xml-conduit-0.5.2: Text/XML/Stream/Parse.hs
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PatternGuards #-}
-- | 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.Stream.Parse
-- > import Data.Text (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_ def "people.xml" $ 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.
module Text.XML.Stream.Parse
( -- * Parsing XML files
parseBytes
, parseText
, detectUtf
, parseFile
, parseLBS
-- ** Parser settings
, ParseSettings
, def
, DecodeEntities
, psDecodeEntities
-- * Event parsing
, tag
, tagPredicate
, tagName
, tagNoAttr
, content
, contentMaybe
-- * Attribute parsing
, AttrParser
, requireAttr
, optionalAttr
, requireAttrRaw
, optionalAttrRaw
, ignoreAttrs
-- * Combinators
, orE
, choose
, many
, force
-- * Exceptions
, XmlException (..)
) where
import Data.Attoparsec.Text
( char, Parser, takeWhile1, skipWhile, string
, manyTill, takeWhile, try, anyChar, endOfInput
)
import qualified Control.Applicative as A
import Data.Conduit.Attoparsec (sinkParser)
import Data.XML.Types
( Name (..), Event (..), Content (..)
, Instruction (..), ExternalID (..)
)
import Filesystem.Path.CurrentOS (FilePath, encodeString)
import Control.Applicative (Applicative(..), Alternative(empty,(<|>)), (<$>))
import Data.Text (Text)
import qualified Data.Text as T
import Data.Text.Read (Reader, decimal, hexadecimal)
import Data.Text.Encoding (decodeUtf32BEWith)
import Data.Text.Encoding.Error (ignore)
import Data.Word (Word32)
import Blaze.ByteString.Builder (fromWord32be, toByteString)
import Text.XML.Stream.Token
import Prelude hiding (takeWhile, FilePath)
import qualified Data.ByteString as S
import qualified Data.ByteString.Lazy as L
import qualified Data.Map as Map
import qualified Data.Conduit as C
import qualified Data.Conduit.Text as CT
import qualified Data.Conduit.List as CL
import Control.Monad (ap, liftM)
import qualified Data.Text as TS
import Data.List (foldl')
import Data.Typeable (Typeable)
import Control.Exception (Exception)
import Data.Conduit.Binary (sourceFile)
import Data.Char (isSpace)
import Data.Default (Default (..))
import Data.Maybe (catMaybes)
import Control.Monad.Trans.Resource (ResourceIO, resourceThrow)
import Control.Monad.Trans.Class (lift)
type Ents = [(Text, Text)]
tokenToEvent :: Ents -> [NSLevel] -> Token -> (Ents, [NSLevel], [Event])
tokenToEvent es n (TokenBeginDocument _) = (es, n, [])
tokenToEvent es n (TokenInstruction i) = (es, n, [EventInstruction i])
tokenToEvent es n (TokenBeginElement name as isClosed _) =
(es, n', if isClosed then [begin, end] else [begin])
where
l0 = case n of
[] -> NSLevel Nothing Map.empty
x:_ -> x
(as', l') = foldl' go (id, l0) as
go (front, l) (TName kpref kname, val)
| kpref == Just "xmlns" =
(front, l { prefixes = Map.insert kname (contentsToText val)
$ prefixes l })
| kpref == Nothing && kname == "xmlns" =
(front, l { defaultNS = if T.null $ contentsToText val
then Nothing
else Just $ contentsToText val })
| otherwise = (front . (:) (TName kpref kname, map resolve val), l)
resolve (ContentEntity e)
| Just t <- lookup e es = ContentText t
resolve c = c
n' = if isClosed then n else l' : n
fixAttName level (name', val) = (tnameToName True level name', val)
begin = EventBeginElement (tnameToName False l' name)
$ map (fixAttName l')
$ as' []
end = EventEndElement $ tnameToName False l' name
tokenToEvent es n (TokenEndElement name) =
(es, n', [EventEndElement $ tnameToName False l name])
where
(l, n') =
case n of
[] -> (NSLevel Nothing Map.empty, [])
x:xs -> (x, xs)
tokenToEvent es n (TokenContent (ContentEntity e))
| Just t <- lookup e es = (es, n, [EventContent $ ContentText t])
tokenToEvent es n (TokenContent c) = (es, n, [EventContent c])
tokenToEvent es n (TokenComment c) = (es, n, [EventComment c])
tokenToEvent es n (TokenDoctype t eid es') = (es ++ es', n, [EventBeginDoctype t eid, EventEndDoctype])
tokenToEvent es n (TokenCDATA t) = (es, n, [EventCDATA t])
tnameToName :: Bool -> NSLevel -> TName -> Name
tnameToName _ _ (TName (Just "xml") name) =
Name name (Just "http://www.w3.org/XML/1998/namespace") (Just "xml")
tnameToName isAttr (NSLevel def' _) (TName Nothing name) =
Name name (if isAttr then Nothing else def') Nothing
tnameToName _ (NSLevel _ m) (TName (Just pref) name) =
case Map.lookup pref m of
Just ns -> Name name (Just ns) (Just pref)
Nothing -> Name name Nothing (Just pref) -- FIXME is this correct?
