Merge branch 'mpickering-errortype'

21 files changed, 314 insertions, 210 deletions

diff --git a/src/Text/Pandoc/Error.hs b/src/Text/Pandoc/Error.hs
new file mode 100644
index 000000000..73d1e8f08
--- /dev/null
+++ b/src/Text/Pandoc/Error.hs
@@ -0,0 +1,64 @@
+{-
+Copyright (C) 2006-2015 John MacFarlane <jgm@berkeley.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.Error
+   Copyright   : Copyright (C) 2006-2015 John MacFarlane
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : John MacFarlane <jgm@berkeley.edu>
+   Stability   : alpha
+   Portability : portable
+
+This module provides a standard way to deal with possible errors encounted
+during parsing.
+
+-}
+module Text.Pandoc.Error (PandocError(..), handleError) where
+
+import Text.Parsec.Error
+import Text.Parsec.Pos hiding (Line)
+import Text.Pandoc.Compat.Except
+
+type Input = String
+
+data PandocError = -- | Generic parse failure
+                   ParseFailure String
+                 -- | Error thrown by a Parsec parser
+                 | ParsecError Input ParseError
+                 deriving (Show)
+
+
+instance Error PandocError where
+  strMsg = ParseFailure
+
+
+-- | An unsafe method to handle `PandocError`s.
+handleError :: Either PandocError a -> a
+handleError (Right r) = r
+handleError (Left err) =
+  case err of
+    ParseFailure string -> error string
+    ParsecError input err' ->
+        let errPos = errorPos err'
+            errLine = sourceLine errPos
+            errColumn = sourceColumn errPos
+            theline = (lines input ++ [""]) !! (errLine - 1)
+        in  error $ "\nError at " ++ show  err' ++ "\n" ++
+                theline ++ "\n" ++ replicate (errColumn - 1) ' ' ++
+                "^"
+
diff --git a/src/Text/Pandoc/ImageSize.hs b/src/Text/Pandoc/ImageSize.hs
index 68b34dcf3..8f0a991ba 100644
--- a/src/Text/Pandoc/ImageSize.hs
+++ b/src/Text/Pandoc/ImageSize.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE OverloadedStrings, ScopedTypeVariables  #-}
+{-# OPTIONS_GHC -fno-warn-type-defaults #-}
 {-
   Copyright (C) 2011-2014 John MacFarlane <jgm@berkeley.edu>
 
@@ -38,8 +39,11 @@ import Control.Monad
 import Data.Bits
 import Data.Binary
 import Data.Binary.Get
-import Text.Pandoc.Shared (safeRead)
+import Text.Pandoc.Shared (safeRead, hush)
 import qualified Data.Map as M
+import Text.Pandoc.Compat.Except
+import Control.Monad.Trans
+import Data.Maybe (fromMaybe)
 
 -- quick and dirty functions to get image sizes
 -- algorithms borrowed from wwwis.pl
@@ -64,7 +68,7 @@ imageType img = case B.take 4 img of
                      "%!PS"
                        | (B.take 4 $ B.drop 1 $ B.dropWhile (/=' ') img) == "EPSF"
                                         -> return Eps
-                     _                  -> fail "Unknown image type"
+                     _                  -> (hush . Left) "Unknown image type"
 
 imageSize :: ByteString -> Maybe ImageSize
 imageSize img = do
@@ -114,7 +118,7 @@ pngSize img = do
                 ([w1,w2,w3,w4,h1,h2,h3,h4] :: [Integer]) -> return
                     ((shift w1 24) + (shift w2 16) + (shift w3 8) + w4,
                      (shift h1 24) + (shift h2 16) + (shift h3 8) + h4)
-                _ -> fail "PNG parse error"
+                _ -> (hush . Left) "PNG parse error"
   let (dpix, dpiy) = findpHYs rest''
   return $ ImageSize { pxX  = x, pxY = y, dpiX = dpix, dpiY = dpiy }
 
@@ -143,7 +147,7 @@ gifSize img = do
                           dpiX = 72,
                           dpiY = 72
                           }
-       _             -> fail "GIF parse error"
+       _             -> (hush . Left) "GIF parse error"
 
 jpegSize :: ByteString -> Maybe ImageSize
 jpegSize img = do
@@ -174,36 +178,37 @@ findJfifSize bs = do
        Just (c,bs'') | c >= '\xc0' && c <= '\xc3' -> do
          case map fromIntegral $ unpack $ B.take 4 $ B.drop 3 bs'' of
               [h1,h2,w1,w2] -> return (shift w1 8 + w2, shift h1 8 + h2)
-              _             -> fail "JPEG parse error"
+              _             -> (hush . Left) "JPEG parse error"
        Just (_,bs'') ->  do
          case map fromIntegral $ unpack $ B.take 2 bs'' of
               [c1,c2] -> do
                 let len = shift c1 8 + c2
                 -- skip variables
                 findJfifSize $ B.drop len bs''
-              _       -> fail "JPEG parse error"
-       Nothing -> fail "Did not find length record"
+              _       -> (hush . Left) "JPEG parse error"
+       Nothing -> (hush . Left) "Did not find length record"
 
 exifSize :: ByteString -> Maybe ImageSize
-exifSize bs = runGet (Just <$> exifHeader bl) bl
+exifSize bs = hush . runGet header $ bl
   where bl = BL.fromChunks [bs]
+        header = runExceptT $ exifHeader bl
 -- NOTE:  It would be nicer to do
 -- runGet ((Just <$> exifHeader) <|> return Nothing)
 -- which would prevent pandoc from raising an error when an exif header can't
 -- be parsed.  But we only get an Alternative instance for Get in binary 0.6,
 -- and binary 0.5 ships with ghc 7.6.
 
