    {-# OPTIONS_GHC -fno-warn-name-shadowing #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    {-# LANGUAGE TypeSynonymInstances #-}

    -- ------------------------------------------------------------

    {- |
       Module     : Text.XML.HXT.Arrow.Pickle.Xml
       Copyright  : Copyright (C) 2005-2012 Uwe Schmidt
       License    : MIT

       Maintainer : Uwe Schmidt (uwe@fh-wedel.de)
       Stability  : stable
       Portability: portable

       Pickler functions for converting between user defined data types
       and XmlTree data. Usefull for persistent storage and retreival
       of arbitray data as XML documents.

       This module is an adaptation of the pickler combinators
       developed by Andrew Kennedy
       ( http:\/\/research.microsoft.com\/~akenn\/fun\/picklercombinators.pdf ).

       The difference to Kennedys approach is that the target is not
       a list of Chars but a list of XmlTrees. The basic picklers will
       convert data into XML text nodes. New are the picklers for
       creating elements and attributes.

       One extension was neccessary: The unpickling may fail.

       Old: Therefore the unpickler has a Maybe result type.
       Failure is used to unpickle optional elements
       (Maybe data) and lists of arbitray length.

       Since hxt-9.2.0: The unpicklers are implemented as
       a parser monad with an Either err val result type.
       This enables appropriate error messages , when unpickling
       XML stuff, that is not generated with the picklers and which contains
       some elements and/or attributes that are not handled when unpickling.

       There is an example program demonstrating the use
       of the picklers for a none trivial data structure.
       (see \"examples\/arrows\/pickle\" directory in the hxt distribution)

    -}

    -- ------------------------------------------------------------

    module Text.XML.HXT.Arrow.Pickle.Xml
    where

    import           Control.Arrow.ArrowList
    import           Control.Arrow.ListArrows
    import           Control.Monad                    ( )
    import           Control.Monad.Error
    import           Control.Monad.State

    import           Data.Char                        (isDigit)
    import           Data.List                        (foldl')
    import           Data.Maybe
    import           Data.Map                         (Map)
    import qualified Data.Map                         as M

    import           Text.XML.HXT.DOM.Interface
    import qualified Text.XML.HXT.DOM.XmlNode         as XN
    import qualified Text.XML.HXT.DOM.ShowXml         as XN
    import           Text.XML.HXT.Arrow.Edit          (xshowEscapeXml)
    import           Text.XML.HXT.Arrow.Pickle.Schema
    import           Text.XML.HXT.Arrow.ReadDocument  (xread)
    import           Text.XML.HXT.Arrow.WriteDocument (writeDocumentToString)
    import           Text.XML.HXT.Arrow.XmlState

    {- just for embedded test cases, prefix with -- to activate
    import           Text.XML.HXT.Arrow.XmlArrow
    import qualified Control.Arrow.ListArrows         as X
    -- -}

    -- ------------------------------------------------------------

    data St         = St { attributes :: [XmlTree]
                         , contents   :: [XmlTree]
                         , nesting    :: Int                -- the remaining 3 fields are used only for unpickling
                         , pname      :: QName              -- to generate appropriate error messages
                         , pelem      :: Bool
                         } deriving (Show)

    data PU a       = PU { appPickle   :: Pickler a         -- (a, St) -> St
                         , appUnPickle :: Unpickler a
                         , theSchema   :: Schema
                         }

    -- --------------------
    --
    -- The pickler

    type Pickler a          = a -> St -> St

    -- --------------------
    --
    -- The unpickler monad, a combination of state and error monad

    newtype Unpickler a     = UP { runUP :: St -> (UnpickleVal a, St) }

    type UnpickleVal a      = Either UnpickleErr a

    type UnpickleErr        = (String, St)

    instance Monad Unpickler where
        return x    = UP $ \ st -> (Right x, st)
        u >>= f     = UP $ \ st ->
                      let (r, st') = runUP u st in
                      case r of
                        Left err -> (Left err, st')
                        Right v  -> runUP (f v) st'
        fail        = throwMsg                              -- don't use fail, use throwError

    instance MonadState St Unpickler where
        get         = UP $ \ st -> (Right st, st)
        put st      = UP $ \ _  -> (Right (), st)

    instance MonadError UnpickleErr Unpickler where
        throwError err
                    = UP $ \ st -> (Left err, st)

        -- redundant, not (yet) used
        catchError u handler
                    = UP $ \ st ->
                      let (r, st') = runUP u st in
                      case r of
                        Left err -> runUP (handler err) st  -- not st', state will be reset in error case
                        _        -> (r, st')

    throwMsg        :: String -> Unpickler a
    throwMsg msg    = UP $ \ st -> (Left (msg, st), st)

    -- | Choice combinator for unpickling
    --
    -- first 2 arguments are applied sequentially, but if the 1. one fails the
    -- 3. arg is applied

    mchoice         :: Unpickler a -> (a -> Unpickler b) -> Unpickler b -> Unpickler b
    mchoice u f v   = UP $ \ st ->
                      let (r, st') = runUP u st in
                      case r of
                        Right x
                            -> runUP (f x) st'                      -- success
                        Left e@(_msg, st'')
                            -> if nesting st'' == nesting st        -- true: failure in parsing curr contents
                               then runUP v st                      -- try the alternative unpickler
                               else (Left e, st')                   -- false: failure in unpickling a subtree of
                                                                    -- the current contents, so the whole unpickler
                                                                    -- must fail

    -- | Lift a Maybe value into the Unpickler monad.
    --
    -- The 1. arg is the attached error message

    liftMaybe       :: String -> Maybe a -> Unpickler a
    liftMaybe e v  = case v of
                        Nothing -> throwMsg e
                        Just x  -> return x

    -- | Lift an Either value into the Unpickler monad

    liftUnpickleVal         :: UnpickleVal a -> Unpickler a
    liftUnpickleVal v       = UP $ \ st -> (v, st)

    -- --------------------

    getCont         :: Unpickler XmlTree
    getCont         = do cs <- gets contents
                         case cs of
                           []       -> throwMsg "no more contents to be read"
                           (x : xs) -> do modify (\ s -> s {contents = xs})
                                          return x

    getAtt          :: QName -> Unpickler XmlTree
    getAtt qn       = do as <- gets attributes
                         case findAtt as of
                           Nothing -> throwMsg $ "no attribute value found for " ++ show qn
                           Just (a, as') -> do modify (\ s -> s {attributes = as'})
                                               return a
        where
          findAtt   = findElem (maybe False (== qn) . XN.getAttrName)

    getNSAtt        :: String -> Unpickler ()
    getNSAtt ns     = do as <- gets attributes
                         case findNS as of
                           Nothing        -> throwMsg $
                                             "no namespace declaration found for namespace " ++ show ns
                           Just (_a, as') -> do modify (\ s -> s {attributes = as'})
                                                return ()
        where
          isNS t    = (fromMaybe False . fmap isNameSpaceName . XN.getAttrName $ t)
                      &&
                      XN.xshow (XN.getChildren t) == ns
          findNS    = findElem isNS

