorableText a => String -> LoadRec (Maybe a)
+loadMbText name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecText x) -> Just <$> fromText x
+    Just _ -> throwError $ "Expecting type text of record item '"++name++"'"
+
+loadTexts :: StorableText a => String -> LoadRec [a]
+loadTexts name = do
+    items <- map snd . filter ((BC.pack name ==) . fst) <$> loadRecItems
+    forM items $ \case RecText x -> fromText x
+                       _ -> throwError $ "Expecting type text of record item '"++name++"'"
+
+loadBinary :: BA.ByteArray a => String -> LoadRec a
+loadBinary name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbBinary name
+
+loadMbBinary :: BA.ByteArray a => String -> LoadRec (Maybe a)
+loadMbBinary name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecBinary x) -> return $ Just $ BA.convert x
+    Just _ -> throwError $ "Expecting type binary of record item '"++name++"'"
+
+loadBinaries :: BA.ByteArray a => String -> LoadRec [a]
+loadBinaries name = do
+    items <- map snd . filter ((BC.pack name ==) . fst) <$> loadRecItems
+    forM items $ \case RecBinary x -> return $ BA.convert x
+                       _ -> throwError $ "Expecting type binary of record item '"++name++"'"
+
+loadDate :: StorableDate a => String -> LoadRec a
+loadDate name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbDate name
+
+loadMbDate :: StorableDate a => String -> LoadRec (Maybe a)
+loadMbDate name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecDate x) -> return $ Just $ fromDate x
+    Just _ -> throwError $ "Expecting type date of record item '"++name++"'"
+
+loadUUID :: StorableUUID a => String -> LoadRec a
+loadUUID name = maybe (throwError $ "Missing record iteem '"++name++"'") return =<< loadMbUUID name
+
+loadMbUUID :: StorableUUID a => String -> LoadRec (Maybe a)
+loadMbUUID name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecUUID x) -> return $ Just $ fromUUID x
+    Just _ -> throwError $ "Expecting type UUID of record item '"++name++"'"
+
+loadRawRef :: String -> LoadRec Ref
+loadRawRef name = maybe (throwError $ "Missing record item '"++name++"'") return =<< loadMbRawRef name
+
+loadMbRawRef :: String -> LoadRec (Maybe Ref)
+loadMbRawRef name = (lookup (BC.pack name) <$> loadRecItems) >>= \case
+    Nothing -> return Nothing
+    Just (RecRef x) -> return (Just x)
+    Just _ -> throwError $ "Expecting type ref of record item '"++name++"'"
+
+loadRawRefs :: String -> LoadRec [Ref]
+loadRawRefs name = do
+    items <- map snd . filter ((BC.pack name ==) . fst) <$> loadRecItems
+    forM items $ \case RecRef x -> return x
+                       _ -> throwError $ "Expecting type ref of record item '"++name++"'"
+
+loadRef :: Storable a => String -> LoadRec a
+loadRef name = load <$> loadRawRef name
+
+loadMbRef :: Storable a => String -> LoadRec (Maybe a)
+loadMbRef name = fmap load <$> loadMbRawRef name
+
+loadRefs :: Storable a => String -> LoadRec [a]
+loadRefs name = map load <$> loadRawRefs name
+
+loadZRef :: ZeroStorable a => String -> LoadRec a
+loadZRef name = loadMbRef name >>= \case
+                    Nothing -> do Ref st _ <- loadRecCurrentRef
+                                  return $ fromZero st
+                    Just x  -> return x
+
+
+type Stored a = Stored' Complete a
+
+instance Storable a => Storable (Stored a) where
+    store st = copyRef st . storedRef
+    store' (Stored _ x) = store' x
+    load' = Stored <$> loadCurrentRef <*> load'
+
+instance ZeroStorable a => ZeroStorable (Stored a) where
+    fromZero st = Stored (zeroRef st) $ fromZero st
+
+fromStored :: Stored a -> a
+fromStored (Stored _ x) = x
+
+storedRef :: Stored a -> Ref
+storedRef (Stored ref _) = ref
+
+wrappedStore :: MonadIO m => Storable a => Storage -> a -> m (Stored a)
+wrappedStore st x = do ref <- liftIO $ store st x
+                       return $ Stored ref x
+
+wrappedLoad :: Storable a => Ref -> Stored a
+wrappedLoad ref = Stored ref (load ref)
+
+copyStored :: forall c c' m a. (StorageCompleteness c, StorageCompleteness c', MonadIO m) =>
+    Storage' c' -> Stored' c a -> m (LoadResult c (Stored' c' a))
+copyStored st (Stored ref' x) = liftIO $ returnLoadResult . fmap (flip Stored x) <$> copyRef' st ref'
