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Serializing and deserializing Haskell data to/from XML

With so called pickler functions and arrows, it becomes rather easy and straightforward to convert native Haskell values to XML and vice versa. The module Text.XML.HXT.Arrow.Pickle and submodules contain a set of picklers (conversion functions) for simple data types and pickler combinators for complex types.

The idea: XML pickler

For conversion of native Haskell data to and from external representations two functions are necessary: One for generating the external representation and one for reading/parsing the representation. Read and Show often form such a pair of functions.

Char

A so-called pickler is a value with two such conversion functions, but it needs to keep track of the external representation during encoding and decoding, too. So the simplest form of a pickler converting between a value of typeand a sequence ofs looks like this:

type St = [ Char ] data PU a = PU { appPickle :: ( a , St ) -> St , appUnPickle :: St -> ( a , St ) }

In a programming pearl paper [1] Andrew Kennedy has described how to define primitive picklers plus a set of pickler combinators to (de-)serialize from and to (Byte-)Strings.

The HXT picklers are an adaptation of these pickler combinators. The difference to Kennedys approach is that the external representation is not a list of Chars but a list of XmlTrees. The basic picklers for the primitve types (Int, Bool,...) will convert simple values into XML text nodes, and the picklers for creating XML element and attribute nodes are new:

data St = St { attributes :: [ XmlTree ] , contents :: [ XmlTree ] } data PU a = PU { appPickle :: ( a , St ) -> St , appUnPickle :: St -> ( Maybe a , St ) , theSchema :: Schema }

In XML there are two places for storing information: The attributes and the element contents. Furthermore, the pickler contains a third component for type information. This enables the derivation of a DTD from a set of picklers, but in the following examples we do not need this component. With the predefined picklers and pickler combinators, we don't have to look very much into these internals. Let's start with an example.

Example: Processing baseball league data

The XML data structure

In this first example we are dealing with baseball league data, taken from the so- called XML Bible. The complete source for this example is included in the HXT distribution in directory examples/arrows/AGentleIntroductionToHXT/PicklerExample/. First let's get some idea about the structure of the XML data. The structure is not defined by a DTD or schema, so we have to guess some things. Here is a part of the example XML file:

<SEASON YEAR="1998"> <LEAGUE NAME="National League"> <DIVISION NAME="East"> <TEAM CITY="Atlanta" NAME="Braves"> <PLAYER GIVEN_NAME="Marty" SURNAME="Malloy" POSITION="Second Base" GAMES="11" GAMES_STARTED="8" AT_BATS="28" RUNS="3" HITS="5" DOUBLES="1" TRIPLES="0" HOME_RUNS="1" RBI="1" STEALS="0" CAUGHT_STEALING="0" SACRIFICE_HITS="0" SACRIFICE_FLIES="0" ERRORS="0" WALKS="2" STRUCK_OUT="2" HIT_BY_PITCH="0"> </PLAYER> <PLAYER GIVEN_NAME="Ozzie" SURNAME="Guillen" POSITION="Shortstop" GAMES="83" GAMES_STARTED="59" AT_BATS="264" RUNS="35" HITS="73" DOUBLES="15" TRIPLES="1" HOME_RUNS="1" RBI="22" STEALS="1" CAUGHT_STEALING="4" SACRIFICE_HITS="4" SACRIFICE_FLIES="2" ERRORS="6" WALKS="24" STRUCK_OUT="25" HIT_BY_PITCH="1"> </PLAYER> <PLAYER GIVEN_NAME="Danny" ... HIT_BY_PITCH="0"> </PLAYER> <PLAYER GIVEN_NAME="Gerald" ...> </PLAYER> ... </TEAM> <TEAM CITY="Florida" NAME="Marlins"> </TEAM> <TEAM CITY="Montreal" NAME="Expos"> </TEAM> <TEAM CITY="New York" NAME="Mets"> </TEAM> <TEAM CITY="Philadelphia" NAME="Phillies"> </TEAM> </DIVISION> ... </LEAGUE> <LEAGUE NAME="American League"> <DIVISION NAME="East"> ... </DIVISION> <DIVISION NAME="Central"> ... </DIVISION> ... </LEAGUE> </SEASON>

The Haskell data model

Let's first analyze the underlying data model and then define an appropriate set of Haskell data types for the internal representation.

