Text.XML.Stream.Parse

This module provides both a native Haskell solution for parsing XML documents into a stream of events, and a set of parser combinators for dealing with a stream of events.

As a simple example, if you have the following XML file:

 <?xml version="1.0" encoding="utf-8"?>
 <people>
     <person age="25">Michael</person>
     <person age="2">Eliezer</person>
 </people>

Then this code:

 {-# LANGUAGE OverloadedStrings #-}
 import Text.XML.Enumerator.Parse
 import Data.Text.Lazy (Text, unpack)
 
 data Person = Person { age :: Int, name :: Text }
     deriving Show
 
 parsePerson = tagName "person" (requireAttr "age") $ \age -> do
     name <- content
     return $ Person (read $ unpack age) name
 
 parsePeople = tagNoAttr "people" $ many parsePerson
 
 main = parseFile_ "people.xml" decodeEntities $ force "people required" parsePeople

will produce:

 [Person {age = 25, name = "Michael"},Person {age = 2, name = "Eliezer"}]

Previous versions of this module contained a number of more sophisticated functions written by Aristid Breitkreuz and Dmitry Olshansky. To keep this package simpler, those functions are being moved to a separate package. This note will be updated with the name of the package(s) when available.

Parsing XML files

parseBytes :: Monad m => ParseSettings -> Enumeratee ByteString Event m a

Parses a byte stream into Events. This function is implemented fully in Haskell using attoparsec-text for parsing. The produced error messages do not give line/column information, so you may prefer to stick with the parser provided by libxml-enumerator. However, this has the advantage of not relying on any C libraries.

This relies on detectUtf to determine character encoding, and parseText to do the actual parsing.

parseText :: Monad m => ParseSettings -> Enumeratee Text Event m a

Parses a character stream into Events. This function is implemented fully in Haskell using attoparsec-text for parsing. The produced error messages do not give line/column information, so you may prefer to stick with the parser provided by libxml-enumerator. However, this has the advantage of not relying on any C libraries.

detectUtf :: Monad m => Enumeratee ByteString Text m a

Automatically determine which UTF variant is being used. This function first checks for BOMs, removing them as necessary, and then check for the equivalent of <?xml for each of UTF-8, UTF-16LEBE, and UTF-32LEBE. It defaults to assuming UTF-8.

parseFile :: ParseSettings -> FilePath -> Iteratee Event IO a -> IO (Either SomeException a)

A helper function which reads a file from disk using enumFile, detects character encoding using detectUtf, parses the XML using parseBytes, and then hands off control to your supplied parser.

parseFile_ :: ParseSettings -> FilePath -> Iteratee Event IO a -> IO a

The same as parseFile, but throws any exceptions.

parseLBS :: ParseSettings -> ByteString -> Iteratee Event IO a -> IO (Either SomeException a)

Parse an event stream from a lazy ByteString.

parseLBS_ :: ParseSettings -> ByteString -> Iteratee Event IO a -> IO a

Same as parseLBS, but throws exceptions.

Parser settings

data ParseSettings

Instances

Default ParseSettings

def :: Default a => a

The default value for this type.

type DecodeEntities = Text -> Content

psDecodeEntities :: ParseSettings -> DecodeEntities

Event parsing

tag :: Monad m => (Name -> Maybe a) -> (a -> AttrParser b) -> (b -> Iteratee Event m c) -> Iteratee Event m (Maybe c)

The most generic way to parse a tag. It takes a predicate for checking if this is the correct tag name, an AttrParser for handling attributes, and then a parser for dealing with content.

This function automatically absorbs its balancing closing tag, and will throw an exception if not all of the attributes or child elements are consumed. If you want to allow extra attributes, see ignoreAttrs.

This function automatically ignores comments, instructions and whitespace.

tagPredicate :: Monad m => (Name -> Bool) -> AttrParser a -> (a -> Iteratee Event m b) -> Iteratee Event m (Maybe b)

A simplified version of tag which matches against boolean predicates.

tagName :: Monad m => Name -> AttrParser a -> (a -> Iteratee Event m b) -> Iteratee Event m (Maybe b)

A simplified version of tag which matches for specific tag names instead of taking a predicate function. This is often sufficient, and when combined with OverloadedStrings and the IsString instance of Name, can prove to be very concise.

tagNoAttr :: Monad m => Name -> Iteratee Event m a -> Iteratee Event m (Maybe a)

A further simplified tag parser, which requires that no attributes exist.

content :: Monad m => Iteratee Event m Text

Grabs the next piece of content. If none if available, returns empty. This is simply a wrapper around contentMaybe.

contentMaybe :: Monad m => Iteratee Event m (Maybe Text)

Grabs the next piece of content if available. This function skips over any comments and instructions and concatenates all content until the next start or end tag.

Attribute parsing

data AttrParser a

A monad for parsing attributes. By default, it requires you to deal with all attributes present on an element, and will throw an exception if there are unhandled attributes. Use the requireAttr, optionalAttr et al functions for handling an attribute, and ignoreAttrs if you would like to skip the rest of the attributes on an element.

Alternative instance behave like First monoid. It chooses first parser which doesn't fail.

Instances

Monad AttrParser

Functor AttrParser

Applicative AttrParser

Alternative AttrParser

requireAttr :: Name -> AttrParser Text

Require that a certain attribute be present and return its value.

optionalAttr :: Name -> AttrParser (Maybe Text)

Return the value for an attribute if present.

requireAttrRaw :: String -> ((Name, [Content]) -> Maybe b) -> AttrParser b

optionalAttrRaw :: ((Name, [Content]) -> Maybe b) -> AttrParser (Maybe b)

ignoreAttrs :: AttrParser ()

Skip the remaining attributes on an element. Since this will clear the list of attributes, you must call this after any calls to requireAttr, optionalAttr, etc.

Combinators

orE :: Monad m => Iteratee Event m (Maybe a) -> Iteratee Event m (Maybe a) -> Iteratee Event m (Maybe a)

Get the value of the first parser which returns Just. If no parsers succeed (i.e., return Just), this function returns Nothing.

 orE a b = choose [a, b]

choose :: Monad m => [Iteratee Event m (Maybe a)] -> Iteratee Event m (Maybe a)

Get the value of the first parser which returns Just. If no parsers succeed (i.e., return Just), this function returns Nothing.

many :: Monad m => Iteratee Event m (Maybe a) -> Iteratee Event m [a]

Keep parsing elements as long as the parser returns Just.

force

Arguments

:: Monad m

=> String

Error message

-> Iteratee Event m (Maybe a)

-> Iteratee Event m a

Force an optional parser into a required parser. All of the tag functions, choose and many deal with Maybe parsers. Use this when you want to finally force something to happen.

Exceptions

data XmlException

Constructors

XmlException

Fields

xmlErrorMessage :: String

xmlBadInput :: Maybe Event

InvalidEndElement Name

InvalidEntity Text

UnparsedAttributes [(Name, [Content])]

Instances

Show XmlException

Typeable XmlException

Exception XmlException