-- | 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-16LE/BE, and UTF-32LE/BE. It
-- defaults to assuming UTF-8.
detectUtf :: C.ResourceThrow m => C.Conduit S.ByteString m TS.Text
detectUtf =
conduit id
where
conduit front = C.Conduit (push front) close
push front bss = do
e <- getEncoding front bss
case e of
Left x -> return $ C.Producing (conduit x) []
Right (bss', decode) -> C.conduitPush decode bss'
close = return []
getEncoding front bs'
| S.length bs < 4 =
return $ Left (bs `S.append`)
| otherwise = do
let decode = CT.decode codec
return $ Right (bsOut, decode)
where
bs = front bs'
bsOut = S.append (S.drop toDrop x) y
(x, y) = S.splitAt 4 bs
(toDrop, codec) =
case S.unpack x of
[0x00, 0x00, 0xFE, 0xFF] -> (4, CT.utf32_be)
[0xFF, 0xFE, 0x00, 0x00] -> (4, CT.utf32_le)
0xFE : 0xFF: _ -> (2, CT.utf16_be)
0xFF : 0xFE: _ -> (2, CT.utf16_le)
0xEF : 0xBB: 0xBF : _ -> (3, CT.utf8)
[0x00, 0x00, 0x00, 0x3C] -> (0, CT.utf32_be)
[0x3C, 0x00, 0x00, 0x00] -> (0, CT.utf32_le)
[0x00, 0x3C, 0x00, 0x3F] -> (0, CT.utf16_be)
[0x3C, 0x00, 0x3F, 0x00] -> (0, CT.utf16_le)
_ -> (0, CT.utf8) -- Assuming UTF-8
-- | Parses a byte stream into 'Event's. 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.
parseBytes :: C.ResourceThrow m
=> ParseSettings -> C.Conduit S.ByteString m Event
parseBytes ps = detectUtf C.=$= parseText ps
dropBOM :: C.Resource m => C.Conduit TS.Text m TS.Text
dropBOM = C.conduitState
False
push
close
where
push a b = do
let (a', b') = go a b
return $ C.StateProducing a' [b']
close _ = return []
go True b = (True, b)
go False b =
case T.uncons b of
Nothing -> (False, b)
Just (c, cs) -> (True,
if c == '\xfeef' then cs else b)
-- | Parses a character stream into 'Event's. 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.
parseText :: C.ResourceThrow m
=> ParseSettings
-> C.Conduit TS.Text m Event
parseText de =
dropBOM
C.=$= tokenize
C.=$= toEventC
C.=$= addBeginEnd
where
tokenize = C.sequenceSink () $ const $ C.Emit () . return <$> sinkToken de
addBeginEnd = C.conduitState
False
push
close
where
go False es = EventBeginDocument : [es]
go True es = [es]
push x es = return $ C.StateProducing True $ go x es
close x = return $ go x EventEndDocument
toEventC :: C.Resource m => C.Conduit (Maybe Token) m Event
toEventC = C.conduitState
([], [])
push
close
where
go es levels [] front = (es, levels, front [])
go es levels (t:ts) front =
let (es', levels', events) = tokenToEvent es levels t
in go es' levels' ts (front . (events ++))
push (es, levels) tokens =
return $ C.StateProducing (es', levels') events
where
(es', levels', events) = go es levels (catMaybes [tokens]) id
close (es, levels) =
return events
where
(_, _, events) = go es levels (catMaybes []) id
data ParseSettings = ParseSettings
{ psDecodeEntities :: DecodeEntities
}
instance Default ParseSettings where
def = ParseSettings
{ psDecodeEntities = decodeEntities
}
sinkToken :: C.ResourceThrow m => ParseSettings -> C.Sink TS.Text m (Maybe Token)
sinkToken de = sinkParser ((endOfInput >> return Nothing) <|> fmap Just (parseToken $ psDecodeEntities de))
parseToken :: DecodeEntities -> Parser Token
parseToken de = (char '<' >> parseLt) <|> TokenContent <$> parseContent de False False
where
parseLt =
(char '?' >> parseInstr) <|>
(char '!' >> (parseComment <|> parseCdata <|> parseDoctype)) <|>
(char '/' >> parseEnd) <|>
parseBegin
parseInstr = do
name <- parseIdent
if name == "xml"
then do
as <- A.many $ parseAttribute de
skipSpace
char' '?'