    -- --------------------

    emptySt         :: St
    emptySt         =  St { attributes = []
                          , contents   = []
                          , nesting    = 0
                          , pname      = mkName "/"
                          , pelem      = True
                          }

    putAtt          :: QName -> [XmlTree] -> St -> St
    putAtt qn v s   = s {attributes = x : attributes s}
                      where
                        x = XN.mkAttr qn v
    {-# INLINE putAtt #-}

    putCont         :: XmlTree -> St -> St
    putCont x s     = s {contents = x : contents s}
    {-# INLINE putCont #-}

    -- --------------------
    --
    -- generally useful function for splitting a value from a list

    findElem       :: (a -> Bool) -> [a] -> Maybe (a, [a])
    findElem p     = find' id
        where
          find' _ []         = Nothing
          find' prefix (x : xs)
              | p x          = Just (x, prefix xs)
              | otherwise    = find' (prefix . (x:)) xs

    -- ------------------------------------------------------------
    --
    -- | Format the context of an error message.

    formatSt                :: St -> String
    formatSt st             = fcx ++
                              fa (attributes st) ++
                              fc (contents   st)
        where
          fcx               = "\n" ++ "context:    " ++
                              ( if pelem st
                                then "element"
                                else "attribute"
                              ) ++
                              " " ++ show (pname st)
          fc []             = ""
          fc cs             = "\n" ++ "contents:   " ++ formatXML cs
          fa []             = ""
          fa as             = "\n" ++ "attributes: " ++ formatXML as
          formatXML         = format 80 . showXML
          showXML           = concat . runLA ( xshowEscapeXml unlistA )
          format n s        = let s' = take (n + 1) s in
                              if length s' <= n then s' else take n s ++ "..."

    -- ------------------------------------------------------------

    -- | conversion of an arbitrary value into an XML document tree.
    --
    -- The pickler, first parameter, controls the conversion process.
    -- Result is a complete document tree including a root node

    pickleDoc       :: PU a -> a -> XmlTree
    pickleDoc p v   = XN.mkRoot (attributes st) (contents st)
        where
          st        = appPickle p v emptySt

    -- | Conversion of an XML document tree into an arbitrary data type
    --
    -- The inverse of 'pickleDoc'.
    -- This law should hold for all picklers: @ unpickle px . pickle px $ v == Just v @.
    -- Not every possible combination of picklers does make sense.
    -- For reconverting a value from an XML tree, is becomes neccessary,
    -- to introduce \"enough\" markup for unpickling the value

    unpickleDoc     :: PU a -> XmlTree -> Maybe a
    unpickleDoc p   = either (const Nothing) Just
                      . unpickleDoc' p

    -- | Like unpickleDoc but with a (sometimes) useful error message, when unpickling failed.

    unpickleDoc'    :: PU a -> XmlTree -> Either String a
    unpickleDoc' p t
        | XN.isRoot t       = mapErr $
                              unpickleElem' p 0              t
        | otherwise         = unpickleDoc'  p (XN.mkRoot [] [t])
        where
          mapErr            = either ( Left .
                                       \ (msg, st) -> msg ++ formatSt st
                                     ) Right

    -- | The main entry for unpickling, called by unpickleDoc

    unpickleElem'   :: PU a -> Int -> XmlTree -> UnpickleVal a
    unpickleElem' p l t
        = fst
          . runUP (appUnPickle p)
          $ St { attributes = fromMaybe [] .
                              XN.getAttrl $  t
               , contents   = XN.getChildren t
               , nesting    = l
               , pname      = fromJust .
                              XN.getName  $  t
               , pelem      = XN.isElem      t
               }

    -- ------------------------------------------------------------

    -- | Pickles a value, then writes the document to a string.

    showPickled :: (XmlPickler a) => SysConfigList -> a -> String
    showPickled a = concat . (pickleDoc xpickle >>> runLA (writeDocumentToString a))

    -- ------------------------------------------------------------

    -- | The zero pickler
    --
    -- Encodes nothing, fails always during unpickling

    xpZero                  :: String -> PU a
    xpZero err              =  PU { appPickle   = const id
                                  , appUnPickle = throwMsg err
                                  , theSchema   = scNull
                                  }

    -- | unit pickler

    xpUnit                  :: PU ()
    xpUnit                  = xpLift ()

    -- | Check EOF pickler.
    --
    -- When pickling, this behaves like the unit pickler.
    -- The unpickler fails, when there is some unprocessed XML contents left.

    xpCheckEmptyContents    :: PU a -> PU a
    xpCheckEmptyContents pa =  PU { appPickle   = appPickle pa
                                  , appUnPickle = do res <- appUnPickle pa
                                                     cs <- gets contents
                                                     if null cs
                                                        then return res
                                                        else contentsLeft
                                  , theSchema   = scNull
                                  }
        where
          contentsLeft      = throwMsg
                              "xpCheckEmptyContents: unprocessed XML content detected"

    -- | Like xpCheckEmptyContents, but checks the attribute list

    xpCheckEmptyAttributes  :: PU a -> PU a
    xpCheckEmptyAttributes pa
                            =  PU { appPickle   = appPickle pa
                                  , appUnPickle = do res <- appUnPickle pa
                                                     as <- gets attributes
                                                     if null as
                                                        then return res
                                                        else attributesLeft
                                  , theSchema   = scNull
                                  }
        where
          attributesLeft    = throwMsg
                              "xpCheckEmptyAttributes: unprocessed XML attribute(s) detected"

    -- | Composition of xpCheckEmptyContents and xpCheckAttributes

    xpCheckEmpty            :: PU a -> PU a
    xpCheckEmpty            = xpCheckEmptyAttributes . xpCheckEmptyContents

    xpLift                  :: a -> PU a
    xpLift x                =  PU { appPickle   = const id
                                  , appUnPickle = return x
                                  , theSchema   = scEmpty
                                  }

    -- | Lift a Maybe value to a pickler.
    --
    -- @Nothing@ is mapped to the zero pickler, @Just x@ is pickled with @xpLift x@.

