Text.XML.Expat.Tree

hexpat-0.20.9: XML parser/formatter based on expat

Safe Haskell | None
Language | Haskell98

Text.XML.Expat.Tree

Contents

- Tree structure
- Generic node manipulation
- Qualified nodes
- Namespaced nodes
- Parse to tree
- Variant that throws exceptions
- Convert from SAX
- Abstraction of string types

Description

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)

Synopsis

- type Node tag text = NodeG [] tag text
- data NodeG c tag text
  - = Element {
    - eName :: !tag
    - eAttributes :: ![(tag, text)]
    - eChildren :: c (NodeG c tag text)
    }
  - | Text !text
- type UNode text = Node text text
- module Text.XML.Expat.Internal.NodeClass
- type QNode text = Node (QName text) text
- module Text.XML.Expat.Internal.Qualified
- type NNode text = Node (NName text) text
- module Text.XML.Expat.Internal.Namespaced
- data ParseOptions tag text = ParseOptions {
  - overrideEncoding :: Maybe Encoding
  - entityDecoder :: Maybe (tag -> Maybe text)
  }
- defaultParseOptions :: ParseOptions tag text
- data Encoding
  - = ASCII
  - | UTF8
  - | UTF16
  - | ISO88591
- parse :: (GenericXMLString tag, GenericXMLString text) => ParseOptions tag text -> ByteString -> (Node tag text, Maybe XMLParseError)
- parse' :: (GenericXMLString tag, GenericXMLString text) => ParseOptions tag text -> ByteString -> Either XMLParseError (Node tag text)
- parseG :: (GenericXMLString tag, GenericXMLString text, List l) => ParseOptions tag text -> l ByteString -> ItemM l (NodeG l tag text)
- data XMLParseError = XMLParseError String XMLParseLocation
- data XMLParseLocation = XMLParseLocation {
  - xmlLineNumber :: Int64
  - xmlColumnNumber :: Int64
  - xmlByteIndex :: Int64
  - xmlByteCount :: Int64
  }
- parseThrowing :: (GenericXMLString tag, GenericXMLString text) => ParseOptions tag text -> ByteString -> Node tag text
- data XMLParseException = XMLParseException XMLParseError
- saxToTree :: GenericXMLString tag => [SAXEvent tag text] -> (Node tag text, Maybe XMLParseError)
- saxToTreeG :: forall tag text l. (GenericXMLString tag, List l) => l (SAXEvent tag text) -> ItemM l (NodeG l tag text)
- class (Monoid s, Eq s) => GenericXMLString s where
  - 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

# 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. `&nbsp;` 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 |

# 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 |