pandoc/src/Text/Pandoc/Readers/Markdown.hs

{-
Copyright (C) 2006 John MacFarlane <jgm at berkeley dot edu>

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
-}

{- |
   Module      : Text.Pandoc.Readers.Markdown
   Copyright   : Copyright (C) 2006 John MacFarlane
   License     : GNU GPL, version 2 or above 

   Maintainer  : John MacFarlane <jgm at berkeley dot edu>
   Stability   : alpha
   Portability : portable

Conversion of markdown-formatted plain text to 'Pandoc' document.
-}
module Text.Pandoc.Readers.Markdown ( 
                                     readMarkdown 
                                    ) where

import Data.List ( findIndex, sortBy, transpose )
import Data.Char ( isAlphaNum )
import Text.ParserCombinators.Pandoc
import Text.Pandoc.Definition
import Text.Pandoc.Readers.LaTeX ( rawLaTeXInline, rawLaTeXEnvironment )
import Text.Pandoc.Shared 
import Text.Pandoc.Readers.HTML ( rawHtmlBlock, 
                                  anyHtmlBlockTag, anyHtmlInlineTag,
                                  anyHtmlTag, anyHtmlEndTag,
                                  htmlEndTag, extractTagType,
                                  htmlBlockElement )
import Text.Pandoc.Entities ( decodeEntities )
import Text.Regex ( matchRegex, mkRegex )
import Text.ParserCombinators.Parsec

-- | Read markdown from an input string and return a Pandoc document.
readMarkdown :: ParserState -> String -> Pandoc
readMarkdown = readWith parseMarkdown

-- | Parse markdown string with default options and print result (for testing).
testString :: String -> IO ()
testString = testStringWith parseMarkdown 

--
-- Constants and data structure definitions
--

spaceChars = " \t"
endLineChars = "\n"
labelStart = '['
labelEnd = ']'
labelSep = ':'
srcStart = '('
srcEnd = ')'
imageStart = '!'
noteStart = '^'
codeStart = '`'
codeEnd = '`'
emphStart = '*'
emphEnd = '*'
emphStartAlt = '_'
emphEndAlt = '_'
autoLinkStart = '<'
autoLinkEnd = '>'
mathStart = '$'
mathEnd = '$'
bulletListMarkers = "*+-"
orderedListDelimiters = ".)"
escapeChar = '\\'
hruleChars = "*-_"
quoteChars = "'\""
atxHChar = '#'
titleOpeners = "\"'("
setextHChars = ['=','-']
blockQuoteChar = '>'
hyphenChar = '-'
ellipsesChar = '.'
listColSepChar = '|'

-- treat these as potentially non-text when parsing inline:
specialChars = [escapeChar, labelStart, labelEnd, emphStart, emphEnd,
                emphStartAlt, emphEndAlt, codeStart, codeEnd, autoLinkEnd,
                autoLinkStart, mathStart, mathEnd, imageStart, noteStart,
                hyphenChar, ellipsesChar] ++ quoteChars

--
-- auxiliary functions
--

-- | Skip a single endline if there is one.
skipEndline = option Space endline

indentSpaces = do
  state <- getState
  let tabStop = stateTabStop state
  count tabStop (char ' ') <|> 
    (do{nonindentSpaces; string "\t"}) <?> "indentation"

nonindentSpaces = do
  state <- getState
  let tabStop = stateTabStop state
  choice (map (\n -> (try (count n (char ' ')))) (reverse [0..(tabStop - 1)]))

-- | Fail if reader is in strict markdown syntax mode.
failIfStrict = do
    state <- getState
    if stateStrict state then fail "Strict markdown mode" else return ()

-- | Fail unless we're at beginning of a line.
failUnlessBeginningOfLine = do
  pos <- getPosition
  if sourceColumn pos == 1 then return () else fail "not beginning of line"

-- | Fail unless we're in "smart typography" mode.
failUnlessSmart = do
  state <- getState
  if stateSmart state then return () else fail "Smart typography feature"