    xpLiftMaybe                     :: Maybe a -> PU a
    xpLiftMaybe v                   = (xpLiftMaybe'' v) { theSchema = scOption scEmpty }
        where
        xpLiftMaybe'' Nothing       = xpZero "xpLiftMaybe: got Nothing"
        xpLiftMaybe'' (Just x)      = xpLift x

    xpLiftEither                    :: Either String a -> PU a
    xpLiftEither v                  = (xpLiftEither'' v) { theSchema = scOption scEmpty }
        where
        xpLiftEither'' (Left err)   = xpZero err
        xpLiftEither'' (Right x)    = xpLift x

    -- | Combine two picklers sequentially.
    --
    -- If the first fails during
    -- unpickling, the whole unpickler fails

    xpSeq           :: (b -> a) -> PU a -> (a -> PU b) -> PU b
    xpSeq f pa k
        = PU { appPickle  = ( \ b ->
                              let a = f b in
                              appPickle pa a . appPickle (k a) b
                             )
             , appUnPickle = appUnPickle pa >>= (appUnPickle . k)
             , theSchema   = undefined
             }

    -- | First apply a fixed pickler/unpickler, then a 2. one
    --
    -- If the first fails during unpickling, the whole pickler fails.
    -- This can be used to check some properties of the input, e.g. whether
    -- a given fixed attribute or a namespace declaration exists ('xpAddFixedAttr', 'xpAddNSDecl')
    -- or to filter the input, e.g. to ignore some elements or attributes ('xpFilterCont', 'xpFilterAttr').
    --
    -- When pickling, this can be used to insert some fixed XML pieces, e.g. namespace declarations,
    -- class attributes or other stuff.

    xpSeq'          :: PU () -> PU a -> PU a
    xpSeq' pa       = xpWrap ( snd
                             , \ y -> ((), y)
                             ) .
                      xpPair pa

    -- | combine tow picklers with a choice
    --
    -- Run two picklers in sequence like with xpSeq.
    -- When during unpickling the first one fails,
    -- an alternative pickler (first argument) is applied.
    -- This pickler is only used as combinator for unpickling.

    xpChoice                :: PU b -> PU a -> (a -> PU b) -> Unpickler b
    xpChoice pb pa k        = mchoice (appUnPickle pa) (appUnPickle . k) (appUnPickle pb)

    -- | map value into another domain and apply pickler there
    --
    -- One of the most often used picklers.

    xpWrap                  :: (a -> b, b -> a) -> PU a -> PU b
    xpWrap (i, j) pa        = (xpSeq j pa (xpLift . i)) { theSchema = theSchema pa }

    -- | like 'xpWrap', but if the inverse mapping is undefined, the unpickler fails
    --
    -- Map a value into another domain. If the inverse mapping is
    -- undefined (Nothing), the unpickler fails
    --
    -- Deprecated: Use xpWrapEither, this gives better error messages

    xpWrapMaybe             :: (a -> Maybe b, b -> a) -> PU a -> PU b
    xpWrapMaybe (i, j) pa   = (xpSeq j pa (xpLiftMaybe . i)) { theSchema = theSchema pa }

    -- | like 'xpWrap', but if the inverse mapping is undefined, the unpickler fails
    --
    -- Map a value into another domain. If the inverse mapping is
    -- undefined, the unpickler fails with an error message in the Left component

    xpWrapEither             :: (a -> Either String b, b -> a) -> PU a -> PU b
    xpWrapEither (i, j) pa   = (xpSeq j pa (xpLiftEither . i)) { theSchema = theSchema pa }

    -- ------------------------------------------------------------

    -- | pickle a pair of values sequentially
    --
    -- Used for pairs or together with wrap for pickling
    -- algebraic data types with two components

    xpPair  :: PU a -> PU b -> PU (a, b)
    xpPair pa pb
        = ( xpSeq fst pa (\ a ->
            xpSeq snd pb (\ b ->
            xpLift (a,b)))
          ) { theSchema = scSeq (theSchema pa) (theSchema pb) }

    -- | Like 'xpPair' but for triples

    xpTriple        :: PU a -> PU b -> PU c -> PU (a, b, c)
    xpTriple pa pb pc
        = xpWrap (toTriple, fromTriple) (xpPair pa (xpPair pb pc))
        where
        toTriple   ~(a, ~(b, c)) = (a,  b, c )
        fromTriple ~(a,   b, c ) = (a, (b, c))

    -- | Like 'xpPair' and 'xpTriple' but for 4-tuples

    xp4Tuple        :: PU a -> PU b -> PU c -> PU d -> PU (a, b, c, d)
    xp4Tuple pa pb pc pd
        = xpWrap (toQuad, fromQuad) (xpPair pa (xpPair pb (xpPair pc pd)))
        where
        toQuad   ~(a, ~(b, ~(c, d))) = (a,  b,  c, d  )
        fromQuad ~(a,   b,   c, d  ) = (a, (b, (c, d)))

    -- | Like 'xpPair' and 'xpTriple' but for 5-tuples

    xp5Tuple        :: PU a -> PU b -> PU c -> PU d -> PU e -> PU (a, b, c, d, e)
    xp5Tuple pa pb pc pd pe
        = xpWrap (toQuint, fromQuint) (xpPair pa (xpPair pb (xpPair pc (xpPair pd pe))))
        where
        toQuint   ~(a, ~(b, ~(c, ~(d, e)))) = (a,  b,  c,  d, e   )
        fromQuint ~(a,   b,   c,   d, e   ) = (a, (b, (c, (d, e))))

    -- | Like 'xpPair' and 'xpTriple' but for 6-tuples

    xp6Tuple        :: PU a -> PU b -> PU c -> PU d -> PU e -> PU f -> PU (a, b, c, d, e, f)
    xp6Tuple pa pb pc pd pe pf
        = xpWrap (toSix, fromSix) (xpPair pa (xpPair pb (xpPair pc (xpPair pd (xpPair pe pf)))))
        where
        toSix   ~(a, ~(b, ~(c, ~(d, ~(e, f))))) = (a,  b,  c,  d,  e, f    )
        fromSix ~(a,   b,   c,   d,   e, f)     = (a, (b, (c, (d, (e, f)))))

    -- ------------------------------------------------------------

    -- | Like 'xpPair' and 'xpTriple' but for 7-tuples
    --
    -- Thanks to Tony Morris for doing xp7Tuple, ..., xp24Tuple.

