Portability | portable |
---|---|
Stability | experimental |
Maintainer | Uwe Schmidt (uwe@fh-wedel.de) |
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)
- xpickleDocument :: PU a -> Attributes -> String -> IOStateArrow s a XmlTree
- xunpickleDocument :: PU a -> Attributes -> String -> IOStateArrow s b a
- xpickleWriteDTD :: PU b -> Attributes -> String -> IOStateArrow s b XmlTree
- xpickleDTD :: PU b -> IOStateArrow s b XmlTree
- checkPickler :: Eq a => PU a -> IOStateArrow s a a
- xpickleVal :: ArrowXml a => PU b -> a b XmlTree
- xunpickleVal :: ArrowXml a => PU b -> a XmlTree b
- thePicklerDTD :: PU b -> XmlTrees
- a_addDTD :: String
- pickleDoc :: PU a -> a -> XmlTree
- unpickleDoc :: PU a -> XmlTree -> Maybe a
- data PU a = PU {}
- class XmlPickler a where
- xp4Tuple :: PU a -> PU b -> PU c -> PU d -> PU (a, b, c, d)
- xp5Tuple :: PU a -> PU b -> PU c -> PU d -> PU e -> PU (a, b, c, d, e)
- xp6Tuple :: PU a -> PU b -> PU c -> PU d -> PU e -> PU f -> PU (a, b, c, d, e, f)
- xpAddFixedAttr :: String -> String -> PU a -> PU a
- xpAlt :: (a -> Int) -> [PU a] -> PU a
- xpAttr :: String -> PU a -> PU a
- xpAttrFixed :: String -> String -> PU ()
- xpAttrImplied :: String -> PU a -> PU (Maybe a)
- xpChoice :: PU b -> PU a -> (a -> PU b) -> PU b
- xpCondSeq :: PU b -> (b -> a) -> PU a -> (a -> PU b) -> PU b
- xpDefault :: Eq a => a -> PU a -> PU a
- xpElem :: String -> PU a -> PU a
- xpElemWithAttrValue :: String -> String -> String -> PU a -> PU a
- xpLift :: a -> PU a
- xpLiftMaybe :: Maybe a -> PU a
- xpList :: PU a -> PU [a]
- xpList1 :: PU a -> PU [a]
- xpMap :: Ord k => String -> String -> PU k -> PU v -> PU (Map k v)
- xpOption :: PU a -> PU (Maybe a)
- xpPair :: PU a -> PU b -> PU (a, b)
- xpPrim :: (Read a, Show a) => PU a
- xpSeq :: (b -> a) -> PU a -> (a -> PU b) -> PU b
- xpText :: PU String
- xpText0 :: PU String
- xpTextDT :: Schema -> PU String
- xpText0DT :: Schema -> PU String
- xpTree :: PU XmlTree
- xpTrees :: PU [XmlTree]
- xpTriple :: PU a -> PU b -> PU c -> PU (a, b, c)
- xpUnit :: PU ()
- xpWrap :: (a -> b, b -> a) -> PU a -> PU b
- xpWrapMaybe :: (a -> Maybe b, b -> a) -> PU a -> PU b
- xpXmlText :: PU String
- xpZero :: PU a
- data Schema
- type Schemas = [Schema]
- data DataTypeDescr
Documentation
xpickleDocument :: PU a -> Attributes -> String -> IOStateArrow s a XmlTreeSource
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.
xunpickleDocument :: PU a -> Attributes -> String -> IOStateArrow s b aSource
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.
xpickleWriteDTD :: PU b -> Attributes -> String -> IOStateArrow s b XmlTreeSource
Write out the DTD generated out of a pickler. Calls xpicklerDTD
xpickleDTD :: PU b -> IOStateArrow s b XmlTreeSource
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.
checkPickler :: Eq a => PU a -> IOStateArrow s a aSource
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.
xpickleVal :: ArrowXml a => PU b -> a b XmlTreeSource
The arrow version of the pickler function
xunpickleVal :: ArrowXml a => PU b -> a XmlTree bSource
The arrow version of the unpickler function
thePicklerDTD :: PU b -> XmlTreesSource
Compute the associated DTD of a pickler
pickleDoc :: PU a -> a -> XmlTreeSource
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
unpickleDoc :: PU a -> XmlTree -> Maybe aSource
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 make sense.