--
-- document structure
--

titleLine = try (do
  char '%'
  skipSpaces
  line <- manyTill inline newline
  return line)

authorsLine = try (do
  char '%'
  skipSpaces
  authors <- sepEndBy (many1 (noneOf ",;\n")) (oneOf ",;")
  newline
  return (map removeLeadingTrailingSpace authors))

dateLine = try (do
  char '%'
  skipSpaces
  date <- many (noneOf "\n")
  newline
  return (removeTrailingSpace date))

titleBlock = try (do
  failIfStrict
  title <- option [] titleLine
  author <- option [] authorsLine
  date <- option "" dateLine
  option "" blanklines
  return (title, author, date))

-- | Returns the number assigned to a Note block
numberOfNote :: Block -> Int
numberOfNote (Note ref _) = (read ref) 
numberOfNote _ = 0 

parseMarkdown = do
  updateState (\state -> state { stateParseRaw = True }) 
  -- need to parse raw HTML, since markdown allows it
  (title, author, date) <- option ([],[],"") titleBlock
  oldState <- getState
  oldInput <- getInput
  -- go through once just to get list of reference keys
  manyTill (referenceKey <|> (do{anyLine; return Null})) eof 
  newState <- getState
  let keysUsed = stateKeysUsed newState
  setInput oldInput
  setState (oldState { stateKeysUsed = keysUsed })
  blocks <- parseBlocks  -- go through again, for real
  let blocks' = filter (/= Null) blocks
  state <- getState
  let keys = reverse $ stateKeyBlocks state
  let notes = reverse $ stateNoteBlocks state
  let sortedNotes = sortBy (\x y -> compare (numberOfNote x) 
                           (numberOfNote y)) notes
  return (Pandoc (Meta title author date) (blocks' ++ sortedNotes ++ keys))

--
-- parsing blocks
--

parseBlocks = manyTill block eof

block = choice [ header, table, codeBlock, note, referenceKey, hrule, list, 
                 blockQuote, htmlBlock, rawLaTeXEnvironment', para,
                 plain, blankBlock, nullBlock ] <?> "block"

--
-- header blocks
--

header = choice [ setextHeader, atxHeader ] <?> "header"

atxHeader = try (do
  lead <- many1 (char atxHChar)
  skipSpaces
  txt <- manyTill inline atxClosing
  return (Header (length lead) (normalizeSpaces txt)))

atxClosing = try (do
  skipMany (char atxHChar)
  skipSpaces
  newline
  option "" blanklines)

setextHeader = choice $ 
               map (\x -> setextH x) (enumFromTo 1 (length setextHChars))

setextH n = try (do
  txt <- many1Till inline newline
  many1 (char (setextHChars !! (n-1)))
  skipSpaces
  newline
  option "" blanklines
  return (Header n (normalizeSpaces txt)))

--
-- hrule block
--

hruleWith chr = try (do
  skipSpaces
  char chr
  skipSpaces
  char chr
  skipSpaces
  char chr
  skipMany (oneOf (chr:spaceChars))
  newline
  option "" blanklines
  return HorizontalRule)

hrule = choice (map hruleWith hruleChars) <?> "hrule"

--
-- code blocks
--

indentedLine = try (do
  indentSpaces
  result <- manyTill anyChar newline
  return (result ++ "\n"))

-- two or more indented lines, possibly separated by blank lines
indentedBlock = try (do 
  res1 <- indentedLine
  blanks <- many blankline 
  res2 <- choice [indentedBlock, indentedLine]
  return (res1 ++ blanks ++ res2))

codeBlock = do
  result <- choice [indentedBlock, indentedLine]
  option "" blanklines
  return (CodeBlock (stripTrailingNewlines result))

--
-- note block
--

rawLine = try (do
  notFollowedBy' blankline
  notFollowedBy' noteMarker
  contents <- many1 nonEndline
  end <- option "" (do
                      newline
                      option "" indentSpaces
                      return "\n")
  return (contents ++ end))

rawLines = do
    lines <- many1 rawLine
    return (concat lines)

note = try (do
  failIfStrict
  ref <- noteMarker
  char ':'
  skipSpaces
  skipEndline 
  raw <- sepBy rawLines (try (do {blankline; indentSpaces}))
  option "" blanklines
  -- parse the extracted text, which may contain various block elements:
  rest <- getInput
  setInput $ (joinWithSep "\n" raw) ++ "\n\n"
  contents <- parseBlocks
  setInput rest
  state <- getState
  let identifiers = stateNoteIdentifiers state
  case (findIndex (== ref) identifiers) of
    Just n  -> updateState (\s -> s {stateNoteBlocks = 
               (Note (show (n+1)) contents):(stateNoteBlocks s)})
    Nothing -> updateState id 
  return Null)