    xp7Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                PU f -> PU g -> PU (a, b, c, d, e, f, g)
    xp7Tuple a b c d e f g
        = xpWrap ( \ (a, (b, c, d, e, f, g)) -> (a, b, c, d, e, f, g)
                 , \ (a, b, c, d, e, f, g)   -> (a, (b, c, d, e, f, g))
                 )
          (xpPair a (xp6Tuple b c d e f g))

    xp8Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                PU f -> PU g -> PU h -> PU (a, b, c, d, e, f, g, h)
    xp8Tuple a b c d e f g h
        = xpWrap ( \ ((a, b), (c, d, e, f, g, h)) -> (a, b, c, d, e, f, g, h)
                 , \ (a, b, c, d, e, f, g, h) -> ((a, b), (c, d, e, f, g, h))
                 )
          (xpPair (xpPair a b) (xp6Tuple c d e f g h))

    xp9Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                PU f -> PU g -> PU h -> PU i -> PU (a, b, c, d, e, f, g, h, i)
    xp9Tuple a b c d e f g h i
        = xpWrap ( \ ((a, b, c), (d, e, f, g, h, i)) -> (a, b, c, d, e, f, g, h, i)
                 , \ (a, b, c, d, e, f, g, h, i) -> ((a, b, c), (d, e, f, g, h, i))
                 )
          (xpPair (xpTriple a b c) (xp6Tuple d e f g h i))

    xp10Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU (a, b, c, d, e, f, g, h, i, j)
    xp10Tuple a b c d e f g h i j
        = xpWrap ( \ ((a, b, c, d), (e, f, g, h, i, j)) -> (a, b, c, d, e, f, g, h, i, j)
                 , \ (a, b, c, d, e, f, g, h, i, j) -> ((a, b, c, d), (e, f, g, h, i, j))
                 )
          (xpPair (xp4Tuple a b c d) (xp6Tuple e f g h i j))

    xp11Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU (a, b, c, d, e, f, g, h, i, j, k)
    xp11Tuple a b c d e f g h i j k
        = xpWrap ( \ ((a, b, c, d, e), (f, g, h, i, j, k)) -> (a, b, c, d, e, f, g, h, i, j, k)
                 , \ (a, b, c, d, e, f, g, h, i, j, k) -> ((a, b, c, d, e), (f, g, h, i, j, k))
                 )
          (xpPair (xp5Tuple a b c d e) (xp6Tuple f g h i j k))

    xp12Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU (a, b, c, d, e, f, g, h, i, j, k, l)
    xp12Tuple a b c d e f g h i j k l
        = xpWrap ( \ ((a, b, c, d, e, f), (g, h, i, j, k, l)) -> (a, b, c, d, e, f, g, h, i, j, k, l)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l) -> ((a, b, c, d, e, f), (g, h, i, j, k, l))
                 )
          (xpPair (xp6Tuple a b c d e f) (xp6Tuple g h i j k l))

    xp13Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m)
    xp13Tuple a b c d e f g h i j k l m
        = xpWrap ( \ (a, (b, c, d, e, f, g), (h, i, j, k, l, m)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m) -> (a, (b, c, d, e, f, g), (h, i, j, k, l, m))
                 )
          (xpTriple a (xp6Tuple b c d e f g) (xp6Tuple h i j k l m))

    xp14Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
    xp14Tuple a b c d e f g h i j k l m n
        = xpWrap ( \ ((a, b), (c, d, e, f, g, h), (i, j, k, l, m, n)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n) -> ((a, b), (c, d, e, f, g, h), (i, j, k, l, m, n))
                 )
          (xpTriple (xpPair a b) (xp6Tuple c d e f g h) (xp6Tuple i j k l m n))

    xp15Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
    xp15Tuple a b c d e f g h i j k l m n o
        = xpWrap ( \ ((a, b, c), (d, e, f, g, h, i), (j, k, l, m, n, o)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) -> ((a, b, c), (d, e, f, g, h, i), (j, k, l, m, n, o))
                 )
          (xpTriple (xpTriple a b c) (xp6Tuple d e f g h i) (xp6Tuple j k l m n o))

    xp16Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p)
    xp16Tuple a b c d e f g h i j k l m n o p
        = xpWrap ( \ ((a, b, c, d), (e, f, g, h, i, j), (k, l, m, n, o, p)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) -> ((a, b, c, d), (e, f, g, h, i, j), (k, l, m, n, o, p))
                 )
          (xpTriple (xp4Tuple a b c d) (xp6Tuple e f g h i j) (xp6Tuple k l m n o p))

    xp17Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q)
    xp17Tuple a b c d e f g h i j k l m n o p q
        = xpWrap ( \ ((a, b, c, d, e), (f, g, h, i, j, k), (l, m, n, o, p, q)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q) -> ((a, b, c, d, e), (f, g, h, i, j, k), (l, m, n, o, p, q))
                 )
          (xpTriple (xp5Tuple a b c d e) (xp6Tuple f g h i j k) (xp6Tuple l m n o p q))

    xp18Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r)
    xp18Tuple a b c d e f g h i j k l m n o p q r
        = xpWrap ( \ ((a, b, c, d, e, f), (g, h, i, j, k, l), (m, n, o, p, q, r)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r) -> ((a, b, c, d, e, f), (g, h, i, j, k, l), (m, n, o, p, q, r))
                 )
          (xpTriple (xp6Tuple a b c d e f) (xp6Tuple g h i j k l) (xp6Tuple m n o p q r))

    xp19Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s)
    xp19Tuple a b c d e f g h i j k l m n o p q r s
        = xpWrap ( \ (a, (b, c, d, e, f, g), (h, i, j, k, l, m), (n, o, p, q, r, s)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s) -> (a, (b, c, d, e, f, g), (h, i, j, k, l, m), (n, o, p, q, r, s))
                 )
          (xp4Tuple a (xp6Tuple b c d e f g) (xp6Tuple h i j k l m) (xp6Tuple n o p q r s))

    xp20Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU t ->
                 PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t)
    xp20Tuple a b c d e f g h i j k l m n o p q r s t
        = xpWrap ( \ ((a, b), (c, d, e, f, g, h), (i, j, k, l, m, n), (o, p, q, r, s, t)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t) -> ((a, b), (c, d, e, f, g, h), (i, j, k, l, m, n), (o, p, q, r, s, t))
                 )
          (xp4Tuple (xpPair a b) (xp6Tuple c d e f g h) (xp6Tuple i j k l m n) (xp6Tuple o p q r s t))

    xp21Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU t ->
                 PU u -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u)
    xp21Tuple a b c d e f g h i j k l m n o p q r s t u
        = xpWrap ( \ ((a, b, c), (d, e, f, g, h, i), (j, k, l, m, n, o), (p, q, r, s, t, u)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u) -> ((a, b, c), (d, e, f, g, h, i), (j, k, l, m, n, o), (p, q, r, s, t, u))
                 )
          (xp4Tuple (xpTriple a b c) (xp6Tuple d e f g h i) (xp6Tuple j k l m n o) (xp6Tuple p q r s t u))

