------------------------------------------------------------------------------
--- This module provides a general interface for databases (persistent
--- predicates) where each entry consists of a key and an info
--- part. The key is an integer and the info is arbitrary. All
--- functions are parameterized with a dynamic predicate that takes an
--- integer key as a first parameter.
---
--- This module is based on the [SQLite](http://sqlite.org/) database engine.
--- In order to use it you need to have `sqlite3` in your
--- `PATH` environment variable or adjust the value of the
--- constant `path'to'sqlite3`.
---
--- @author Sebastian Fischer with changes by Michael Hanus
--- @version August 2011
--- @category database
------------------------------------------------------------------------------

module Database.KeyDatabaseSQLite (

  Key, KeyPred,

  Query, runQ, transformQ, getDB,

  Transaction, TError(..), TErrorKind(..), showTError, runT, runJustT,
  returnT, doneT, errorT, failT, (|>>=), (|>>),
  sequenceT, sequenceT_, mapT, mapT_,

  Dynamic, persistentSQLite, closeDBHandles,

  existsDBKey,

  allDBKeys, allDBInfos, allDBKeyInfos,

  ColVal, (@=), someDBKeys, someDBInfos, someDBKeyInfos, someDBKeyProjections,

  getDBInfo, getDBInfos,

  deleteDBEntry, deleteDBEntries, updateDBEntry, newDBEntry, newDBKeyEntry,
  cleanDB

  ) where

import Global       ( Global, GlobalSpec(..), global, readGlobal, writeGlobal )
import IO           ( Handle, hPutStrLn, hGetLine, hFlush, hClose, stderr )
import IOExts       ( connectToCommand )
import List         ( intersperse, insertBy )
import Maybe        ( mapMMaybe )
import ReadNumeric  ( readInt )
import ReadShowTerm ( readQTerm, showQTerm, readsQTerm )
import System       ( system )

infixl 1 |>>, |>>=

-- adjust this if 'sqlite3' is not in the PATH
path'to'sqlite3 :: String
path'to'sqlite3 = "sqlite3"

-- Query and Transaction types

--- Queries can read but not write to the database.
data Query a = Query (IO a)

--- Runs a database query in the IO monad.
runQ :: Query a -> IO a
runQ (Query a) = a

--- Applies a function to the result of a database query.
transformQ :: (a -> b) -> Query a -> Query b
transformQ f query = Query (runQ query >>= return . f)

--- Transactions can modify the database and are executed
--- atomically.
data Transaction a = Trans (IO (TransResult a))
data TransResult a = OK a | Error TError

unTrans :: Transaction a -> IO (TransResult a)
unTrans (Trans action) = action

--- Runs a transaction atomically in the IO monad.
---
--- Transactions are <em>immediate</em>, which means that locks are
--- acquired on all databases as soon as the transaction is
--- started. After one transaction is started, no other database
--- connection will be able to write to the database or start a
--- transaction. Other connections <em>can</em> read the database
--- during a transaction of another process.
---
--- The choice to use immediate rather than deferred transactions is
--- conservative. It might also be possible to allow multiple
--- simultaneous transactions that lock tables on the first database
--- access (which is the default in SQLite). However this leads to
--- unpredictable order in which locks are taken when multiple
--- databases are involved. The current implementation fixes the
--- locking order by sorting databases by their name and locking them
--- in order immediately when a transaction begins.
---
--- More information on 
--- <a href="http://sqlite.org/lang_transaction.html">transactions</a>
--- in SQLite is available online.
---
runT :: Transaction a -> IO (Either a TError)
runT trans =
  do beginTransaction
     result <- catchTrans $ unTrans trans
     case result of
       Error err ->
         do rollbackTransaction
            return (Right err)
       OK res ->
         do commitTransaction
            return (Left res)

catchTrans :: IO (TransResult a) -> IO (TransResult a)
catchTrans action =
  action `catch` \terr ->
    do err <- readGlobal lastQueryError
       writeGlobal lastQueryError Nothing
       return . Error $ maybe (TError ExecutionError (showError terr)) id err

--- Executes a possibly composed transaction on the current state
--- of dynamic predicates as a single transaction.
--- Similar to <code>runT</code> but a run-time error is raised
--- if the execution of the transaction fails.
runJustT :: Transaction a -> IO a
runJustT t =
  runT t  >>=
  return . either id
              (\e -> error ("Transaction failed: " ++ showTError e))