The root type is a Season , consisting of a year an a set of League s

, consisting of a an a set of s The League s are all identified by a String and consist of a set of Division s, so it's a Map .

s are all identified by a and consist of a set of s, so it's a . The Division s are also identified by a String and consist of a list of Team s, so it's again a Map

s are also identified by a and consist of a list of s, so it's again a A Team has three components, a teamName , a city , and a list of Player s

has three components, a , a , and a list of s A Player has a lot of attributes, for simplicity of the example in the internal model we will not take all fields into account. Just six fields are included, the firstName, the lastName, the position, atBats, hits and era. All others will be ignored.

So the Haskell data model looks like this:

import Data.Map data Season = Season { sYear :: Int , sLeagues :: Leagues } deriving ( Show , Eq ) type Leagues = Map String Divisions type Divisions = Map String [ Team ] data Team = Team { teamName :: String , city :: String , players :: [ Player ] } deriving ( Show , Eq ) data Player = Player { firstName :: String , lastName :: String , position :: String , atBats :: Maybe Int , hits :: Maybe Int , era :: Maybe Float } deriving ( Show , Eq )

The predefined picklers

HXT contains a class XmlPickler defining a single function xpickle for overloading the xpickle function name:

class XmlPickler a where xpickle :: PU a

For simple data types there is an instance for XmlPickler which uses the primitive pickler xpPrim for conversion from and to XML text nodes. This primitive pickler is available for all types supporting Read and Show:

instance XmlPickler Int where xpickle = xpPrim instance XmlPickler Integer where xpickle = xpPrim ...

For the composite data type tuples, lists and Maybe there are predefined pickler combinators:

instance ( XmlPickler a , XmlPickler b ) => XmlPickler ( a , b ) where xpickle = xpPair xpickle xpickle -- similar instances for (,,), (,,,), ... instance XmlPickler a => XmlPickler [ a ] where xpickle = xpList xpickle instance XmlPickler a => XmlPickler ( Maybe a ) where xpickle = xpOption xpickle

xpPair take two picklers and builds up a pickler for a tuple type. There are also pickler combinators for triples, 4- and 5- tuples.

take two picklers and builds up a pickler for a tuple type. There are also pickler combinators for triples, 4- and 5- tuples. xpList takes a pickler for an element type and gives a list pickler

takes a pickler for an element type and gives a list pickler xpOption takes a pickler and returns a pickler for optional values.

In addition to those picklers, we need picklers for generating/reading element and attribute nodes:

xpElem generates/parses an XML element node

generates/parses an XML element node xpAttr generates/parses an attribute node

Most of the other structured data is pickled/unpickled by converting the data to/from tuples, lists and options. This is done by a wrapper pickler xpWrap.

Constructing the example picklers

For every Haskell type we will define a pickler.

For the own data types we will declare instances of the XmlPickler class.

instance XmlPickler Season where xpickle = xpSeason instance XmlPickler Team where xpickle = xpTeam instance XmlPickler Player where xpickle = xpPlayer



Then the picklers are developed top down starting with xpSeason.

xpSeason :: PU Season xpSeason = xpElem "SEASON" $ xpWrap ( uncurry Season , \ s -> ( sYear s , sLeagues s )) $ xpPair ( xpAttr "YEAR" xpickle ) xpLeagues

A Season value is mapped onto an element SEASON with xpElem. This constructs/reads the XML SEASON element. The two components of Season are wrapped into a pair with xpWrap. xpWrap needs a pair of functions for a 1-1 mapping between Season and (Int, Leagues). The first component of the pair, the year is mapped onto an attribute YEAR. The attribute value is handled with the predefined pickler for Int. The second one, the Leagues are handled by xpLeagues.

xpLeagues :: PU Leagues xpLeagues = xpWrap ( fromList , toList ) $ xpList $ xpElem "LEAGUE" $ xpPair ( xpAttr "NAME" xpText ) xpDivisions

xpLeagues has to deal with a Map value. This can't done directly, but the Map value is converted to/from a list of pairs with xpWrap and (fromList, toList). Then the xpList is applied for the list of pairs. Each pair will be represented by an LEAGUE element, the name is mapped to an attribute NAME, the divisions are handled by xpDivisions.

