Text.XML.Expat.Tree

This module provides functions to parse an XML document to a tree structure, either strictly or lazily.

The GenericXMLString type class allows you to use any string type. Three string types are provided for here: String, ByteString and Text.

Here is a complete example to get you started:

-- | A "hello world" example of hexpat that lazily parses a document, printing
-- it to standard out.

import Text.XML.Expat.Tree
import Text.XML.Expat.Format
import System.Environment
import System.Exit
import System.IO
import qualified Data.ByteString.Lazy as L

main = do
    args <- getArgs
    case args of
        [filename] -> process filename
        otherwise  -> do
            hPutStrLn stderr "Usage: helloworld <file.xml>"
            exitWith $ ExitFailure 1

process :: String -> IO ()
process filename = do
    inputText <- L.readFile filename
    -- Note: Because we're not using the tree, Haskell can't infer the type of
    -- strings we're using so we need to tell it explicitly with a type signature.
    let (xml, mErr) = parse defaultParseOptions inputText :: (UNode String, Maybe XMLParseError)
    -- Process document before handling error, so we get lazy processing.
    L.hPutStr stdout $ format xml
    putStrLn ""
    case mErr of
        Nothing -> return ()
        Just err -> do
            hPutStrLn stderr $ "XML parse failed: "++show err
            exitWith $ ExitFailure 2

Error handling in strict parses is very straightforward - just check the Either return value. Lazy parses are not so simple. Here are two working examples that illustrate the ways to handle errors. Here they are:

Way no. 1 - Using a Maybe value

import Text.XML.Expat.Tree
import qualified Data.ByteString.Lazy as L
import Data.ByteString.Internal (c2w)

-- This is the recommended way to handle errors in lazy parses
main = do
    let (tree, mError) = parse defaultParseOptions
                   (L.pack $ map c2w $ "<top><banana></apple></top>")
    print (tree :: UNode String)

    -- Note: We check the error _after_ we have finished our processing
    -- on the tree.
    case mError of
        Just err -> putStrLn $ "It failed : "++show err
        Nothing -> putStrLn "Success!"

Way no. 2 - Using exceptions

parseThrowing can throw an exception from pure code, which is generally a bad way to handle errors, because Haskell's lazy evaluation means it's hard to predict where it will be thrown from. However, it may be acceptable in situations where it's not expected during normal operation, depending on the design of your program.

...
import Control.Exception.Extensible as E

-- This is not the recommended way to handle errors.
main = do
    do
        let tree = parseThrowing defaultParseOptions
                       (L.pack $ map c2w $ "<top><banana></apple></top>")
        print (tree :: UNode String)
        -- Because of lazy evaluation, you should not process the tree outside
        -- the 'do' block, or exceptions could be thrown that won't get caught.
    `E.catch` (\exc ->
        case E.fromException exc of
            Just (XMLParseException err) -> putStrLn $ "It failed : "++show err
            Nothing -> E.throwIO exc)

Tree structure

type Node tag text = NodeG [] tag text

A pure tree representation that uses a list as its container type.

In the hexpat package, a list of nodes has the type [Node tag text], but note that you can also use the more general type function ListOf to give a list of any node type, using that node's associated list type, e.g. ListOf (UNode Text).

data NodeG c tag text

The tree representation of the XML document.

c is the container type for the element's children, which would normally be [], but could potentially be a monadic list type to allow for chunked I/O.

tag is the tag type, which can either be one of several string types, or a special type from the Text.XML.Expat.Namespaced or Text.XML.Expat.Qualified modules.

text is the string type for text content.

Constructors

Element

Fields

eName :: !tag

eAttributes :: ![(tag, text)]

eChildren :: c (NodeG c tag text)

Text !text

Instances

(Functor c, List c) => MkElementClass NodeG c

(Functor c, List c) => NodeClass NodeG c

(Eq tag, Eq text) => Eq (NodeG [] tag text)

(Show tag, Show text) => Show (NodeG [] tag text)

(NFData tag, NFData text) => NFData (NodeG [] tag text)

type ListOf (NodeG c tag text) = c (NodeG c tag text)

type UNode text = Node text text

Type alias for a node with unqualified tag names where tag and text are the same string type.