char' '>'
newline <|> return ()
return $ TokenBeginDocument as
else do
skipSpace
x <- T.pack <$> manyTill anyChar (try $ string "?>")
return $ TokenInstruction $ Instruction name x
parseComment = do
char' '-'
char' '-'
c <- T.pack <$> manyTill anyChar (string "-->") -- FIXME use takeWhile instead
return $ TokenComment c
parseCdata = do
_ <- string "[CDATA["
t <- T.pack <$> manyTill anyChar (string "]]>") -- FIXME use takeWhile instead
return $ TokenCDATA t
parseDoctype = do
_ <- string "DOCTYPE"
skipSpace
i <- parseIdent
skipSpace
eid <- fmap Just parsePublicID <|>
fmap Just parseSystemID <|>
return Nothing
skipSpace
ents <- (do
char' '['
ents <- parseEntities id
skipSpace
return ents) <|> return []
char' '>'
newline <|> return ()
return $ TokenDoctype i eid ents
parseEntities front =
(char ']' >> return (front [])) <|>
(parseEntity >>= \e -> parseEntities (front . (e:))) <|>
(char '<' >> parseEntities front) <|>
(skipWhile (\t -> t /= ']' && t /= '<') >> parseEntities front)
parseEntity = try $ do
_ <- string "<!ENTITY"
skipSpace
i <- parseIdent
t <- quotedText
skipSpace
char' '>'
return (i, t)
parsePublicID = do
_ <- string "PUBLIC"
x <- quotedText
y <- quotedText
return $ PublicID x y
parseSystemID = do
_ <- string "SYSTEM"
x <- quotedText
return $ SystemID x
quotedText = do
skipSpace
between '"' <|> between '\''
between c = do
char' c
x <- takeWhile (/=c)
char' c
return x
parseEnd = do
skipSpace
n <- parseName
skipSpace
char' '>'
return $ TokenEndElement n
parseBegin = do
skipSpace
n <- parseName
as <- A.many $ parseAttribute de
skipSpace
isClose <- (char '/' >> skipSpace >> return True) <|> return False
char' '>'
return $ TokenBeginElement n as isClose 0
parseAttribute :: DecodeEntities -> Parser TAttribute
parseAttribute de = do
skipSpace
key <- parseName
skipSpace
char' '='
skipSpace
val <- squoted <|> dquoted
return (key, val)
where
squoted = do
char' '\''
manyTill (parseContent de False True) (char '\'')
dquoted = do
char' '"'
manyTill (parseContent de True False) (char '"')
parseName :: Parser TName
parseName = do
i1 <- parseIdent
mi2 <- (char ':' >> fmap Just parseIdent) <|> return Nothing
return $
case mi2 of
Nothing -> TName Nothing i1
Just i2 -> TName (Just i1) i2
parseIdent :: Parser Text
parseIdent =
takeWhile1 valid
where
valid '&' = False
valid '<' = False
valid '>' = False
valid ':' = False
valid '?' = False
valid '=' = False
valid '"' = False
valid '\'' = False
valid '/' = False
valid c = not $ isSpace c
parseContent :: DecodeEntities
-> Bool -- break on double quote
-> Bool -- break on single quote
-> Parser Content
parseContent de breakDouble breakSingle =
parseEntity <|> parseText'
where
parseEntity = do
char' '&'
t <- takeWhile1 (/= ';')
char' ';'
return $ de t
parseText' = do
bs <- takeWhile1 valid
return $ ContentText bs
valid '"' = not breakDouble
valid '\'' = not breakSingle
valid '&' = False -- amp
valid '<' = False -- lt
valid _ = True
skipSpace :: Parser ()
skipSpace = skipWhile isSpace
newline :: Parser ()
newline = ((char '\r' >> char '\n') <|> char '\n') >> return ()
char' :: Char -> Parser ()
char' c = char c >> return ()
data ContentType =
Ignore | IsContent Text | IsError String | NotContent
-- | 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.