--- Lifts a database query to the transaction type such that it can be
--- composed with other transactions. Run-time errors that occur
--- during the execution of the given query are transformed into
--- transaction errors.
getDB :: Query a -> Transaction a
getDB query = Trans . catchTrans $ runQ query >>= return . OK

-- not exported
transIO :: IO a -> Transaction a
transIO action = Trans (action >>= return . OK)

--- Returns the given value in a transaction that does not access the
--- database.
returnT :: a -> Transaction a
returnT = transIO . return

--- Returns the unit value in a transaction that does not access the
--- database. Useful to ignore results when composing transactions.
doneT :: Transaction ()
doneT = transIO done

--- Aborts a transaction with an error.
errorT :: TError -> Transaction a
errorT = Trans . return . Error

--- Aborts a transaction with a user-defined error message.
failT :: String -> Transaction a
failT = errorT . TError UserDefinedError

--- Combines two transactions into a single transaction that executes
--- both in sequence. The first transaction is executed, its result
--- passed to the function which computes the second transaction,
--- which is then executed to compute the final result.
---
--- If the first transaction is aborted with an error, the second
--- transaction is not executed.
(|>>=) :: Transaction a -> (a -> Transaction b) -> Transaction b
Trans action |>>= f = Trans $
  do result <- action
     case result of
       Error err -> return $ Error err
       OK res    -> unTrans $ f res

--- Combines two transactions to execute them in sequence. The result of
--- the first transaction is ignored.
(|>>) :: Transaction _ -> Transaction a -> Transaction a
t1 |>> t2 = t1 |>>= const t2

--- Executes a list of transactions sequentially and computes a list
--- of all results.
sequenceT :: [Transaction a] -> Transaction [a]
sequenceT = foldr seqT (returnT [])
 where
  -- seqT t ts = t |>>= \x -> ts |>>= \xs -> returnT (x:xs)
  seqT t ts = do x  <- t
                 xs <- ts
                 return (x:xs)

--- Executes a list of transactions sequentially, ignoring their
--- results.
sequenceT_ :: [Transaction _] -> Transaction ()
sequenceT_ = foldr (|>>) doneT

--- Applies a function that yields transactions to all elements of a
--- list, executes the transaction sequentially, and collects their
--- results.
mapT :: (a -> Transaction b) -> [a] -> Transaction [b]
mapT f = sequenceT . map f

--- Applies a function that yields transactions to all elements of a
--- list, executes the transactions sequentially, and ignores their
--- results.
mapT_ :: (a -> Transaction _) -> [a] -> Transaction ()
mapT_ f = sequenceT_ . map f

-- Interface based on keys

type DBFile = String
type TableName = String
type ColName = String

--- Result type of database predicates.
data Dynamic = DBInfo DBFile TableName [ColName]

--- The general type of database keys.
type Key = Int

type KeyPred a = Key -> a -> Dynamic -- for interface compatibility

dbInfo :: KeyPred a -> (DBFile,(TableName,[ColName]))
dbInfo keyPred = (db,(table,cols))
 where
  DBInfo db table cols = keyPred ignored ignored

ignored :: a
ignored = error "unexpected access to argument of database predicate"

dbFile :: KeyPred _ -> DBFile
dbFile = fst . dbInfo

tableName :: KeyPred _ -> TableName
tableName = fst . snd . dbInfo

colNames :: KeyPred _ -> [ColName]
colNames = snd . snd . dbInfo

--- This function is used instead of <code>dynamic</code> or
--- <code>persistent</code> to declare predicates whose facts are stored
--- in an SQLite database.
---
--- If the provided database or the table do not exist they are created
--- automatically when the declared predicate is accessed for the first time.
---
--- Multiple column names can be provided if the second argument of
--- the predicate is a tuple with a matching arity. Other record types
--- are not supported. If no column names are provided a table with a
--- single column called <code>info</code> is created. Columns of name
--- <code>_rowid_</code> are not supported and lead to a run-time
--- error.
---
--- @param dbFile - the name of the associated database file
--- @param tableName - the name of the associated database table
--- @param colNames - the column names of the associated database table
persistentSQLite :: DBFile -> TableName -> [ColName] -> KeyPred a
persistentSQLite db table cols _ _
  | null cols             = DBInfo db table ["info"]
  | "_rowid_" `elem` cols = error "columns must not be called _rowid_"
  | otherwise             = DBInfo db table cols