Generic node manipulation

module Text.XML.Expat.Internal.NodeClass

Qualified nodes

type QNode text = Node (QName text) text

Type alias for a node where qualified names are used for tags

module Text.XML.Expat.Internal.Qualified

Namespaced nodes

type NNode text = Node (NName text) text

Type alias for a node where namespaced names are used for tags

module Text.XML.Expat.Internal.Namespaced

Parse to tree

data ParseOptions tag text

Constructors

ParseOptions

Fields

overrideEncoding :: Maybe Encoding

The encoding parameter, if provided, overrides the document's encoding declaration.

entityDecoder :: Maybe (tag -> Maybe text)

If provided, entity references (i.e.   and friends) will be decoded into text using the supplied lookup function

defaultParseOptions :: ParseOptions tag text

data Encoding

Constructors

ASCII

UTF8

UTF16

ISO88591

parse

Arguments

:: (GenericXMLString tag, GenericXMLString text)

=> ParseOptions tag text

Parse options

-> ByteString

Input text (a lazy ByteString)

-> (Node tag text, Maybe XMLParseError)

Lazily parse XML to tree. Note that forcing the XMLParseError return value will force the entire parse. Therefore, to ensure lazy operation, don't check the error status until you have processed the tree.

parse'

Arguments

:: (GenericXMLString tag, GenericXMLString text)

=> ParseOptions tag text

Parse options

-> ByteString

Input text (a strict ByteString)

-> Either XMLParseError (Node tag text)

Strictly parse XML to tree. Returns error message or valid parsed tree.

parseG

Arguments

:: (GenericXMLString tag, GenericXMLString text, List l)

=> ParseOptions tag text

Parse options

-> l ByteString

Input text as a generalized list of blocks

-> ItemM l (NodeG l tag text)

Parse a generalized list to a tree, ignoring parse errors. This function allows for a parse from an enumerator/iteratee to a "lazy" tree structure using the List-enumerator package.

data XMLParseError

Parse error, consisting of message text and error location

Constructors

XMLParseError String XMLParseLocation

Instances

Eq XMLParseError

Show XMLParseError

NFData XMLParseError

data XMLParseLocation

Specifies a location of an event within the input text

Constructors

XMLParseLocation

Fields

xmlLineNumber :: Int64

Line number of the event

xmlColumnNumber :: Int64

Column number of the event

xmlByteIndex :: Int64

Byte index of event from start of document

xmlByteCount :: Int64

The number of bytes in the event

Instances

Eq XMLParseLocation

Show XMLParseLocation

NFData XMLParseLocation

Variant that throws exceptions

parseThrowing

Arguments

:: (GenericXMLString tag, GenericXMLString text)

=> ParseOptions tag text

Parse options

-> ByteString

Input text (a lazy ByteString)

-> Node tag text

Lazily parse XML to tree. In the event of an error, throw XMLParseException.

parseThrowing can throw an exception from pure code, which is generally a bad way to handle errors, because Haskell's lazy evaluation means it's hard to predict where it will be thrown from. However, it may be acceptable in situations where it's not expected during normal operation, depending on the design of your program.

data XMLParseException

An exception indicating an XML parse error, used by the ..Throwing variants.

Constructors

XMLParseException XMLParseError

Instances

Eq XMLParseException

Show XMLParseException

Exception XMLParseException

Typeable * XMLParseException

Convert from SAX

saxToTree :: GenericXMLString tag => [SAXEvent tag text] -> (Node tag text, Maybe XMLParseError)

A lower level function that lazily converts a SAX stream into a tree structure.

saxToTreeG :: forall tag text l. (GenericXMLString tag, List l) => l (SAXEvent tag text) -> ItemM l (NodeG l tag text)

A lower level function that converts a generalized SAX stream into a tree structure. Ignores parse errors.

Abstraction of string types

class (Monoid s, Eq s) => GenericXMLString s where

An abstraction for any string type you want to use as xml text (that is, attribute values or element text content). If you want to use a new string type with hexpat, you must make it an instance of GenericXMLString.

Methods

gxNullString :: s -> Bool

gxToString :: s -> String

gxFromString :: String -> s

gxFromChar :: Char -> s

gxHead :: s -> Char

gxTail :: s -> s

gxBreakOn :: Char -> s -> (s, s)

gxFromByteString :: ByteString -> s

gxToByteString :: s -> ByteString

Instances

GenericXMLString String

GenericXMLString ByteString

GenericXMLString Text