contentMaybe :: C.ResourceThrow m => C.Sink Event m (Maybe Text)
contentMaybe = do
x <- CL.peek
case pc' x of
Ignore -> CL.drop 1 >> contentMaybe
IsContent t -> CL.drop 1 >> fmap Just (takeContents (t:))
IsError e -> lift $ C.resourceThrow $ XmlException e x
NotContent -> return Nothing
where
pc' Nothing = NotContent
pc' (Just x) = pc x
pc (EventContent (ContentText t)) = IsContent t
pc (EventContent (ContentEntity e)) = IsError $ "Unknown entity: " ++ show e
pc (EventCDATA t) = IsContent t
pc EventBeginElement{} = NotContent
pc EventEndElement{} = NotContent
pc EventBeginDocument{} = Ignore
pc EventEndDocument = Ignore
pc EventBeginDoctype{} = Ignore
pc EventEndDoctype = Ignore
pc EventInstruction{} = Ignore
pc EventComment{} = Ignore
takeContents front = do
x <- CL.peek
case pc' x of
Ignore -> CL.drop 1 >> takeContents front
IsContent t -> CL.drop 1 >> takeContents (front . (:) t)
IsError e -> lift $ C.resourceThrow $ XmlException e x
NotContent -> return $ T.concat $ front []
-- | Grabs the next piece of content. If none if available, returns 'T.empty'.
-- This is simply a wrapper around 'contentMaybe'.
content :: C.ResourceThrow m => C.Sink Event m Text
content = do
x <- contentMaybe
case x of
Nothing -> return T.empty
Just y -> return y
-- | 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.
tag :: C.ResourceThrow m
=> (Name -> Maybe a)
-> (a -> AttrParser b)
-> (b -> C.Sink Event m c)
-> C.Sink Event m (Maybe c)
tag checkName attrParser f = do
x <- dropWS
case x of
Just (EventBeginElement name as) ->
case checkName name of
Just y ->
case runAttrParser' (attrParser y) as of
Left e -> lift $ C.resourceThrow e
Right z -> do
CL.drop 1
z' <- f z
a <- dropWS
case a of
Just (EventEndElement name')
| name == name' -> CL.drop 1 >> return (Just z')
_ -> lift $ C.resourceThrow $ XmlException ("Expected end tag for: " ++ show name) a
Nothing -> return Nothing
_ -> return Nothing
where
dropWS = do
x <- CL.peek
let isWS =
case x of
Just EventBeginDocument -> True
Just EventEndDocument -> True
Just EventBeginDoctype{} -> True
Just EventEndDoctype -> True
Just EventInstruction{} -> True
Just EventBeginElement{} -> False
Just EventEndElement{} -> False
Just (EventContent (ContentText t))
| T.all isSpace t -> True
| otherwise -> False
Just (EventContent ContentEntity{}) -> False
Just EventComment{} -> True
Just EventCDATA{} -> False
Nothing -> False
if isWS then CL.drop 1 >> dropWS else return x
runAttrParser' p as =
case runAttrParser p as of
Left e -> Left e
Right ([], x) -> Right x
Right (attr, _) -> Left $ UnparsedAttributes attr
-- | A simplified version of 'tag' which matches against boolean predicates.
tagPredicate :: C.ResourceThrow m
=> (Name -> Bool)
-> AttrParser a
-> (a -> C.Sink Event m b)
-> C.Sink Event m (Maybe b)
tagPredicate p attrParser = tag (\x -> if p x then Just () else Nothing) (const attrParser)
-- | 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.
tagName :: C.ResourceThrow m
=> Name
-> AttrParser a
-> (a -> C.Sink Event m b)
-> C.Sink Event m (Maybe b)
tagName name = tagPredicate (== name)
-- | A further simplified tag parser, which requires that no attributes exist.
tagNoAttr :: C.ResourceThrow m => Name -> C.Sink Event m a -> C.Sink Event m (Maybe a)
tagNoAttr name f = tagName name (return ()) $ const f
-- | 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]
orE :: C.Resource m => C.Sink Event m (Maybe a) -> C.Sink Event m (Maybe a) -> C.Sink Event m (Maybe a)
orE a b = do
x <- a
case x of
Nothing -> b
_ -> return x
-- | Get the value of the first parser which returns 'Just'. If no parsers
-- succeed (i.e., return 'Just'), this function returns 'Nothing'.
choose :: C.Resource m
=> [C.Sink Event m (Maybe a)]
-> C.Sink Event m (Maybe a)
choose [] = return Nothing
choose (i:is) = do
x <- i
case x of
Nothing -> choose is
Just a -> return $ Just 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.