--- Checks whether the predicate has an entry with the given key.
existsDBKey :: KeyPred _ -> Key -> Query Bool
existsDBKey keyPred key = Query $
  do n <- selectInt keyPred "count(*)" $ "where _rowid_ = " ++ show key
     return $! n > 0

--- Returns a list of all stored keys. Do not use this function unless
--- the database is small.
allDBKeys :: KeyPred _ -> Query [Key]
allDBKeys keyPred = Query $
  do rows <- selectRows keyPred "_rowid_" ""
     mapIO readIntOrExit rows

--- Returns a list of all info parts of stored entries. Do not use this
--- function unless the database is small.
allDBInfos :: KeyPred a -> Query [a]
allDBInfos keyPred = Query $
  do rows <- selectRows keyPred "*" ""
     return $!! map readInfo rows

readInfo :: String -> a
readInfo str = readQTerm $ "(" ++ str ++ ")"

--- Returns a list of all stored entries. Do not use this function
--- unless the database is small.
allDBKeyInfos :: KeyPred a -> Query [(Key,a)]
allDBKeyInfos keyPred = Query $
  do rows <- selectRows keyPred "_rowid_,*" ""
     mapIO readKeyInfo rows

readKeyInfo :: String -> IO (Key,a)
readKeyInfo row =
  do key <- readIntOrExit keyStr
     return $!! (key, readInfo infoStr)
 where
  (keyStr,_:infoStr) = break (','==) row

--- Abstract type for value restrictions
data ColVal = ColVal Int String

--- Constructs a value restriction for the column given as first argument
(@=) :: Int -> a -> ColVal
n @= x = ColVal n . quote $ showQTerm x

--- Returns a list of those stored keys where the corresponding info
--- part matches the gioven value restriction. Safe to use even on
--- large databases if the number of results is small.
someDBKeys :: KeyPred _ -> [ColVal] -> Query [Key]
someDBKeys keyPred cvs = Query $
  do rows <- selectSomeRows keyPred cvs "_rowid_"
     mapIO readIntOrExit rows

--- Returns a list of those info parts of stored entries that match
--- the given value restrictions for columns. Safe to use even on
--- large databases if the number of results is small.
someDBInfos :: KeyPred a -> [ColVal] -> Query [a]
someDBInfos keyPred cvs = Query $
  do rows <- selectSomeRows keyPred cvs "*"
     return $!! map readInfo rows

--- Returns a list of those entries that match the given value
--- restrictions for columns. Safe to use even on large databases if
--- the number of results is small.
someDBKeyInfos :: KeyPred a -> [ColVal] -> Query [(Key,a)]
someDBKeyInfos keyPred cvs = Query $
  do rows <- selectSomeRows keyPred cvs "_rowid_,*"
     mapIO readKeyInfo rows

--- Returns a list of column projections on
--- those entries that match the given value
--- restrictions for columns. Safe to use even on large databases if
--- the number of results is small.
someDBKeyProjections :: KeyPred a -> [Int] -> [ColVal] -> Query [(Key,b)]
someDBKeyProjections keyPred cols cvs = Query $
  do let colnames = commaSep (map ((colNames keyPred) !!) cols)
     rows <- selectSomeRows keyPred cvs ("_rowid_,"++colnames)
     mapIO readKeyInfo rows

--- Queries the information stored under the given key. Yields
--- <code>Nothing</code> if the given key is not present.
getDBInfo :: KeyPred a -> Key -> Query (Maybe a)
getDBInfo keyPred key = Query $
  do rows <- selectRows keyPred "*" $ "where _rowid_ = " ++ show key
     readHeadIfExists rows
 where
  readHeadIfExists []    = return Nothing
  readHeadIfExists (x:_) = return $!! Just (readInfo x)

--- Queries the information stored under the given keys. Yields
--- <code>Nothing</code> if a given key is not present.
getDBInfos :: KeyPred a -> [Key] -> Query (Maybe [a])
getDBInfos keyPred keys = Query $
  do rows <- selectRows keyPred "_rowid_,*" $
               "where _rowid_ in (" ++ commaSep (map show keys) ++ ")"
     sortByIndexInGivenList rows
 where
  sortByIndexInGivenList rows =
    do keyInfos <- mapIO readKeyInfo rows
       return $ mapMMaybe (\key -> lookup key keyInfos) keys

commaSep :: [String] -> String
commaSep = concat . intersperse ", "