    xp22Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU t ->
                 PU u -> PU v -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v)
    xp22Tuple a b c d e f g h i j k l m n o p q r s t u v
        = xpWrap ( \ ((a, b, c, d), (e, f, g, h, i, j), (k, l, m, n, o, p), (q, r, s, t, u, v)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v) -> ((a, b, c, d), (e, f, g, h, i, j), (k, l, m, n, o, p), (q, r, s, t, u, v))
                 )
          (xp4Tuple (xp4Tuple a b c d) (xp6Tuple e f g h i j) (xp6Tuple k l m n o p) (xp6Tuple q r s t u v))

    xp23Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU t ->
                 PU u -> PU v -> PU w -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w)
    xp23Tuple a b c d e f g h i j k l m n o p q r s t u v w
        = xpWrap ( \ ((a, b, c, d, e), (f, g, h, i, j, k), (l, m, n, o, p, q), (r, s, t, u, v, w)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w) -> ((a, b, c, d, e), (f, g, h, i, j, k), (l, m, n, o, p, q), (r, s, t, u, v, w))
                 )
          (xp4Tuple (xp5Tuple a b c d e) (xp6Tuple f g h i j k) (xp6Tuple l m n o p q) (xp6Tuple r s t u v w))

    -- | Hopefully no one needs a xp25Tuple

    xp24Tuple :: PU a -> PU b -> PU c -> PU d -> PU e ->
                 PU f -> PU g -> PU h -> PU i -> PU j ->
                 PU k -> PU l -> PU m -> PU n -> PU o ->
                 PU p -> PU q -> PU r -> PU s -> PU t ->
                 PU u -> PU v -> PU w -> PU x -> PU (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x)
    xp24Tuple a b c d e f g h i j k l m n o p q r s t u v w x
        = xpWrap ( \ ((a, b, c, d, e, f), (g, h, i, j, k, l), (m, n, o, p, q, r), (s, t, u, v, w, x)) -> (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x)
                 , \ (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x) -> ((a, b, c, d, e, f), (g, h, i, j, k, l), (m, n, o, p, q, r), (s, t, u, v, w, x))
                 )
          (xp4Tuple (xp6Tuple a b c d e f) (xp6Tuple g h i j k l) (xp6Tuple m n o p q r) (xp6Tuple s t u v w x))

    -- ------------------------------------------------------------

    -- | Pickle a string into an XML text node
    --
    -- One of the most often used primitive picklers. Attention:
    -- For pickling empty strings use 'xpText0'. If the text has a more
    -- specific datatype than xsd:string, use 'xpTextDT'

    xpText  :: PU String
    xpText  = xpTextDT scString1
    {-# INLINE xpText #-}

    -- | Pickle a string into an XML text node
    --
    -- Text pickler with a description of the structure of the text
    -- by a schema. A schema for a data type can be defined by 'Text.XML.HXT.Arrow.Pickle.Schema.scDT'.
    -- In 'Text.XML.HXT.Arrow.Pickle.Schema' there are some more functions for creating
    -- simple datatype descriptions.

    xpTextDT        :: Schema -> PU String
    xpTextDT sc     = PU { appPickle   = putCont . XN.mkText
                         , appUnPickle = do t <- getCont
                                            liftMaybe "xpText: XML text expected" $ XN.getText t
                         , theSchema   = sc
                         }

    -- | Pickle a possibly empty string into an XML node.
    --
    -- Must be used in all places, where empty strings are legal values.
    -- If the content of an element can be an empty string, this string disapears
    -- during storing the DOM into a document and reparse the document.
    -- So the empty text node becomes nothing, and the pickler must deliver an empty string,
    -- if there is no text node in the document.

    xpText0         :: PU String
    xpText0         = xpText0DT scString1
    {-# INLINE xpText0 #-}

    -- | Pickle a possibly empty string with a datatype description into an XML node.
    --
    -- Like 'xpText0' but with extra Parameter for datatype description as in 'xpTextDT'.

    xpText0DT       :: Schema -> PU String
    xpText0DT sc    = xpWrap (fromMaybe "", emptyToNothing) $
                      xpOption $
                      xpTextDT sc
        where
        emptyToNothing "" = Nothing
        emptyToNothing x  = Just x

    -- | Pickle an arbitrary value by applyling show during pickling
    -- and read during unpickling.
    --
    -- Real pickling is then done with 'xpText'.
    -- One of the most often used pimitive picklers. Applicable for all
    -- types which are instances of @Read@ and @Show@

    xpPrim                  :: (Read a, Show a) => PU a
    xpPrim                  = xpWrapEither (readMaybe, show) xpText
        where
        readMaybe           :: Read a => String -> Either String a
        readMaybe str       = val (reads str)
            where
              val [(x,"")]  = Right x
              val _         = Left $ "xpPrim: reading string " ++ show str ++ " failed"

    -- | Pickle an Int
    xpInt                   :: PU Int
    xpInt                   = xpWrapEither (readMaybe, show) xpText
        where
          readMaybe xs
              | all isDigit xs = Right . foldl' (\ r c -> 10 * r + (fromEnum c - fromEnum '0')) 0 $ xs
          readMaybe ('-' : xs) = fmap (0 -) . readMaybe $ xs
          readMaybe ('+' : xs) =              readMaybe $ xs
          readMaybe        xs  = Left $ "xpInt: reading an Int from string " ++ show xs ++ " failed"

    -- ------------------------------------------------------------

    -- | Pickle an XmlTree by just adding it
    --
    -- Usefull for components of type XmlTree in other data structures

    xpTree          :: PU XmlTree
    xpTree          = PU { appPickle   = putCont
                         , appUnPickle = getCont
                         , theSchema   = Any
                         }

    -- | Pickle a whole list of XmlTrees by just adding the list, unpickle is done by taking all element contents.
    --
    -- This pickler should always be combined with 'xpElem' for taking the whole contents of an element.

    xpTrees         :: PU [XmlTree]
    xpTrees         = (xpList xpTree) { theSchema = Any }

    -- | Pickle a string representing XML contents by inserting the tree representation into the XML document.
    --
    -- Unpickling is done by converting the contents with
    -- 'Text.XML.HXT.Arrow.Edit.xshowEscapeXml' into a string,
    -- this function will escape all XML special chars, such that pickling the value back becomes save.
    -- Pickling is done with 'Text.XML.HXT.Arrow.ReadDocument.xread'

    xpXmlText       :: PU String
    xpXmlText       = xpWrap ( showXML, readXML ) $ xpTrees
        where
          showXML   = concat . runLA ( xshowEscapeXml unlistA )
          readXML   = runLA xread