force :: C.ResourceThrow m
=> String -- ^ Error message
-> C.Sink Event m (Maybe a)
-> C.Sink Event m a
force msg i = do
x <- i
case x of
Nothing -> lift $ resourceThrow $ XmlException msg Nothing
Just a -> return 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 :: (ResourceIO m, C.ResourceThrow m)
=> ParseSettings
-> FilePath
-> C.Source m Event
parseFile ps fp = sourceFile (encodeString fp) C.$= parseBytes ps
-- | Parse an event stream from a lazy 'L.ByteString'.
parseLBS :: C.ResourceThrow m
=> ParseSettings
-> L.ByteString
-> C.Source m Event
parseLBS ps lbs = CL.sourceList (L.toChunks lbs) C.$= parseBytes ps
data XmlException = XmlException
{ xmlErrorMessage :: String
, xmlBadInput :: Maybe Event
}
| InvalidEndElement Name
| InvalidEntity Text
| UnparsedAttributes [(Name, [Content])]
deriving (Show, Typeable)
instance Exception XmlException
-- | 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.
newtype AttrParser a = AttrParser { runAttrParser :: [(Name, [Content])] -> Either XmlException ([(Name, [Content])], a) }
instance Monad AttrParser where
return a = AttrParser $ \as -> Right (as, a)
(AttrParser f) >>= g = AttrParser $ \as ->
case f as of
Left e -> Left e
Right (as', f') -> runAttrParser (g f') as'
instance Functor AttrParser where
fmap = liftM
instance Applicative AttrParser where
pure = return
(<*>) = ap
instance Alternative AttrParser where
empty = AttrParser $ const $ Left $ XmlException "AttrParser.empty" Nothing
AttrParser f <|> AttrParser g = AttrParser $ \x ->
case f x of
Left _ -> g x
res -> res
optionalAttrRaw :: ((Name, [Content]) -> Maybe b) -> AttrParser (Maybe b)
optionalAttrRaw f =
AttrParser $ go id
where
go front [] = Right (front [], Nothing)
go front (a:as) =
case f a of
Nothing -> go (front . (:) a) as
Just b -> Right (front as, Just b)
requireAttrRaw :: String -> ((Name, [Content]) -> Maybe b) -> AttrParser b
requireAttrRaw msg f = do
x <- optionalAttrRaw f
case x of
Just b -> return b
Nothing -> AttrParser $ const $ Left $ XmlException msg Nothing
-- | Require that a certain attribute be present and return its value.
requireAttr :: Name -> AttrParser Text
requireAttr n = requireAttrRaw
("Missing attribute: " ++ show n)
(\(x, y) -> if x == n then Just (contentsToText y) else Nothing)
-- | Return the value for an attribute if present.
optionalAttr :: Name -> AttrParser (Maybe Text)
optionalAttr n = optionalAttrRaw
(\(x, y) -> if x == n then Just (contentsToText y) else Nothing)
contentsToText :: [Content] -> Text
contentsToText =
T.concat . map toText
where
toText (ContentText t) = t
toText (ContentEntity e) = T.concat ["&", e, ";"]
-- | 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.
ignoreAttrs :: AttrParser ()
ignoreAttrs = AttrParser $ \_ -> Right ([], ())
-- | Keep parsing elements as long as the parser returns 'Just'.
many :: C.Resource m => C.Sink Event m (Maybe a) -> C.Sink Event m [a]
many i =
go id
where
go front = do
x <- i
case x of
Nothing -> return $ front []
Just y -> go $ front . (:) y
type DecodeEntities = Text -> Content
-- | Default implementation of 'DecodeEntities': handles numeric entities and
-- the five standard character entities (lt, gt, amp, quot, apos).
decodeEntities :: DecodeEntities
decodeEntities "lt" = ContentText "<"
decodeEntities "gt" = ContentText ">"
decodeEntities "amp" = ContentText "&"
decodeEntities "quot" = ContentText "\""
decodeEntities "apos" = ContentText "'"
decodeEntities t = let backup = ContentEntity t in
case T.uncons t of
Just ('#', t') ->
case T.uncons t' of
Just ('x', t'')
| T.length t'' > 6 -> backup
| otherwise -> decodeChar hexadecimal backup t''
_
| T.length t' > 7 -> backup
| otherwise -> decodeChar decimal backup t'
_ -> backup
decodeChar :: Reader Word32 -> Content -> Text -> Content
decodeChar readNum backup = either (const backup) toContent . readNum
where
toContent (num, extra) | T.null extra =
case decodeUtf32BEWith ignore . toByteString $ fromWord32be num of
c | T.length c == 1 -> ContentText c
| otherwise -> backup
toContent _ = backup