--
-- block quotes
--

emacsBoxQuote = try (do
  failIfStrict
  string ",----"
  manyTill anyChar newline
  raw <- manyTill (try (do 
                          char '|'
                          option ' ' (char ' ')
                          result <- manyTill anyChar newline
                          return result))
                   (string "`----")
  manyTill anyChar newline
  option "" blanklines
  return raw)

emailBlockQuoteStart = try (do
  nonindentSpaces
  char blockQuoteChar
  option ' ' (char ' ')
  return "> ")

emailBlockQuote = try (do
  emailBlockQuoteStart
  raw <- sepBy (many (choice [nonEndline, 
                              (try (do 
                                      endline
                                      notFollowedBy' emailBlockQuoteStart
                                      return '\n'))]))
               (try (do {newline; emailBlockQuoteStart}))
  newline <|> (do{ eof; return '\n' })
  option "" blanklines
  return raw)

blockQuote = do 
  raw <- choice [ emailBlockQuote, emacsBoxQuote ]
  -- parse the extracted block, which may contain various block elements:
  rest <- getInput
  setInput $ (joinWithSep "\n" raw) ++ "\n\n"
  contents <- parseBlocks
  setInput rest
  return (BlockQuote contents)
 
--
-- list blocks
--

list = choice [ bulletList, orderedList ] <?> "list"

bulletListStart = try (do
  option ' ' newline -- if preceded by a Plain block in a list context
  nonindentSpaces
  notFollowedBy' hrule  -- because hrules start out just like lists
  oneOf bulletListMarkers
  spaceChar
  skipSpaces)

orderedListStart = try (do
  option ' ' newline -- if preceded by a Plain block in a list context
  nonindentSpaces
  many1 digit <|> (do{failIfStrict; count 1 letter})
  delim <- oneOf orderedListDelimiters
  if delim /= '.' then failIfStrict else return ()
  oneOf spaceChars
  skipSpaces)

-- parse a line of a list item (start = parser for beginning of list item)
listLine start = try (do
  notFollowedBy' start
  notFollowedBy blankline
  notFollowedBy' (do 
                    indentSpaces
                    many (spaceChar)
                    choice [bulletListStart, orderedListStart])
  line <- manyTill anyChar newline
  return (line ++ "\n"))

-- parse raw text for one list item, excluding start marker and continuations
rawListItem start = try (do
  start
  result <- many1 (listLine start)
  blanks <- many blankline
  return ((concat result) ++ blanks))

-- continuation of a list item - indented and separated by blankline 
-- or (in compact lists) endline.
-- note: nested lists are parsed as continuations
listContinuation start = try (do
  followedBy' indentSpaces
  result <- many1 (listContinuationLine start)
  blanks <- many blankline
  return ((concat result) ++ blanks))

listContinuationLine start = try (do
  notFollowedBy' blankline
  notFollowedBy' start
  option "" indentSpaces
  result <- manyTill anyChar newline
  return (result ++ "\n"))

listItem start = try (do 
  first <- rawListItem start
  continuations <- many (listContinuation start)
  -- parsing with ListItemState forces markers at beginning of lines to
  -- count as list item markers, even if not separated by blank space.
  -- see definition of "endline"
  state <- getState
  let oldContext = stateParserContext state
  setState $ state {stateParserContext = ListItemState}
  -- parse the extracted block, which may contain various block elements:
  rest <- getInput
  let raw = concat (first:continuations)
  setInput $ raw
  contents <- parseBlocks
  setInput rest
  updateState (\st -> st {stateParserContext = oldContext})
  return contents)

orderedList = try (do
  items <- many1 (listItem orderedListStart)
  let items' = compactify items
  return (OrderedList items'))

bulletList = try (do
  items <- many1 (listItem bulletListStart)
  let items' = compactify items
  return (BulletList items'))

--
-- paragraph block
--

para = try (do 
  result <- many1 inline
  newline
  st <- getState
  if stateStrict st
     then choice [followedBy' blockQuote, followedBy' header, 
                  (do{blanklines; return ()})]
     else choice [followedBy' emacsBoxQuote, 
                  (do{blanklines; return ()})]
  let result' = normalizeSpaces result
  return (Para result'))

plain = do
  result <- many1 inline
  let result' = normalizeSpaces result
  return (Plain result')