--- Deletes the information stored under the given key. If the given
--- key does not exist this transaction is silently ignored and no
--- error is raised.
deleteDBEntry :: KeyPred _ -> Key -> Transaction ()
deleteDBEntry keyPred key =
  modify keyPred "delete from" $ "where _rowid_ = " ++ show key

--- Deletes the information stored under the given keys. No error is
--- raised if (some of) the keys do not exist.
deleteDBEntries :: KeyPred _ -> [Key] -> Transaction ()
deleteDBEntries keyPred keys =
  modify keyPred "delete from" $
    "where _rowid_ in (" ++ commaSep (map show keys) ++ ")"

--- Updates the information stored under the given key. The
--- transaction is aborted with a <code>KeyNotExistsError</code> if
--- the given key is not present in the database.
updateDBEntry :: KeyPred a -> Key -> a -> Transaction ()
updateDBEntry keyPred key info =
  errorUnlessKeyExists keyPred key ("updateDBEntry, " ++ show key) |>>
  modify keyPred "update"
    ("set " ++ commaSep (colVals keyPred info) ++
     " where _rowid_ = " ++ show key)

errorUnlessKeyExists :: KeyPred a -> Key -> String -> Transaction ()
errorUnlessKeyExists keyPred key msg =
  getDB (existsDBKey keyPred key) |>>= \exists ->
  if not exists
    then errorT $ TError KeyNotExistsError msg
    else doneT

colVals :: KeyPred a -> a -> [String]
colVals keyPred info =
  zipWith (\c v -> c ++ " = " ++ v) (colNames keyPred) (infoVals keyPred info)

infoVals :: KeyPred a -> a -> [String]
infoVals keyPred info
  | null . tail $ colNames keyPred = [quote $ showQTerm info]
  | otherwise                      = map quote $ showTupleArgs info

quote :: String -> String
quote s = "'" ++ concatMap quoteChar s ++ "'"
 where
  quoteChar c = if c == ''' then "''" else [c]

--- Stores new information in the database and yields the newly
--- generated key.
newDBEntry :: KeyPred a -> a -> Transaction Key
newDBEntry keyPred info =
  modify keyPred "insert into"
    ("values (" ++ commaSep (infoVals keyPred info) ++ ")") |>>
  getDB (Query $ selectInt keyPred "last_insert_rowid()" "")

--- Stores a new entry in the database under a given key.
--- The transaction fails if the key already exists.
--- @param db - the database (a dynamic predicate)
--- @param key - the key of the new entry (an integer)
--- @param info - the information to be stored in the new entry
newDBKeyEntry :: KeyPred a -> Key -> a -> Transaction ()
newDBKeyEntry keyPred key info =
  getDB (existsDBKey keyPred key) |>>= \b ->
  if b
   then errorT . TError DuplicateKeyError $
                   "database already contains entry with key: " ++ show key
   else modify keyPred "insert into"
          ("values (" ++ commaSep (infoVals keyPred info) ++ ")") |>>
        getDB (Query $ selectInt keyPred "last_insert_rowid()" "") |>>= \k ->
        modify keyPred "update" $
          "set _rowid_ = " ++ show key ++ " where _rowid_ = " ++ show k

--- Deletes all entries from the database associated with a predicate.
cleanDB :: KeyPred _ -> Transaction ()
cleanDB keyPred = modify keyPred "delete from" ""

-- SQL access functions

-- These functions are not exported and abstract common functionality
-- used in the library functions above. Each database access is one of
-- the following: a modification, a selection of a numeric aggregate,
-- or a selection of rows.

sqlite3 :: KeyPred _ -> String -> IO Handle
sqlite3 keyPred sql =
  do h <- getDBHandle keyPred
     hPutAndFlush h $ sql ++ ";"
     return h

hPutAndFlush :: Handle -> String -> IO ()
hPutAndFlush h s = hPutStrLn h s >> hFlush h

modify :: KeyPred _ -> String -> String -> Transaction ()
modify keyPred before after = transIO $
  do sqlite3 keyPred $
       before ++ " " ++ tableName keyPred ++ " " ++ after
     done

selectInt :: KeyPred _ -> String -> String -> IO Int
selectInt keyPred aggr cond =
  do h <- sqlite3 keyPred $
       "select distinct " ++ aggr ++
       " from " ++ tableName keyPred ++ " " ++ cond
     hGetLine h >>= readIntOrExit