-exifHeader :: BL.ByteString -> Get ImageSize
+exifHeader :: BL.ByteString -> ExceptT String Get ImageSize
 exifHeader hdr = do
-  _app1DataSize <- getWord16be
-  exifHdr <- getWord32be
-  unless (exifHdr == 0x45786966) $ fail "Did not find exif header"
-  zeros <- getWord16be
-  unless (zeros == 0) $ fail "Expected zeros after exif header"
+  _app1DataSize <- lift getWord16be
+  exifHdr <- lift getWord32be
+  unless (exifHdr == 0x45786966) $ throwError "Did not find exif header"
+  zeros <- lift getWord16be
+  unless (zeros == 0) $ throwError "Expected zeros after exif header"
   -- beginning of tiff header -- we read whole thing to use
   -- in getting data from offsets:
   let tiffHeader = BL.drop 8 hdr
-  byteAlign <- getWord16be
+  byteAlign <- lift getWord16be
   let bigEndian = byteAlign == 0x4d4d
   let (getWord16, getWord32, getWord64) =
         if bigEndian
@@ -213,17 +218,17 @@ exifHeader hdr = do
         num <- getWord32
         den <- getWord32
         return $ fromIntegral num / fromIntegral den
-  tagmark <- getWord16
-  unless (tagmark == 0x002a) $ fail "Failed alignment sanity check"
-  ifdOffset <- getWord32
-  skip (fromIntegral ifdOffset - 8) -- skip to IDF
-  numentries <- getWord16
-  let ifdEntry = do
-       tag <- getWord16 >>= \t ->
-                maybe (return UnknownTagType) return
-                (M.lookup t tagTypeTable)
-       dataFormat <- getWord16
-       numComponents <- getWord32
+  tagmark <- lift getWord16
+  unless (tagmark == 0x002a) $ throwError "Failed alignment sanity check"
+  ifdOffset <- lift getWord32
+  lift $ skip (fromIntegral ifdOffset - 8) -- skip to IDF
+  numentries <- lift  getWord16
+  let ifdEntry :: ExceptT String Get (TagType, DataFormat)
+      ifdEntry = do
+       tag <- fromMaybe UnknownTagType . flip M.lookup tagTypeTable
+                <$> lift getWord16
+       dataFormat <- lift getWord16
+       numComponents <- lift getWord32
        (fmt, bytesPerComponent) <-
              case dataFormat of
                   1  -> return (UnsignedByte . runGet getWord8, 1)
@@ -238,9 +243,10 @@ exifHeader hdr = do
                   10 -> return (SignedRational . runGet getRational, 8)
                   11 -> return (SingleFloat . runGet getWord32 {- TODO -}, 4)
                   12 -> return (DoubleFloat . runGet getWord64 {- TODO -}, 8)
-                  _  -> fail $ "Unknown data format " ++ show dataFormat
+                  _  -> throwError $ "Unknown data format " ++ show dataFormat
        let totalBytes = fromIntegral $ numComponents * bytesPerComponent
-       payload <- if totalBytes <= 4 -- data is right here
+       payload <- lift $
+                    if totalBytes <= 4 -- data is right here
                      then fmt <$>
                           (getLazyByteString (fromIntegral totalBytes) <*
                           skip (4 - totalBytes))
@@ -252,9 +258,9 @@ exifHeader hdr = do
   entries <- sequence $ replicate (fromIntegral numentries) ifdEntry
   subentries <- case lookup ExifOffset entries of
                       Just (UnsignedLong offset) -> do
-                        pos <- bytesRead
-                        skip (fromIntegral offset - (fromIntegral pos - 8))
-                        numsubentries <- getWord16
+                        pos <- lift bytesRead
+                        lift $ skip (fromIntegral offset - (fromIntegral pos - 8))
+                        numsubentries <- lift getWord16
                         sequence $
                            replicate (fromIntegral numsubentries) ifdEntry
                       _ -> return []
diff --git a/src/Text/Pandoc/Parsing.hs b/src/Text/Pandoc/Parsing.hs
index aebdcae4c..c18aa331f 100644
--- a/src/Text/Pandoc/Parsing.hs
+++ b/src/Text/Pandoc/Parsing.hs
@@ -190,6 +190,8 @@ import Control.Applicative ((<$>), (<*>), (*>), (<*), (<$))
 import Data.Monoid
 import Data.Maybe (catMaybes)
 