-- 
-- raw html
--

htmlElement = choice [strictHtmlBlock,
                      htmlBlockElement] <?> "html element"

htmlBlock = do
  st <- getState
  if stateStrict st
    then do
           failUnlessBeginningOfLine
           first <- htmlElement
           finalSpace <- many (oneOf spaceChars)
           finalNewlines <- many newline
           return (RawHtml (first ++ finalSpace ++ finalNewlines))
    else rawHtmlBlocks

-- True if tag is self-closing
selfClosing tag = case (matchRegex (mkRegex "\\/[[:space:]]*>$") tag) of
                      Just _  -> True
                      Nothing -> False

strictHtmlBlock = try (do
  tag <- anyHtmlBlockTag 
  let tag' = extractTagType tag
  if selfClosing tag || tag' == "hr" 
     then return tag
     else do
            contents <- many (do{notFollowedBy' (htmlEndTag tag'); 
                                 htmlElement <|> (count 1 anyChar)})
            end <- htmlEndTag tag'
            return $ tag ++ (concat contents) ++ end)

rawHtmlBlocks = try (do
  htmlBlocks <- many1 rawHtmlBlock    
  let combined = concatMap (\(RawHtml str) -> str) htmlBlocks
  let combined' = if (last combined == '\n')
                     then init combined  -- strip extra newline 
                     else combined 
  return (RawHtml combined'))

-- 
-- reference key
--

referenceKey = try (do
  nonindentSpaces
  label <- reference
  char labelSep
  skipSpaces
  option ' ' (char autoLinkStart)
  src <- many (noneOf (titleOpeners ++ [autoLinkEnd] ++ endLineChars))
  option ' ' (char autoLinkEnd)
  tit <- option "" title 
  blanklines 
  state <- getState
  let keysUsed = stateKeysUsed state
  setState state { stateKeysUsed = (label:keysUsed) }
  return $ Key label (Src (removeTrailingSpace src) tit))

--
-- LaTeX
--

rawLaTeXEnvironment' = do
  failIfStrict
  rawLaTeXEnvironment

--
-- Tables
-- 

-- Parse a dashed line with optional trailing spaces; return its length
-- and the length including trailing space.
dashedLine ch = do
    dashes <- many1 (char ch)
    sp     <- many spaceChar
    return $ (length dashes, length $ dashes ++ sp)

-- Parse a table header with dashed lines of '-' preceded by 
-- one line of text.
simpleTableHeader = do
    rawContent  <- anyLine
    initSp      <- nonindentSpaces
    dashes      <- many1 (dashedLine '-')
    newline
    let (lengths, lines) = unzip dashes
    let indices  = scanl (+) (length initSp) lines
    let rawHeads = tail $ splitByIndices (init indices) rawContent
    let aligns   = zipWith alignType (map (\a -> [a]) rawHeads) lengths
    return $ (rawHeads, aligns, indices)

-- Parse a table footer - dashed lines followed by blank line.
tableFooter = try $ do
    nonindentSpaces
    many1 (dashedLine '-')
    blanklines

-- Parse a table separator - dashed line.
tableSep = try $ do
    nonindentSpaces
    many1 (dashedLine '-')
    string "\n"

-- Parse a raw line and split it into chunks by indices.
rawTableLine indices = do
    notFollowedBy' (blanklines <|> tableFooter)
    line <- many1Till anyChar newline
    return $ map removeLeadingTrailingSpace $ tail $ 
             splitByIndices (init indices) line

-- Parse a table line and return a list of lists of blocks (columns).
tableLine indices = try $ do
    rawline <- rawTableLine indices
    mapM (parseFromStr (many plain)) rawline

-- Parse a multiline table row and return a list of blocks (columns).
multilineRow indices = try $ do
    colLines <- many1 (rawTableLine indices)
    option "" blanklines
    let cols = map unlines $ transpose colLines
    mapM (parseFromStr (many plain)) cols