+
+-- |Passed function needs to preserve the object representation to be safe
+unsafeMapStored :: (a -> b) -> Stored a -> Stored b
+unsafeMapStored f (Stored ref x) = Stored ref (f x)
+
+
+data StoreInfo = StoreInfo
+    { infoDate :: ZonedTime
+    , infoNote :: Maybe Text
+    }
+    deriving (Show)
+
+makeStoreInfo :: IO StoreInfo
+makeStoreInfo = StoreInfo
+    <$> getZonedTime
+    <*> pure Nothing
+
+storeInfoRec :: StoreInfo -> StoreRec c
+storeInfoRec info = do
+    storeDate "date" $ infoDate info
+    storeMbText "note" $ infoNote info
+
+loadInfoRec :: LoadRec StoreInfo
+loadInfoRec = StoreInfo
+    <$> loadDate "date"
+    <*> loadMbText "note"
+
+
+data History a = History StoreInfo (Stored a) (Maybe (StoredHistory a))
+    deriving (Show)
+
+type StoredHistory a = Stored (History a)
+
+instance Storable a => Storable (History a) where
+    store' (History si x prev) = storeRec $ do
+        storeInfoRec si
+        storeMbRef "prev" prev
+        storeRef "item" x
+
+    load' = loadRec $ History
+        <$> loadInfoRec
+        <*> loadRef "item"
+        <*> loadMbRef "prev"
+
+fromHistory :: StoredHistory a -> a
+fromHistory = fromStored . storedFromHistory
+
+fromHistoryAt :: ZonedTime -> StoredHistory a -> Maybe a
+fromHistoryAt zat = fmap (fromStored . snd) . listToMaybe . dropWhile ((at<) . zonedTimeToUTC . fst) . storedHistoryTimedList
+    where at = zonedTimeToUTC zat
+
+storedFromHistory :: StoredHistory a -> Stored a
+storedFromHistory sh = let History _ item _ = fromStored sh
+                        in item
+
+storedHistoryList :: StoredHistory a -> [Stored a]
+storedHistoryList = map snd . storedHistoryTimedList
+
+storedHistoryTimedList :: StoredHistory a -> [(ZonedTime, Stored a)]
+storedHistoryTimedList sh = let History hinfo item prev = fromStored sh
+                             in (infoDate hinfo, item) : maybe [] storedHistoryTimedList prev
+
+beginHistory :: Storable a => Storage -> StoreInfo -> a -> IO (StoredHistory a)
+beginHistory st si x = do sx <- wrappedStore st x
+                          wrappedStore st $ History si sx Nothing
+
+modifyHistory :: Storable a => StoreInfo -> (a -> a) -> StoredHistory a -> IO (StoredHistory a)
+modifyHistory si f prev@(Stored (Ref st _) _) = do
+    sx <- wrappedStore st $ f $ fromHistory prev
+    wrappedStore st $ History si sx (Just prev)
+
+
+showRatio :: Rational -> String
+showRatio r = case decimalRatio r of
+                   Just (n, 1) -> show n
+                   Just (n', d) -> let n = abs n'
+                                    in (if n' < 0 then "-" else "") ++ show (n `div` d) ++ "." ++
+                                       (concatMap (show.(`mod` 10).snd) $ reverse $ takeWhile ((>1).fst) $ zip (iterate (`div` 10) d) (iterate (`div` 10) (n `mod` d)))
+                   Nothing -> show (numerator r) ++ "/" ++ show (denominator r)
+
+decimalRatio :: Rational -> Maybe (Integer, Integer)
+decimalRatio r = do
+    let n = numerator r
+        d = denominator r
+        (c2, d') = takeFactors 2 d
+        (c5, d'') = takeFactors 5 d'
+    guard $ d'' == 1
+    let m = if c2 > c5 then 5 ^ (c2 - c5)
+                       else 2 ^ (c5 - c2)
+    return (n * m, d * m)
+
+takeFactors :: Integer -> Integer -> (Integer, Integer)
+takeFactors f n | n `mod` f == 0 = let (c, n') = takeFactors f (n `div` f)
+                                    in (c+1, n')
+                | otherwise = (0, n)
+
+parseRatio :: ByteString -> Maybe Rational
+parseRatio bs = case BC.groupBy ((==) `on` isNumber) bs of
+                     (m:xs) | m == BC.pack "-" -> negate <$> positive xs
+                     xs                        -> positive xs
+    where positive = \case
+              [bx] -> fromInteger . fst <$> BC.readInteger bx
+              [bx, op, by] -> do
+                  (x, _) <- BC.readInteger bx
+                  (y, _) <- BC.readInteger by
+                  case BC.unpack op of
+                       "." -> return $ (x % 1) + (y % (10 ^ BC.length by))
+                       "/" -> return $ x % y
+                       _   -> Nothing
+              _ -> Nothing
-- 
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