    -- ------------------------------------------------------------

    -- | Encoding of optional data by ignoring the Nothing case during pickling
    -- and relying on failure during unpickling to recompute the Nothing case
    --
    -- The default pickler for Maybe types

    xpOption        :: PU a -> PU (Maybe a)
    xpOption pa     = PU { appPickle  = ( \ a ->
                                          case a of
                                            Nothing -> id
                                            Just x  -> appPickle pa x
                                        )

                         , appUnPickle = xpChoice (xpLift Nothing) pa (xpLift . Just)

                         , theSchema   = scOption (theSchema pa)
                         }

    -- | Optional conversion with default value
    --
    -- The default value is not encoded in the XML document,
    -- during unpickling the default value is inserted if the pickler fails

    xpDefault       :: (Eq a) => a -> PU a -> PU a
    xpDefault df    = xpWrap ( fromMaybe df
                             , \ x -> if x == df then Nothing else Just x
                             ) .
                      xpOption

    -- ------------------------------------------------------------

    -- | Encoding of list values by pickling all list elements sequentially.
    --
    -- Unpickler relies on failure for detecting the end of the list.
    -- The standard pickler for lists. Can also be used in combination with 'xpWrap'
    -- for constructing set and map picklers

    xpList          :: PU a -> PU [a]
    xpList pa       = PU { appPickle  = ( \ a ->
                                          case a of
                                            []  -> id
                                            _:_ -> appPickle pc a
                                        )
                         , appUnPickle = xpChoice
                                         (xpLift [])
                                         pa
                                         (\ x -> xpSeq id (xpList pa) (\xs -> xpLift (x:xs)))

                         , theSchema   = scList (theSchema pa)
                         }
          where
          pc        = xpSeq head  pa         (\ x  ->
                      xpSeq tail (xpList pa) (\ xs ->
                      xpLift (x:xs)          ))

    -- | Encoding of a none empty list of values
    --
    -- Attention: when calling this pickler with an empty list,
    -- an internal error \"head of empty list is raised\".

    xpList1         :: PU a -> PU [a]
    xpList1 pa      = ( xpWrap (\ (x, xs) -> x : xs
                               ,\ x -> (head x, tail x)
                               ) $
                        xpPair pa (xpList pa)
                      ) { theSchema = scList1 (theSchema pa) }

    -- ------------------------------------------------------------

    -- | Standard pickler for maps
    --
    -- This pickler converts a map into a list of pairs.
    -- All key value pairs are mapped to an element with name (1.arg),
    -- the key is encoded as an attribute named by the 2. argument,
    -- the 3. arg is the pickler for the keys, the last one for the values

    xpMap           :: Ord k => String -> String -> PU k -> PU v -> PU (Map k v)
    xpMap en an xpk xpv
                    = xpWrap ( M.fromList
                             , M.toList
                             ) $
                      xpList $
                      xpElem en $
                      xpPair ( xpAttr an $ xpk ) xpv

    -- ------------------------------------------------------------

    -- | Pickler for sum data types.
    --
    -- Every constructor is mapped to an index into the list of picklers.
    -- The index is used only during pickling, not during unpickling, there the 1. match is taken

    xpAlt           :: (a -> Int) -> [PU a] -> PU a
    xpAlt tag ps    = PU { appPickle   = \ a ->
                                         appPickle (ps !! tag a) a

                         , appUnPickle = case ps of
                                           []     -> throwMsg "xpAlt: no matching unpickler found for a sum datatype"
                                           pa:ps1 -> xpChoice (xpAlt tag ps1) pa xpLift

                         , theSchema   = scAlts (map theSchema ps)
                         }

    -- ------------------------------------------------------------

    -- | Pickler for wrapping\/unwrapping data into an XML element
    --
    -- Extra parameter is the element name given as a QName. THE pickler for constructing
    -- nested structures
    --
    -- Example:
    --
    -- > xpElemQN (mkName "number") $ xpickle
    --
    -- will map an (42::Int) onto
    --
    -- > <number>42</number>

    xpElemQN        :: QName -> PU a -> PU a
    xpElemQN qn pa  = PU { appPickle   = ( \ a ->
                                           let st' = appPickle pa a emptySt in
                                           putCont (XN.mkElement qn (attributes st') (contents st'))
                                         )
                         , appUnPickle = upElem
                         , theSchema   = scElem (qualifiedName qn) (theSchema pa)
                         }
          where
          upElem    = do t <- getCont
                         n <- liftMaybe "xpElem: XML element expected" $ XN.getElemName t
                         if n /= qn
                            then throwMsg ("xpElem: got element name " ++ show n ++ ", but expected " ++ show qn)
                            else do l <- gets nesting
                                    liftUnpickleVal $ unpickleElem' (xpCheckEmpty pa) (l + 1) t

    -- | convenient Pickler for xpElemQN
    --
    -- > xpElem n = xpElemQN (mkName n)

    xpElem          :: String -> PU a -> PU a
    xpElem          = xpElemQN . mkName

    -- | convenient Pickler for xpElemQN
    --   for pickling elements with respect to namespaces
    --
    -- > xpElemNS ns px lp = xpElemQN (mkQName px lp ns)

    xpElemNS        :: String -> String -> String -> PU a -> PU a
    xpElemNS ns px lp
                    = xpElemQN $ mkQName px lp ns

    -- ------------------------------------------------------------

    -- | Pickler for wrapping\/unwrapping data into an XML element with an attribute with given value
    --
    -- To make XML structures flexible but limit the number of different elements, it's sometimes
    -- useful to use a kind of generic element with a key value structure
    --
    -- Example:
    --
    -- > <attr name="key1">value1</attr>
    -- > <attr name="key2">value2</attr>
    -- > <attr name="key3">value3</attr>
    --
    -- the Haskell datatype may look like this
    --
    -- > type T = T { key1 :: Int ; key2 :: String ; key3 :: Double }
    --
    -- Then the picker for that type looks like this
    --
    -- > xpT :: PU T
    -- > xpT = xpWrap ( uncurry3 T, \ t -> (key1 t, key2 t, key3 t) ) $
    -- >       xpTriple (xpElemWithAttrValue "attr" "name" "key1" $ xpickle)
    -- >                (xpElemWithAttrValue "attr" "name" "key2" $ xpText0)
    -- >                (xpElemWithAttrValue "attr" "name" "key3" $ xpickle)

    xpElemWithAttrValue     :: String -> String -> String -> PU a -> PU a
    xpElemWithAttrValue name an av pa
                    = xpElem name $
                      xpAddFixedAttr an av $
                      pa