-- Calculate relative widths of table columns, based on indices
widthsFromIndices :: Int     -- Number of columns on terminal
                  -> [Int]   -- Indices
                  -> [Float] -- Fractional relative sizes of columns
widthsFromIndices _ [] = []  
widthsFromIndices numColumns indices = 
    let lengths = zipWith (-) indices (0:indices)
        totLength = sum lengths
        quotient = if totLength > numColumns
                     then fromIntegral totLength
                     else fromIntegral numColumns
        fracs = map (\l -> (fromIntegral l) / quotient) lengths in
    tail fracs

-- Parses a table caption:  inlines beginning with 'Table:'
-- and followed by blank lines.
tableCaption = try $ do
    nonindentSpaces
    string "Table:"
    result <- many1 inline
    blanklines
    return $ normalizeSpaces result

-- Parse a table using 'headerParser', 'lineParser', and 'footerParser'.
tableWith headerParser lineParser footerParser = try $ do
    (rawHeads, aligns, indices) <- headerParser
    lines <- many1Till (lineParser indices) footerParser
    caption <- option [] tableCaption
    heads <- mapM (parseFromStr (many plain)) rawHeads
    state <- getState
    let numColumns = stateColumns state
    let widths = widthsFromIndices numColumns indices
    return $ Table caption aligns widths heads lines

-- Parse a simple table with '---' header and one line per row.
simpleTable = tableWith simpleTableHeader tableLine blanklines

-- Parse a multiline table:  starts with row of '-' on top, then header
-- (which may be multiline), then the rows,
-- which may be multiline, separated by blank lines, and
-- ending with a footer (dashed line followed by blank line).
multilineTable = tableWith multilineTableHeader multilineRow tableFooter

multilineTableHeader = try $ do
    tableSep 
    rawContent  <- many1 (do{notFollowedBy' tableSep; 
                             many1Till anyChar newline})
    initSp      <- nonindentSpaces
    dashes      <- many1 (dashedLine '-')
    newline
    let (lengths, lines) = unzip dashes
    let indices  = scanl (+) (length initSp) lines
    let rawHeadsList = transpose $ map 
                       (\ln -> tail $ splitByIndices (init indices) ln)
                       rawContent
    let rawHeads = map (joinWithSep " ") rawHeadsList
    let aligns   = zipWith alignType rawHeadsList lengths
    return $ ((map removeLeadingTrailingSpace rawHeads),
             aligns, indices)

-- Returns the longest of a list of strings.
longest :: [String] -> String
longest [] = ""
longest [x] = x
longest (x:xs) =
    if (length x) >= (maximum $ map length xs)
      then x
      else longest xs

-- Returns an alignment type for a table, based on a list of strings
-- (the rows of the column header) and a number (the length of the
-- dashed line under the rows.
alignType :: [String] -> Int -> Alignment
alignType []  len = AlignDefault
alignType strLst len =
    let str        = longest $ map removeTrailingSpace strLst
        leftSpace  = if null str then False else ((str !! 0) `elem` " \t")
        rightSpace = (length str < len || (str !! (len - 1)) `elem` " \t") in
    case (leftSpace, rightSpace) of
        (True,  False)   -> AlignRight
        (False, True)    -> AlignLeft
        (True, True)     -> AlignCenter
        (False, False)   -> AlignDefault

table = do
    failIfStrict
    result <- simpleTable <|> multilineTable <?> "table"
    return result

-- 
-- inline
--

text = choice [ escapedChar, math, strong, emph, smartPunctuation,
                code, ltSign, symbol,
                str, linebreak, tabchar, whitespace, endline ] <?> "text"

inline = choice [ rawLaTeXInline', escapedChar, special, text ] <?> "inline"

special = choice [ noteRef, inlineNote, link, referenceLink, rawHtmlInline', 
                   autoLink, image ] <?> "link, inline html, note, or image"

escapedChar = try $ do
  char '\\'
  state <- getState
  result <- if stateStrict state 
              then oneOf "\\`*_{}[]()>#+-.!"
              else satisfy (not . isAlphaNum)
  return (Str [result])

ltSign = try (do
  notFollowedBy (noneOf "<")   -- continue only if it's a <
  notFollowedBy' rawHtmlBlocks -- don't return < if it starts html
  char '<'
  return (Str ['<']))

specialCharsMinusLt = filter (/= '<') specialChars

symbol = do 
  result <- oneOf specialCharsMinusLt
  return (Str [result])