-- yields 1 for "1a" and exits for ""
readIntOrExit :: String -> IO Int
readIntOrExit s = maybe err (return . fst) $ readInt s
 where
  err = dbError ExecutionError $
    "readIntOrExit: cannot parse integer from string '" ++ show s ++ "'"

-- When selecting an unknown number of rows it is necessary to know
-- when to stop. One way to be able to stop is to select 'count(*)'
-- instead of the actual colums before the query. As it is potentially
-- inefficient to execute the query twice, this implementation takes a
-- different approach: generate a random string before the query and
-- select it afterwards, then read all lines up to this random string.

type Row = String

selectRows :: KeyPred _ -> String -> String -> IO [Row]
selectRows keyPred cols cond =
  do h <- sqlite3 keyPred "select hex(randomblob(8))"
     rnd <- hGetLine h -- 8 random bytes = 16 random hex chars
     hPutAndFlush h $
       "select " ++ cols ++ " from " ++ tableName keyPred ++
       " " ++ cond ++ "; select " ++ quote rnd ++ ";"
     hGetLinesBefore h rnd

hGetLinesBefore :: Handle -> String -> IO [String]
hGetLinesBefore h stop =
  do line <- hGetLine h
     if line == stop
       then return []
       else do rest <- hGetLinesBefore h stop
               return (line : rest)

selectSomeRows :: KeyPred _ -> [ColVal] -> String -> IO [Row]
selectSomeRows keyPred cvs cols =
  selectRows keyPred cols $ if null cvs then ""
                            else "where " ++ showColVals keyPred cvs

showColVals :: KeyPred a -> [ColVal] -> String
showColVals _       []     = "1"
showColVals keyPred (c:vs) = concat . intersperse " AND " $ map showCV (c:vs)
 where
  showCV (ColVal n s) = colNames keyPred !! n ++ " = " ++ s

--- Closes all database connections. Should be called when no more
--- database access will be necessary.
closeDBHandles :: IO ()
closeDBHandles =
  do withAllDBHandles hClose
     writeGlobal openDBHandles []

-- helper functions and globaly stored information

dbError :: TErrorKind -> String -> IO a
dbError kind msg =
  do writeGlobal lastQueryError . Just $ TError kind msg
     error msg

lastQueryError :: Global (Maybe TError)
lastQueryError = global Nothing Temporary

getDBHandle :: KeyPred _ -> IO Handle
getDBHandle keyPred = 
  do ensureDBFor keyPred
     readDBHandle $ dbFile keyPred

-- Initializes the database and table for the given predicate. This
-- function must be called before the database for this predicate is
-- accessed and before a transaction that uses this predicate is
-- started.
ensureDBFor :: KeyPred _ -> IO ()
ensureDBFor keyPred =
  do ensureDBHandle db
     ensureDBTable db table cols
 where
  (db,(table,cols)) = dbInfo keyPred

readDBHandle :: DBFile -> IO Handle
readDBHandle db = readGlobal openDBHandles >>= maybe err return . lookup db
 where
  err = dbError ExecutionError $ "readDBHandle: no handle for '" ++ db ++ "'"

openDBHandles :: Global [(DBFile,Handle)]
openDBHandles = global [] Temporary

withAllDBHandles :: (Handle -> IO _) -> IO ()
withAllDBHandles f =
  do dbHandles <- readGlobal openDBHandles
     mapIO_ (f . snd) dbHandles

ensureDBHandle :: DBFile -> IO ()
ensureDBHandle db =
  do dbHandles <- readGlobal openDBHandles
     unless (db `elem` map fst dbHandles) $ addNewDBHandle dbHandles
 where
  addNewDBHandle dbHandles = do
    exsqlite3 <- system $ "which " ++ path'to'sqlite3 ++ " > /dev/null"
    when (exsqlite3>0) $
      error "Database interface `sqlite3' not found. Please install package `sqlite3'!"
    h <- connectToCommand $ path'to'sqlite3 ++ " " ++ db
    hPutAndFlush h ".separator ','"
    writeGlobal openDBHandles $ -- sort against deadlock
      insertBy ((<=) `on` fst) (db,h) dbHandles
    isTrans <- readGlobal currentlyInTransaction
    unless (not isTrans) $ hPutStrLn h "begin immediate;"

unless :: Bool -> IO () -> IO ()
unless False action = action
unless True  _      = done

on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
on f g x y = f (g x) (g y)