    -- ------------------------------------------------------------

    -- | Pickler for storing\/retreiving data into\/from an attribute value
    --
    -- The attribute is inserted in the surrounding element constructed by the 'xpElem' pickler

    xpAttrQN        :: QName -> PU a -> PU a
    xpAttrQN qn pa  = PU { appPickle   = ( \ a ->
                                           let st' = appPickle pa a emptySt in
                                           putAtt qn (contents st')
                                         )
                         , appUnPickle = upAttr
                         , theSchema   = scAttr (qualifiedName qn) (theSchema pa)
                         }
          where
          upAttr    = do a <- getAtt qn
                         l <- gets nesting
                         liftUnpickleVal $ unpickleElem' (xpCheckEmptyContents pa) l a

    -- | convenient Pickler for xpAttrQN
    --
    -- > xpAttr n = xpAttrQN (mkName n)

    xpAttr          :: String -> PU a -> PU a
    xpAttr          = xpAttrQN . mkName

    -- | convenient Pickler for xpAttrQN
    --
    -- > xpAttr ns px lp = xpAttrQN (mkQName px lp ns)

    xpAttrNS        :: String -> String -> String -> PU a -> PU a
    xpAttrNS ns px lp
                    = xpAttrQN (mkQName px lp ns)

    -- | A text attribute.
    xpTextAttr      :: String -> PU String
    xpTextAttr      = flip xpAttr xpText

    -- | Add an optional attribute for an optional value (Maybe a).

    xpAttrImplied   :: String -> PU a -> PU (Maybe a)
    xpAttrImplied name pa
                    = xpOption $ xpAttr name pa

    xpAttrFixed     :: String -> String -> PU ()
    xpAttrFixed name val
                    = ( xpWrapEither ( \ v ->
                                       if v == val
                                       then Right ()
                                       else Left ( "xpAttrFixed: value "
                                                   ++ show val
                                                   ++ " expected, but got "
                                                   ++ show v
                                                 )
                                     , const val
                                     ) $
                        xpAttr name xpText
                      ) { theSchema   = scAttr name (scFixed val) }

    -- | Add/Check an attribute with a fixed value.
    --

    xpAddFixedAttr  :: String -> String -> PU a -> PU a
    xpAddFixedAttr name val
                    = xpSeq' $ xpAttrFixed name val

    -- | Add a namespace declaration.
    --
    -- When generating XML the namespace decl is added,
    -- when reading a document, the unpickler checks
    -- whether there is a namespace declaration for the given
    -- namespace URI (2. arg)

    xpAddNSDecl  :: String -> String -> PU a -> PU a
    xpAddNSDecl name val
                    = xpSeq' $ xpAttrNSDecl name' val
        where
          name'
              | null name = "xmlns"
              | otherwise = "xmlns:" ++ name

    xpAttrNSDecl     :: String -> String -> PU ()
    xpAttrNSDecl name ns
                     = PU { appPickle   = const $ putAtt (mkName name) [XN.mkText ns]
                          , appUnPickle = getNSAtt ns
                          , theSchema   = scAttr name (scFixed ns)
                          }

    -- ------------------------------------------------------------

    xpIgnoreCont    :: LA XmlTree XmlTree -> PU ()
    xpIgnoreCont    = xpIgnoreInput $ \ mf s -> s {contents   = mf $ contents   s}

    xpIgnoreAttr    :: LA XmlTree XmlTree -> PU ()
    xpIgnoreAttr    = xpIgnoreInput $ \ mf s -> s {attributes = mf $ attributes s}

    -- | When unpickling, filter the contents of the element currently processed,
    -- before applying the pickler argument
    --
    -- Maybe useful to ignore some stuff in the input, or to do some cleanup before unpickling.

    xpFilterCont    :: LA XmlTree XmlTree -> PU a -> PU a
    xpFilterCont f  = xpSeq' $ xpIgnoreCont f

    -- | Same as 'xpFilterCont' but for the  attribute list of the element currently processed.
    --
    -- Maybe useful to ignore some stuff in the input, e.g. class attributes, or to do some cleanup before unpickling.

    xpFilterAttr    :: LA XmlTree XmlTree -> PU a -> PU a
    xpFilterAttr f  = xpSeq' $ xpIgnoreAttr f

    xpIgnoreInput   :: (([XmlTree] -> [XmlTree]) -> St -> St) -> LA XmlTree XmlTree -> PU ()
    xpIgnoreInput m f
                    =  PU { appPickle   = const id
                          , appUnPickle = do modify (m filterCont)
                                             return ()
                          , theSchema   = scNull
                          }
        where
          filterCont = runLA (unlistA >>> f)

    -- ------------------------------------------------------------

    -- | The class for overloading 'xpickle', the default pickler

    class XmlPickler a where
        xpickle :: PU a

    instance XmlPickler Int where
        xpickle = xpPrim

    instance XmlPickler Integer where
        xpickle = xpPrim

    {-
      no instance of XmlPickler Char
      because then every text would be encoded
      char by char, because of the instance for lists

    instance XmlPickler Char where
        xpickle = xpPrim
    -}

    instance XmlPickler () where
        xpickle = xpUnit

    instance (XmlPickler a, XmlPickler b) => XmlPickler (a,b) where
        xpickle = xpPair xpickle xpickle

    instance (XmlPickler a, XmlPickler b, XmlPickler c) => XmlPickler (a,b,c) where
        xpickle = xpTriple xpickle xpickle xpickle

    instance (XmlPickler a, XmlPickler b, XmlPickler c, XmlPickler d) => XmlPickler (a,b,c,d) where
        xpickle = xp4Tuple xpickle xpickle xpickle xpickle

    instance (XmlPickler a, XmlPickler b, XmlPickler c, XmlPickler d, XmlPickler e) => XmlPickler (a,b,c,d,e) where
        xpickle = xp5Tuple xpickle xpickle xpickle xpickle xpickle

    instance XmlPickler a => XmlPickler [a] where
        xpickle = xpList xpickle

    instance XmlPickler a => XmlPickler (Maybe a) where
        xpickle = xpOption xpickle