-- parses inline code, between n codeStarts and n codeEnds
code = try (do 
  starts <- many1 (char codeStart)
  let num = length starts
  result <- many1Till anyChar (try (count num (char codeEnd)))
  -- get rid of any internal newlines
  let result' = removeLeadingTrailingSpace $ joinWithSep " " $ lines result
  return (Code result'))

mathWord = many1 (choice [ (noneOf (" \t\n\\" ++ [mathEnd])), 
                           (try (do
                                   c <- char '\\'
                                   notFollowedBy (char mathEnd)
                                   return c))])

math = try (do
  failIfStrict
  char mathStart
  notFollowedBy space
  words <- sepBy1 mathWord (many1 space)
  char mathEnd
  return (TeX ("$" ++ (joinWithSep " " words) ++ "$")))

emph = do
  result <- choice [ (enclosed (char emphStart) (char emphEnd) inline), 
                     (enclosed (char emphStartAlt) (char emphEndAlt) inline) ]
  return (Emph (normalizeSpaces result))

strong = do
  result <- choice [ (enclosed (count 2 (char emphStart)) 
                               (count 2 (char emphEnd)) inline), 
                     (enclosed (count 2 (char emphStartAlt)) 
                               (count 2 (char emphEndAlt)) inline) ]
  return (Strong (normalizeSpaces result))

smartPunctuation = do
  failUnlessSmart
  choice [ quoted, apostrophe, dash, ellipses ]

apostrophe = do
  char '\'' <|> char '\8217'
  return Apostrophe

quoted = do
  doubleQuoted <|> singleQuoted 

withQuoteContext context parser = do
  oldState <- getState
  let oldQuoteContext = stateQuoteContext oldState
  setState oldState { stateQuoteContext = context }
  result <- parser
  newState <- getState
  setState newState { stateQuoteContext = oldQuoteContext }
  return result

singleQuoted = try $ do
  singleQuoteStart
  withQuoteContext InSingleQuote $ do
    notFollowedBy space
    result <- many1Till inline singleQuoteEnd
    return $ Quoted SingleQuote $ normalizeSpaces result

doubleQuoted = try $ do 
  doubleQuoteStart
  withQuoteContext InDoubleQuote $ do
    notFollowedBy space
    result <- many1Till inline doubleQuoteEnd
    return $ Quoted DoubleQuote $ normalizeSpaces result

failIfInQuoteContext context = do
  st <- getState
  if (stateQuoteContext st == context)
    then fail "already inside quotes"
    else return ()

singleQuoteStart = do 
  failIfInQuoteContext InSingleQuote
  char '\'' <|> char '\8216'

singleQuoteEnd = try (do
  char '\'' <|> char '\8217'
  notFollowedBy alphaNum)

doubleQuoteStart = do
  failIfInQuoteContext InDoubleQuote
  char '"' <|> char '\8220'

doubleQuoteEnd = char '"' <|> char '\8221'

ellipses = try (do
  oneOfStrings ["...", " . . . ", ". . .", " . . ."]
  return Ellipses)

dash = enDash <|> emDash

enDash = try (do
  char '-'
  notFollowedBy (noneOf "0123456789")
  return EnDash) 

emDash = try (do
  skipSpaces
  oneOfStrings ["---", "--"]
  skipSpaces
  return EmDash)

whitespace = do
  many1 (oneOf spaceChars) <?> "whitespace"
  return Space

tabchar = do
  tab
  return (Str "\t")

-- hard line break
linebreak = try (do
  oneOf spaceChars
  many1 (oneOf spaceChars) 
  endline
  return LineBreak )

nonEndline = noneOf endLineChars

str = do 
  result <- many1 ((noneOf (specialChars ++ spaceChars ++ endLineChars))) 
  return (Str (decodeEntities result))

-- an endline character that can be treated as a space, not a structural break
endline = try (do
  newline
  notFollowedBy blankline
  st <- getState
  if stateStrict st 
    then do
           notFollowedBy' emailBlockQuoteStart
           notFollowedBy (char atxHChar)  -- atx header
           notFollowedBy (try (do{manyTill anyChar newline; 
                                  oneOf setextHChars}))  -- setext header
    else return () 
  -- parse potential list-starts differently if in a list:
  if (stateParserContext st) == ListItemState
     then notFollowedBy' (orderedListStart <|> bulletListStart)
     else return ()
  return Space)