(xpText is used to encode attribute or tag text, but note that you must use xpText0 instead wherever the empty string is a legal value, because xpText doesn't handle the case of unpickling 'nothing' from the XML.)

xpDivisions :: PU Divisions xpDivisions = xpWrap ( fromList , toList ) $ xpList $ xpElem "DIVISION" $ xpPair ( xpAttr "NAME" xpText ) xpickle

The divisions are pickled by the same pattern as the leagues.

xpTeam :: PU Team xpTeam = xpElem "TEAM" $ xpWrap ( uncurry3 Team , \ t -> ( teamName t , city t , players t ) ) $ xpTriple ( xpAttr "NAME" xpText ) ( xpAttr "CITY" xpText ) ( xpList xpickle )

With the teams we have to wrap the three components into a 3-tuple with xpWrap and then pickle a triple of two attributes and a list of players.

xpPlayer :: PU Player xpPlayer = xpElem "PLAYER" $ xpWrap ( \ (( f , l , p , a , h , e )) -> Player f l p a h e , \ t -> ( firstName t , lastName t , position t , atBats t , hits t , era t ) ) $ xp6Tuple ( xpAttr "GIVEN_NAME" xpText ) ( xpAttr "SURNAME" xpText ) ( xpAttr "POSITION" xpText ) ( xpOption ( xpAttr "AT_BATS" xpickle )) ( xpOption ( xpAttr "HITS" xpickle )) ( xpOption ( xpAttr "ERA" xpPrim ))

The Player pickler looks a bit clumsy, because of the six fields. A Player is mapped to an element PLAYER with 3 mandatory attributes and 3 optional attributes

Since HXT-9 tuples are supported until 24 components.

New in this case is the use of xpOption for mapping Maybe values onto optional attributes.

The other attributes used in the input, are ignored during unpickling the XML, but this is the only place where the pickler is tolerant with wrong XML.

A simple application

import Text.XML.HXT.Core -- ... main :: IO () main = do runX ( xunpickleDocument xpSeason [ withValidate no , withTrace 1 , withRemoveWS yes , withPreserveComment no ] "simple2.xml" >>> processSeason >>> xpickleDocument xpSeason [ withIndent yes ] "new-simple2.xml" ) return () -- the dummy for processing the unpickled data processSeason :: IOSArrow Season Season processSeason = arrIO ( \ x -> do { print x ; return x })

This application reads in the complete data used in HXT/Practical/Simple2 from file simple2.xml and unpickles it into a Season value. This value is processed (dummy: print out) by processSeason and pickled again into new-simple2.xml

The unpickled value, when formatted a bit, looks like this

Season { sYear = 1998 , sLeagues = fromList [ ( "American League" , fromList [ ( "Central" , [ Team { teamName = "White Sox" , city = "Chicago" , players = [] } , ... ]) , ( "East" , [ Team { teamName = "Orioles" , city = "Baltimore" , players = [] } , ... ]) , ( "West" , [ Team { teamName = "Angels" , city = "Anaheim" , players = [] } , ... ]) ]) , ( "National League" , fromList [ ( "Central" , [ Team { teamName = "Cubs" , city = "Chicago" , players = [] } , ... ]) , ( "East" , [ Team { teamName = "Braves" , city = "Atlanta" , players = [ Player { firstName = "Marty" , lastName = "Malloy" , position = "Second Base" , atBats = Just 28 , hits = Just 5 , era = Nothing } , Player { firstName = "Ozzie" , lastName = "Guillen" , position = "Shortstop" , atBats = Just 264 , hits = Just 73 , era = Nothing } , ... ]} , ... ]) , ( "West" , [ Team { teamName = "Diamondbacks" , city = "Arizona" , players = [] } , ... ]) ]) ] }

2. Example: A toy programming language

In this second example we will develop the picklers the other way round. We start with a given data model and derive an XML document structure.

The complete source of this example is included in the HXT distribution in directory examples/arrows/pickle/

The abstract syntax for the programming 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 )

A program is a statement, and four variants of statement are defined, assignments, sequences, branches and loops. The expressions have five variants, constants, identifiers, unary and binary expressions. The operators are realized as enumeration types.

For developing the picklers, there are two new aspects. This example contains sum data types and it's a recursive structure.

The pickler definitions

xpProgram :: PU Program xpProgram = xpElem "program" $ xpAddFixedAttr "xmlns" "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" $ 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 ) ]

The root pickler is xpProgram which wraps the main statement in a program element. The program element is decorated with a fixed attribute, defining a name space declaration, just for demonstrating the use of the xpAddFixedAttr.