    -- ------------------------------------------------------------

    {- begin embeded test cases

    -- ------------------------------------------------------------
    --
    -- a somewhat complex data structure
    -- for representing programs of a simple
    -- imperative language

    type Program    = Stmt

    type StmtList   = [Stmt]

    data Stmt
        = Assign  Ident  Expr
        | Stmts   StmtList
        | If      Expr  Stmt (Maybe Stmt)
        | While   Expr  Stmt
          deriving (Eq, Show)

    type Ident      = String

    data Expr
        = IntConst  Int
        | BoolConst Bool
        | Var       Ident
        | UnExpr    UnOp  Expr
        | BinExpr   Op    Expr  Expr
          deriving (Eq, Show)

    data Op
        = Add | Sub | Mul | Div | Mod | Eq | Neq
          deriving (Eq, Ord, Enum, Show)

    data UnOp
        = UPlus | UMinus | Neg
          deriving (Eq, Ord, Read, Show)

    -- ------------------------------------------------------------
    --
    -- the pickler definition for the data types

    -- the main pickler

    xpProgram :: PU Program
    xpProgram = xpElem "program" $
                xpAddNSDecl "" "program42" $
                xpickle

    xpMissingRootElement    :: PU Program
    xpMissingRootElement    = xpickle

    instance XmlPickler UnOp where
        xpickle = xpPrim

    instance XmlPickler Op where
        xpickle = xpWrap (toEnum, fromEnum) xpPrim

    instance XmlPickler Expr where
        xpickle = xpAlt tag ps
            where
            tag (IntConst _    ) = 0
            tag (BoolConst _   ) = 1
            tag (Var _         ) = 2
            tag (UnExpr _ _    ) = 3
            tag (BinExpr _ _ _ ) = 4
            ps = [ xpWrap ( IntConst
                          , \ (IntConst i ) -> i
                          ) $
                   ( xpElem "int"   $
                     xpAttr "value" $
                     xpickle
                   )

                 , xpWrap ( BoolConst
                          , \ (BoolConst b) -> b
                          ) $
                   ( xpElem "bool"  $
                     xpAttr "value" $
                     xpWrap (toEnum, fromEnum) xpickle
                   )

                 , xpWrap ( Var
                          , \ (Var n)       -> n
                          ) $
                   ( xpElem "var"   $
                     xpAttr "name"  $
                     xpText
                   )

                 , xpWrap ( uncurry UnExpr
                          , \ (UnExpr op e) -> (op, e)
                          ) $
                   ( xpElem "unex" $
                     xpPair (xpAttr "op" xpickle)
                             xpickle
                   )

                 , xpWrap ( uncurry3 $ BinExpr
                          , \ (BinExpr op e1 e2) -> (op, e1, e2)
                          ) $
                   ( xpElem "binex" $
                     xpTriple (xpAttr "op" xpickle)
                               xpickle
                               xpickle
                   )
                 ]

    instance XmlPickler Stmt where
        xpickle = xpAlt tag ps
            where
            tag ( Assign _ _ ) = 0
            tag ( Stmts _ )    = 1
            tag ( If _ _ _ )   = 2
            tag ( While _ _ )  = 3
            ps = [ xpWrap ( uncurry Assign
                          , \ (Assign n v) -> (n, v)
                          ) $
                   ( xpElem "assign" $
                     xpFilterCont (neg $ hasName "comment" <+> isText) $  -- test case test7: remove uninteresting stuff
                     xpPair (xpAttr "name" xpText)
                             xpickle
                   )
                 , xpWrap ( Stmts
                          , \ (Stmts sl) -> sl
                          ) $
                   ( xpElem "block" $
                     xpList xpickle
                   )
                 , xpWrap ( uncurry3 If
                          , \ (If c t e) -> (c, t, e)
                          ) $
                   ( xpElem "if" $
                     xpTriple xpickle
                              xpickle
                              xpickle
                   )
                 , xpWrap ( uncurry While
                          , \ (While c b) -> (c, b)
                          ) $
                   ( xpElem "while" $
                     xpPair xpickle
                            xpickle
                   )
                 ]

    -- ------------------------------------------------------------
    --
    -- example programs

    progs   :: [Program]
    progs   = [p0, p1, p2]

    p0, p1, p2 :: Program

    p0 = Stmts []           -- the empty program

    p1 = Stmts
         [ Assign i ( UnExpr UMinus ( IntConst (-22) ) )
         , Assign j ( IntConst 20 )
         , While
           ( BinExpr Neq ( Var i ) ( IntConst 0 ) )
           ( Stmts
             [ Assign i ( BinExpr Sub ( Var i ) ( IntConst 1 ) )
             , Assign j ( BinExpr Add ( Var j ) ( IntConst 1 ) )
             , If ( IntConst 0 ) (Stmts []) Nothing
             ]
           )
         ]
        where
        i = "i"
        j = "j"

    p2 = Stmts
         [ Assign x (IntConst 6)
         , Assign y (IntConst 7)
         , Assign p (IntConst 0)
         , While
           ( BinExpr Neq (Var x) (IntConst 0) )
           ( If ( BinExpr Neq ( BinExpr Mod (Var x) (IntConst 2) ) (IntConst 0) )
                ( Stmts
                  [ Assign x ( BinExpr Sub (Var x) (IntConst 1) )
                  , Assign p ( BinExpr Add (Var p) (Var y) )
                  ]
                )
                ( Just ( Stmts
                         [ Assign x ( BinExpr Div (Var x) (IntConst 2) )
                         , Assign y ( BinExpr Mul (Var y) (IntConst 2) )
                         ]
                       )
                )
           )
         ]
        where
        x = "x"
        y = "y"
        p = "p"

    -- ------------------------------------------------------------

    test0 = putStrLn . head . runLA
            ( xshow (arr (pickleDoc xpProgram)
                     >>> getChildren
                    )
            )

    test0' f = runLA
            ( xshow (arr (pickleDoc xpProgram)
                     >>> getChildren
                    )
              >>>
              root [] [xread]
              >>>
              f
            )

    test1' f = runLA
            ( xshow (arr (pickleDoc xpProgram)
                     >>> getChildren
                    )
              >>>
              root [] [xread]
              >>>
              f
              >>>
              arr (unpickleDoc' xpProgram)
            )

    test1 = test0' (processTopDown (setQName (mkName "real") `X.when` hasName "int"))
    test2 = test1' this
    test3 = test1' (processTopDown (setQName (mkName "real") `X.when` hasName "int"))
    test4 = test1' (processTopDown (setQName (mkName "xxx")  `X.when` hasName "program"))
    test5 = test1' (processTopDown (setQName (mkName "xxx")  `X.when` hasName "assign"))
    test6 = test1' (processTopDownWithAttrl  (txt "xxx"      `X.when` hasText (== "UMinus")))
    test7 = test1' (processTopDown (insertComment            `X.when` hasName "assign"))
        where insertComment = replaceChildren (getChildren <+> eelem "comment" <+> txt "zzz")

    -- ------------------------------------------------------------

    -- end embeded test cases -}