--
-- links
--

rawLabel = try $ do
  char labelStart
  -- allow for embedded brackets:
  raw <- manyTill (do{res <- rawLabel; return ("[" ++ res ++ "]")} <|> 
                   count 1 anyChar) (char labelEnd)
  return $ concat raw 

-- a reference label for a link
reference = try $ do
  raw <- rawLabel
  oldInput <- getInput
  setInput raw
  label <- many inline
  setInput oldInput
  return (normalizeSpaces label)
 
-- source for a link, with optional title
source = try (do 
  char srcStart
  option ' ' (char autoLinkStart)
  src <- many (noneOf ([srcEnd, autoLinkEnd] ++ titleOpeners))
  option ' ' (char autoLinkEnd)
  tit <- option "" title
  skipSpaces
  char srcEnd
  return (Src (removeTrailingSpace src) tit))

titleWith startChar endChar = try (do
  skipSpaces
  option ' ' newline -- a title can be on the next line from the source
  skipSpaces
  char startChar
  tit <- manyTill anyChar (try (do
                                  char endChar
                                  skipSpaces
                                  notFollowedBy (noneOf ")\n")))
  let tit' = gsub "\"" "&quot;" tit
  return tit')

title = choice [ titleWith '(' ')', 
                 titleWith '"' '"', 
                 titleWith '\'' '\''] <?> "title"

link = choice [explicitLink, referenceLink] <?> "link"

explicitLink = try (do
  label <- reference
  src <- source 
  return (Link label src)) 

referenceLink = choice [referenceLinkDouble, referenceLinkSingle]

-- a link like [this][ref]
referenceLinkDouble = try (do
  label <- reference
  skipSpaces
  option ' ' newline
  skipSpaces
  ref <- reference 
  let ref' = if null ref then label else ref
  state <- getState
  if ref' `elem` (stateKeysUsed state)
     then return ()
     else fail "no corresponding key"
  return (Link label (Ref ref'))) 

-- a link like [this]
referenceLinkSingle = try (do
  label <- reference
  state <- getState
  if label `elem` (stateKeysUsed state)
     then return ()
     else fail "no corresponding key"
  return (Link label (Ref label))) 

-- a link <like.this.com>
autoLink = try (do
  notFollowedBy' (anyHtmlTag <|> anyHtmlEndTag)
  src <- between (char autoLinkStart) (char autoLinkEnd) 
         (many (noneOf (spaceChars ++ endLineChars ++ [autoLinkEnd])))
  case (matchRegex emailAddress src) of
    Just _  -> return (Link [Str src] (Src ("mailto:" ++ src) ""))
    Nothing -> return (Link [Str src] (Src src ""))) 

emailAddress = 
    mkRegex "([^@:/]+)@(([^.]+[.]?)*([^.]+))"  -- presupposes no whitespace

image = try (do
  char imageStart
  (Link label src) <- link
  return (Image label src)) 

noteMarker = try (do
  char labelStart
  char noteStart
  manyTill (noneOf " \t\n") (char labelEnd))

noteRef = try (do
  failIfStrict
  ref <- noteMarker
  state <- getState
  let identifiers = (stateNoteIdentifiers state) ++ [ref] 
  setState state {stateNoteIdentifiers = identifiers}
  return (NoteRef (show (length identifiers))))

inlineNote = try (do
  failIfStrict
  char noteStart
  char labelStart
  contents <- manyTill inline (char labelEnd)
  state <- getState
  let identifiers = stateNoteIdentifiers state
  let ref = show $ (length identifiers) + 1
  let noteBlocks = stateNoteBlocks state
  setState state {stateNoteIdentifiers = (identifiers ++ [ref]),
                        stateNoteBlocks = 
                                 (Note ref [Para contents]):noteBlocks}
  return (NoteRef ref))

rawLaTeXInline' = do
  failIfStrict
  rawLaTeXInline

rawHtmlInline' = do
  st <- getState
  result <- if stateStrict st
              then choice [htmlBlockElement, anyHtmlTag, anyHtmlEndTag] 
              else choice [htmlBlockElement, anyHtmlInlineTag]
  return (HtmlInline result)