For the operators two variants are shown. The UnOp is converted with read/show (xpPrim), The Op is in XML represented by a number (xpWrap (toEnum, fromEnum)).

The Expr and Stmt picklers are a bit more interesting. We have to select a pickler for every constructor of the data type. This is done by mapping each variant to a number and then index a list of picklers with this number. For all variants the values are converted with xpWrap into simple values or tuples, and then these values are mapped to XML elements. The simple fields are encoded in attributes, the complex (and recursive) are encoded as child elements.

The complete pickler definitions consist of about 60 lines of code.

A simple program as Haskell value

p2 :: Program 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"

An example program with all variants of statements and expressions.

The serialized program as XML

<program xmlns="program42"> <block> <assign name="x"> <int value="6"/> </assign> <assign name="y"> <int value="7"/> </assign> <assign name="p"> <int value="0"/> </assign> <while> <binex op="6"> <var name="x"/> <int value="0"/> </binex> <if> <binex op="6"> <binex op="4"> <var name="x"/> <int value="2"/> </binex> <int value="0"/> </binex> <block> <assign name="x"> <binex op="1"> <var name="x"/> <int value="1"/> </binex> </assign> <assign name="p"> <binex op="0"> <var name="p"/> <var name="y"/> </binex> </assign> </block> <block> <assign name="x"> <binex op="3"> <var name="x"/> <int value="2"/> </binex> </assign> <assign name="y"> <binex op="2"> <var name="y"/> <int value="2"/> </binex> </assign> </block> </if> </while> </block> </program>

This document is generated by executing the following piece of code

storeProgram :: IO () storeProgram = do runX ( constA p2 >>> xpickleDocument xpProgram [ withIndent yes -- indent XML ] "pickle.xml" ) return ()

It's loaded from a file with

loadProgram :: IO Program loadProgram = do [ p2 ] <- runX ( xunpickleDocument xpProgram [ withRemoveWS yes -- remove redundant whitespace , withValidate no -- don't validate source ] "pickle.xml" ) return p2

The withRemoveWS configuration option is necessary because the XML document was formatted and filled up with redundant whitespace when written.

A few words of advice

These picklers are a powerful tool for de-/serializing from/to XML. Only a few lines of code are needed for serializing as well as for deserializing. But they are absolutely intolerant when dealing with invalid XML. They are intended to read machine generated XML, ideally generated by the same pickler. When unpickling hand written XML or XML generated by foreign tools, please validate the XML before reading, preferably with RelaxNG or XML Schema, because of the more powerful validation schema than DTDs.

When designing picklers, one must be careful to put enough markup into the XML structure, to read the XML back without the need for a lookahead and without any ambiguities. The simplest case of a not working pickler is a pair of primitve picklers e.g. for some text. In this case the text is written out and concatenated into a single string, when parsing the XML, there will only be a single string and the pickler will fail because of a missing value for the second component. So at least every primitive pickler must be combined with an xpElem or xpAttr.

It's possible to define various picklers per data type, and picklers can be used one way, just for serializing into XML/HTML. So this approach can also be used to easily generate parts of a HTML document.

Please do not try to convert a whole large database into a single XML file with this approach. This will run into memory problems when reading the data, because of the DOM approach used in HXT. In the HXT distribution, there is a test case in the examples dir performance, where the pickling and unpickling is done with XML documents containing 2 million elements. This is the limit for a 1G Intel box (tested with ghc 6.8).

There are two strategies to overcome these limitations. The first is a SAX like approach, reading in simple tags and text elements and not building a tree structure, but writing the data instantly into a database. For this approach the Tagsoup package can be useful. The disadvantage is the programming effort for collecting and converting the data.

The second and recommended way is, to split the whole bunch of data into smaller pieces, unpickle these and link the resulting documents together by the use of 'hrefs.

More Examples

Exxamples dealing with direct conversion to/from XML without the use of picklers can be found under HXT/Practical.

Reading/writing between XML and Haskell data types without XML picklers

This is an example for reading and writing XML without the use of picklers. It was developed before the picklers were added to HXT. The code shows that it's much more effort to implement a conversion than with the technique described above.