For reconverting a value from an XML tree, is becomes neccessary,
to introduce "enough" markup for unpickling the value
class XmlPickler a whereSource
The class for overloading xpickle
, the default pickler
XmlPickler Int | |
XmlPickler Integer | |
XmlPickler () | |
XmlPickler a => XmlPickler [a] | |
XmlPickler a => XmlPickler (Maybe a) | |
(XmlPickler a, XmlPickler b) => XmlPickler (a, b) | |
(XmlPickler a, XmlPickler b, XmlPickler c) => XmlPickler (a, b, c) | |
(XmlPickler a, XmlPickler b, XmlPickler c, XmlPickler d) => XmlPickler (a, b, c, d) | |
(XmlPickler a, XmlPickler b, XmlPickler c, XmlPickler d, XmlPickler e) => XmlPickler (a, b, c, d, e) |
xpAddFixedAttr :: String -> String -> PU a -> PU aSource
Add an attribute with a fixed value.
Useful e.g. to declare namespaces. Is implemented by xpAttrFixed
xpAlt :: (a -> Int) -> [PU a] -> PU aSource
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
xpAttr :: String -> PU a -> PU aSource
convenient Pickler for xpAttrQN
xpAttr n = xpAttrQN (mkName n)
xpAttrImplied :: String -> PU a -> PU (Maybe a)Source
Add an optional attribute for an optional value (Maybe a).
xpChoice :: PU b -> PU a -> (a -> PU b) -> PU bSource
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.
xpCondSeq :: PU b -> (b -> a) -> PU a -> (a -> PU b) -> PU bSource
pickle/unpickle combinator for sequence and choice.
When the first unpickler fails,
the second one is taken, else the third one configured with the result from the first
is taken. This pickler is a generalisation for xpSeq
and xpChoice
.
The schema must be attached later, e.g. in xpPair or other higher level combinators
xpDefault :: Eq a => a -> PU a -> PU aSource
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
xpElem :: String -> PU a -> PU aSource
convenient Pickler for xpElemQN
xpElem n = xpElemQN (mkName n)
xpElemWithAttrValue :: String -> String -> String -> PU a -> PU aSource
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)
Lift a value to a pickler
When pickling, nothing is encoded, when unpickling, the given value is inserted. This pickler always succeeds.
xpLiftMaybe :: Maybe a -> PU aSource
Lift a Maybe value to a pickler.
Nothing
is mapped to the zero pickler, Just x
is pickled with xpLift x
.
xpList :: PU a -> PU [a]Source
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
xpList1 :: PU a -> PU [a]Source
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".
xpMap :: Ord k => String -> String -> PU k -> PU v -> PU (Map k v)Source
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
xpOption :: PU a -> PU (Maybe a)Source
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
xpPair :: PU a -> PU b -> PU (a, b)Source
pickle a pair of values sequentially
Used for pairs or together with wrap for pickling algebraic data types with two components
xpPrim :: (Read a, Show a) => PU aSource
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
xpSeq :: (b -> a) -> PU a -> (a -> PU b) -> PU bSource
Combine two picklers sequentially.
If the first fails during unpickling, the whole unpickler fails
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.
xpTextDT :: Schema -> PU StringSource
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 scDT
.
In Text.XML.HXT.Arrow.Pickle.Schema
there are some more functions for creating
simple datatype descriptions.
Pickle an XmlTree by just adding it
Usefull for components of type XmlTree in other data structures
Pickle a whole list of XmlTrees by just adding the list, unpickle is done by taking all element contens.
This pickler should always combined with xpElem
for taking the whole contents of an element.
xpWrap :: (a -> b, b -> a) -> PU a -> PU bSource
map value into another domain and apply pickler there
One of the most often used picklers.
xpWrapMaybe :: (a -> Maybe b, b -> a) -> PU a -> PU bSource
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
Pickle a string representing XML contents by inserting the tree representation into the XML document.
Unpickling is done by converting the contents with
xshowEscapeXml
into a string,
this function will escape all XML special chars, such that pickling the value back becomes save.
Pickling is done with xread