ensureDBTable :: DBFile -> TableName -> [ColName] -> IO ()
ensureDBTable db table cols =
  do dbTables <- readGlobal knownDBTables
     unless ((db,table) `elem` dbTables) $
       do h <- readDBHandle db
          hPutAndFlush h $
            "create table if not exists " ++ table ++
            " (" ++ commaSep cols ++ ");"
          writeGlobal knownDBTables $ (db,table) : dbTables

knownDBTables :: Global [(DBFile,TableName)]
knownDBTables = global [] Temporary

beginTransaction :: IO ()
beginTransaction =
  do writeGlobal currentlyInTransaction True
     withAllDBHandles (`hPutAndFlush` "begin immediate;")

commitTransaction :: IO ()
commitTransaction =
  do withAllDBHandles (`hPutAndFlush` "commit;")
     writeGlobal currentlyInTransaction False

rollbackTransaction :: IO ()
rollbackTransaction =
  do withAllDBHandles (`hPutAndFlush` "rollback;")
     writeGlobal currentlyInTransaction False

currentlyInTransaction :: Global Bool
currentlyInTransaction = global False Temporary

-- converting arguments of a tuple to strings

showTupleArgs :: a -> [String]
showTupleArgs = splitTLC . removeOuterParens . showQTerm

removeOuterParens :: String -> String
removeOuterParens ('(':cs) = init cs

init :: [a] -> [a]
init = reverse . tail . reverse

-- split at top-level commas
splitTLC :: String -> [String]
splitTLC = parse ""

type Stack  = String

parse :: Stack -> String -> [String]
parse _  ""     = []
parse st (c:cs) =
  case (st,c:cs) of -- Curry allows ''' for '\''
    ('\'':xs,'\'':'\'':ys) -> '\'' <: ('\'' <: parse xs ys)
    _ -> next c st $ parse (updStack c st) cs

next :: Char -> Stack -> [String] -> [String]
next c []    xs = if c==',' then [] : xs else c <: xs
next c (_:_) xs = c <: xs

(<:) :: Char -> [String] -> [String]
c <: []     = [[c]]
c <: (x:xs) = (c:x):xs

updStack :: Char -> Stack -> Stack
updStack char stack =
  case (char,stack) of
    -- char is an escaped character
    (_   ,'\\':xs) -> xs      -- the next character is not

    -- char is the escape character
    ('\\',     xs) -> '\\':xs -- push it on the stack

    -- char is the string terminator
    ('"' , '"':xs) -> xs      -- closes current string literal
    ('"' , ''':xs) -> ''':xs  -- ignored inside character
    ('"' ,     xs) -> '"':xs  -- opens a new string

    -- char is the character terminator
    (''' , ''':xs) -> xs      -- closes current character literal
    (''' , '"':xs) -> '"':xs  -- ignored inside string
    (''' ,     xs) -> ''':xs  -- opens a new character

    -- parens and brackets
    (_   , '"':xs) -> '"':xs  -- are ignored inside strings
    (_   , ''':xs) -> ''':xs  -- and characters
    ('(' ,     xs) -> '(':xs  -- new opening paren
    (')' , '(':xs) -> xs      -- closing paren
    ('[' ,     xs) -> '[':xs  -- opening bracket
    (']' , '[':xs) -> xs      -- closing bracket

    -- other characters don't modify the stack
    (_   ,     xs) -> xs

-- for debugging

-- hPutStrLn h s =
--   do IO.hPutStrLn stderr $ " > " ++ s
--      IO.hPutStrLn h s

-- hGetLine h =
--   do l <- IO.hGetLine h
--      IO.hPutStrLn stderr $ "<  " ++ l
--      return l

-- copied from Database:

--- The type of errors that might occur during a transaction.
data TError = TError TErrorKind String
 deriving (Eq,Show)

--- The various kinds of transaction errors.
data TErrorKind = KeyNotExistsError
                | NoRelationshipError
                | DuplicateKeyError
                | KeyRequiredError
                | UniqueError
                | MinError
                | MaxError
                | UserDefinedError
                | ExecutionError
 deriving (Eq,Show)

--- Transforms a transaction error into a string.
showTError :: TError -> String
showTError (TError k s) = "Transaction error " ++ show k ++ ": " ++ s

------------------------------------------------------------------------------
instance Monad Transaction where
  a1 >>= a2 = a1 |>>= a2
  a1 >>  a2 = a1 |>>  a2
  return x = returnT x