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

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

       Maintainer : Uwe Schmidt (uwe@fh-wedel.de)
       Stability  : experimental
       Portability: portable
       Version    : $Id$

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

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

    -}

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

    module Text.XML.HXT.Arrow.Pickle
        ( xpickleDocument		 -- from this module Text.XML.HXT.Arrow.Pickle
        , xunpickleDocument
        , xpickleWriteDTD
        , xpickleDTD
        , checkPickler
        , xpickleVal
        , xunpickleVal
        , thePicklerDTD
        , a_addDTD

          -- from Text.XML.HXT.Arrow.Pickle.Xml
        , pickleDoc
        , unpickleDoc

        , PU(..)
        , XmlPickler

        , xp4Tuple
        , xp5Tuple
        , xp6Tuple
        , xpAddFixedAttr
        , xpAlt
        , xpAttr
        , xpAttrFixed
        , xpAttrImplied
        , xpChoice
        , xpCondSeq
        , xpDefault
        , xpElem
        , xpElemWithAttrValue
        , xpickle
        , xpLift
        , xpLiftMaybe
        , xpList
        , xpList1
        , xpMap
        , xpOption
        , xpPair
        , xpPrim
        , xpSeq
        , xpText
        , xpText0
        , xpTextDT
        , xpText0DT
        , xpTree
        , xpTrees
        , xpTriple
        , xpUnit
        , xpWrap
        , xpWrapMaybe
        , xpXmlText
        , xpZero

          -- from Text.XML.HXT.Arrow.Pickle.Schema
        , Schema
        , Schemas
        , DataTypeDescr
        )
    where

    import           Data.Maybe

    import           Control.Arrow.ListArrows

    import           Text.XML.HXT.DOM.Interface
    import           Text.XML.HXT.Arrow.XmlArrow
    import           Text.XML.HXT.Arrow.XmlIOStateArrow
    import           Text.XML.HXT.Arrow.ReadDocument
    import           Text.XML.HXT.Arrow.WriteDocument

    import           Text.XML.HXT.Arrow.Pickle.Xml
    import           Text.XML.HXT.Arrow.Pickle.Schema
    import           Text.XML.HXT.Arrow.Pickle.DTD

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

    -- the arrow interface for pickling and unpickling

    -- | store an arbitray value in a persistent XML document
    --
    -- The pickler converts a value into an XML tree, this is written out with
    -- 'Text.XML.HXT.Arrow.writeDocument'. The option list is passed to 'Text.XML.HXT.Arrow.writeDocument'
    --
    -- An option evaluated by this arrow is 'a_addDTD'.
    -- If 'a_addDTD' is set ('v_1'), the pickler DTD is added as an inline DTD into the document.

    xpickleDocument		:: PU a -> Attributes -> String -> IOStateArrow s a XmlTree
    xpickleDocument xp al dest
        = xpickleVal xp
          >>>
          traceMsg 1 "xpickleVal applied"
          >>>
          ( if lookup1 a_addDTD al == v_1
    	then replaceChildren ( (constA undefined >>> xpickleDTD xp >>> getChildren)
    			       <+>
    			       getChildren
    			     )
    	else this
          )
          >>>
          writeDocument al dest

    -- | Option for generating and adding DTD when document is pickled

    a_addDTD	:: String
    a_addDTD	= "addDTD"

    -- | read an arbitray value from an XML document
    --
    -- The document is read with 'Text.XML.HXT.Arrow.readDocument'. Options are passed
    -- to 'Text.XML.HXT.Arrow.readDocument'. The conversion from XmlTree is done with the
    -- pickler.
    --
    -- @ xpickleDocument xp al dest >>> xunpickleDocument xp al' dest @ is the identity arrow
    -- when applied with the appropriate options. When during pickling indentation is switched on,
    -- the whitespace must be removed during unpickling.

    xunpickleDocument	:: PU a -> Attributes -> String -> IOStateArrow s b a
    xunpickleDocument xp al src
    			= readDocument al src
    			  >>>
    			  traceMsg 1 ("xunpickleVal for " ++ show src ++ " started")
    			  >>>
    			  xunpickleVal xp
    			  >>>
    			  traceMsg 1 ("xunpickleVal for " ++ show src ++ " finished")

    -- | Write out the DTD generated out of a pickler. Calls 'xpicklerDTD'

    xpickleWriteDTD		:: PU b -> Attributes -> String -> IOStateArrow s b XmlTree
    xpickleWriteDTD	xp al dest
    			= xpickleDTD xp
    			  >>>
    			  writeDocument al dest

    -- | The arrow for generating the DTD out of a pickler
    --
    -- A DTD is generated from a pickler and check for consistency.
    -- Errors concerning the DTD are issued.

    xpickleDTD		:: PU b -> IOStateArrow s b XmlTree
    xpickleDTD xp		= root [] [ constL (thePicklerDTD xp)
    				    >>>
    				    filterErrorMsg
    				  ]

    -- | An arrow for checking picklers
    --
    -- A value is transformed into an XML document by a given pickler,
    -- the associated DTD is extracted from the pickler and checked,
    -- the document including the DTD is tranlated into a string,
    -- this string is read and validated against the included DTD,
    -- and unpickled.
    -- The last step is the equality with the input.
    --
    -- If the check succeeds, the arrow works like this, else it fails.

    checkPickler		:: Eq a => PU a -> IOStateArrow s a a
    checkPickler xp		= ( ( ( ( xpickleVal xp
    				  >>>
    				  replaceChildren ( (constA undefined >>> xpickleDTD xp >>> getChildren)
    						    <+>
    						    getChildren
    						  )
    				  >>>
    				  writeDocumentToString []
    				  >>>
    				  readFromString [(a_validate, v_1)]
    				  >>>
    				  ( xunpickleVal xp
    				    `orElse`
    				    ( issueErr "unpickling the document failed"
    				      >>>
    				      none
    				    )
    				  )
    				)
    				&&&
    				this
    			      )
    			      >>> isA (uncurry (==))
    			    )
    			    `guards` this
    			  )
    			  `orElse` issueErr "pickle/unpickle combinators failed"

    -- | The arrow version of the pickler function

    xpickleVal		:: ArrowXml a => PU b -> a b XmlTree
    xpickleVal xp		= arr (pickleDoc xp)

    -- | The arrow version of the unpickler function

    xunpickleVal		:: ArrowXml a => PU b -> a XmlTree b
    xunpickleVal xp		= arrL (maybeToList . unpickleDoc xp)

    -- | Compute the associated DTD of a pickler

    thePicklerDTD		:: PU b -> XmlTrees
    thePicklerDTD		= dtdDescrToXml . dtdDescr . theSchema

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