Serializing to Xml

We can create an HXT tree from a single-layer data class as follows:

import IO import Char import Text.XML.HXT.Arrow import Data.Generics -- our data class we'll convert into xml data Config = Config { username :: String , logNumDays :: Int , oleDbString :: String } deriving ( Show , Typeable , Data ) -- helper function adapted from http://www.defmacro.org/ramblings/haskell-web.html -- (gshow replaced by gshow') introspectData :: Data a => a -> [( String , String )] introspectData a = zip fields ( gmapQ gshow' a ) where fields = constrFields $ toConstr a gshow' :: Data a => a -> String gshow' t = fromMaybe ( showConstr ( toConstr t )) ( cast t ) -- function to create xml string from single-layer Haskell data type xmlSerialize object = "<" ++ show ( toConstr object ) ++ ">" ++ foldr ( \ ( a , b ) x -> x ++ "<" ++ a ++ ">" ++ b ++ "</" ++ a ++ ">" ) "" ( introspectData object ) ++ "</" ++ show ( toConstr object ) ++ ">" -- function to create HXT tree arrow from single-layer Haskell data type: createHxtArrow object = runLA ( constA ( xmlSerialize object ) >>> xread ) -- create a config object to serialize: createConfig = Config { username = "test" , logNumDays = 3 , oleDbString = "qsdf" } -- test function, using our Config data type testConversion = createHxtArrow ( createConfig ) ()

-- hughperkins

Deserializing from Xml

Here's a solution to deserialize a simple Haskell data type containing Strings and Ints.

It's not really pretty, but it works.

Basically, we just convert the incoming xml into gread-compatible format, then use gread :-D

Currently it works for a simple single-layer Haskell data type containing Ints and Strings. You can add new child data types by adding to the case statement in xmlToGShowFormat.

If someone has a more elegant solution, please let me know ( hughperkins@gmail.com )

module ParseXml where import IO import Char import List import Maybe import Data.Generics hiding ( Unit ) import Text.XML.HXT.Arrow hiding ( when ) data Config = Config { name :: String , age :: Int } --data Config = Config{ age :: Int } deriving ( Data , Show , Typeable , Ord , Eq , Read ) createConfig = Config "qsdfqsdf" 3 --createConfig = Config 3 gshow' :: Data a => a -> String gshow' t = fromMaybe ( showConstr ( toConstr t )) ( cast t ) -- helper function from http://www.defmacro.org/ramblings/haskell-web.html introspectData :: Data a => a -> [( String , String )] introspectData a = zip fields ( gmapQ gshow' a ) where fields = constrFields $ toConstr a -- function to create xml string from single-layer Haskell data type xmlSerialize object = "<" ++ show ( toConstr object ) ++ ">" ++ foldr ( \ ( a , b ) x -> x ++ "<" ++ a ++ ">" ++ b ++ "</" ++ a ++ ">" ) "" ( introspectData object ) ++ "</" ++ show ( toConstr object ) ++ ">" -- parse xml to HXT tree, and obtain the value of node "fieldname" -- returns a string getValue xml fieldname = listToMaybe resultlist where resultlist = runLA ( constA xml >>> xread >>> deep ( hasName fieldname ) >>> getChildren >>> getText ) [] -- parse templateobject to get list of field names -- apply these to xml to get list of values -- return (fieldnames list, value list) xmlToGShowFormat :: Data a => String -> a -> String xmlToGShowFormat xml templateobject = go where mainconstructorname = ( showConstr $ toConstr templateobject ) fields = constrFields $ toConstr templateobject values = map ( getValue xml ) fields datatypes = gmapQ ( dataTypeOf ) templateobject constrs = gmapQ ( toConstr ) templateobject datatypereps = gmapQ ( dataTypeRep . dataTypeOf ) templateobject fieldtogshowformat value IntRep = "(" ++ fromJust value ++ ")" fieldtogshowformat value _ = show ( fromJust value ) formattedfieldlist = zipWith fieldtogshowformat values datatypereps go = "(" ++ mainconstructorname ++ " " ++ unwords formattedfieldlist ++ ")" xmlDeserialize xml templateobject = fst $ head $ gread ( xmlToGShowFormat xml templateobject ) dotest = xmlDeserialize ( xmlSerialize createConfig ) createConfig :: Config dotest' = xmlDeserialize ( "<Config><age>12</age><name>test name!</name></Config>" ) createConfig :: Config



