Safe Haskell | None |
---|---|
Language | Haskell98 |
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)
- type Node tag text = NodeG [] tag text
- data NodeG c tag 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
- 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 {}
- 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
Tree structure
type Node tag text = NodeG [] tag text Source #
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 Source #
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.
(Functor c, List c) => MkElementClass NodeG c Source # | |
(Functor c, List c) => NodeClass NodeG c Source # | |
(Eq tag, Eq text) => Eq (NodeG [] tag text) Source # | |
(Show tag, Show text) => Show (NodeG [] tag text) Source # | |
(NFData tag, NFData text) => NFData (NodeG [] tag text) Source # | |
type ListOf (NodeG c tag text) Source # | |
type UNode text = Node text text Source #
Type alias for a node with unqualified tag names where tag and text are the same string type.
Generic node manipulation
Qualified nodes
type QNode text = Node (QName text) text Source #
Type alias for a node where qualified names are used for tags
Namespaced nodes
type NNode text = Node (NName text) text Source #
Type alias for a node where namespaced names are used for tags
Parse to tree
data ParseOptions tag text Source #
ParseOptions | |
|
defaultParseOptions :: ParseOptions tag text Source #
:: (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.
:: (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.
:: (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 Source #
Parse error, consisting of message text and error location
data XMLParseLocation Source #
Specifies a location of an event within the input text
XMLParseLocation | |
|
Variant that throws exceptions
:: (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 Source #
An exception indicating an XML parse error, used by the ..Throwing variants.
Convert from SAX
saxToTree :: GenericXMLString tag => [SAXEvent tag text] -> (Node tag text, Maybe XMLParseError) Source #
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) Source #
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 Source #
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
.
gxNullString, gxToString, gxFromString, gxFromChar, gxHead, gxTail, gxBreakOn, gxFromByteString, gxToByteString
gxNullString :: s -> Bool Source #
gxToString :: s -> String Source #
gxFromString :: String -> s Source #
gxFromChar :: Char -> s Source #
gxBreakOn :: Char -> s -> (s, s) Source #
gxFromByteString :: ByteString -> s Source #
gxToByteString :: s -> ByteString Source #