+import Text.Pandoc.Error
+
 type Parser t s = Parsec t s
 
 type ParserT = ParsecT
@@ -845,25 +847,16 @@ readWithM :: (Monad m, Functor m)
           => ParserT [Char] st m a       -- ^ parser
           -> st                       -- ^ initial state
           -> String                   -- ^ input
-          -> m a
+          -> m (Either PandocError a)
 readWithM parser state input =
-    handleError <$> (runParserT parser state "source" input)
-    where
-      handleError (Left err') =
-        let errPos = errorPos err'
-            errLine = sourceLine errPos
-            errColumn = sourceColumn errPos
-            theline = (lines input ++ [""]) !! (errLine - 1)
-        in  error $ "\nError at " ++ show  err' ++ "\n" ++
-                theline ++ "\n" ++ replicate (errColumn - 1) ' ' ++
-                "^"
-      handleError (Right result) = result
+    mapLeft (ParsecError input) <$> runParserT parser state "source" input
+
 
 -- | Parse a string with a given parser and state
 readWith :: Parser [Char] st a
          -> st
          -> String
-         -> a
+         -> Either PandocError a
 readWith p t inp = runIdentity $ readWithM p t inp
 
 returnWarnings :: (Stream s m c)
diff --git a/src/Text/Pandoc/Pretty.hs b/src/Text/Pandoc/Pretty.hs
index 2f2656086..9a97dfc21 100644
--- a/src/Text/Pandoc/Pretty.hs
+++ b/src/Text/Pandoc/Pretty.hs
@@ -250,6 +250,11 @@ renderDoc :: (IsString a, Monoid a)
           => Doc -> DocState a
 renderDoc = renderList . toList . unDoc
 
+data IsBlock = IsBlock Int [String]
+
+-- This would be nicer with a pattern synonym
+-- pattern VBlock i s <- mkIsBlock -> Just (IsBlock ..)
+
 renderList :: (IsString a, Monoid a)
            => [D] -> DocState a
 renderList [] = return ()
@@ -323,11 +328,11 @@ renderList (BreakingSpace : xs) = do
           outp 1 " "
           renderList xs'
 
-renderList (b1@Block{} : b2@Block{} : xs) =
-  renderList (mergeBlocks False b1 b2 : xs)
+renderList (Block i1 s1 : Block i2 s2  : xs) =
+  renderList (mergeBlocks False (IsBlock i1 s1) (IsBlock i2 s2) : xs)
 
-renderList (b1@Block{} : BreakingSpace : b2@Block{} : xs) =
-  renderList (mergeBlocks True b1 b2 : xs)
+renderList (Block i1 s1 : BreakingSpace : Block i2 s2 : xs) =
+  renderList (mergeBlocks True (IsBlock i1 s1) (IsBlock i2 s2) : xs)
 
 renderList (Block width lns : xs) = do
   st <- get
@@ -339,15 +344,14 @@ renderList (Block width lns : xs) = do
   modify $ \s -> s{ prefix = oldPref }
   renderList xs
 
-mergeBlocks :: Bool -> D -> D -> D
-mergeBlocks addSpace (Block w1 lns1) (Block w2 lns2) =
+mergeBlocks :: Bool -> IsBlock -> IsBlock -> D
+mergeBlocks addSpace (IsBlock w1 lns1) (IsBlock w2 lns2) =
   Block (w1 + w2 + if addSpace then 1 else 0) $
      zipWith (\l1 l2 -> pad w1 l1 ++ l2) (lns1 ++ empties) (map sp lns2 ++ empties)
     where empties = replicate (abs $ length lns1 - length lns2) ""
           pad n s = s ++ replicate (n - realLength s) ' '
           sp "" = ""
           sp xs = if addSpace then (' ' : xs) else xs
-mergeBlocks _ _ _ = error "mergeBlocks tried on non-Block!"
 
 blockToDoc :: Int -> [String] -> Doc
 blockToDoc _ lns = text $ intercalate "\n" lns
diff --git a/src/Text/Pandoc/Readers/CommonMark.hs b/src/Text/Pandoc/Readers/CommonMark.hs
index f8a2ec28e..51a35c8ad 100644
--- a/src/Text/Pandoc/Readers/CommonMark.hs
+++ b/src/Text/Pandoc/Readers/CommonMark.hs
@@ -37,10 +37,11 @@ import Data.Text (unpack, pack)
 import Data.List (groupBy)
 import Text.Pandoc.Definition
 import Text.Pandoc.Options
+import Text.Pandoc.Error
 
 -- | Parse a CommonMark formatted string into a 'Pandoc' structure.
-readCommonMark :: ReaderOptions -> String -> Pandoc
-readCommonMark opts = nodeToPandoc . commonmarkToNode opts' . pack
+readCommonMark :: ReaderOptions -> String -> Either PandocError Pandoc
+readCommonMark opts = Right . nodeToPandoc . commonmarkToNode opts' . pack
   where opts' = if readerSmart opts
                    then [optNormalize, optSmart]
                    else [optNormalize]
diff --git a/src/Text/Pandoc/Readers/DocBook.hs b/src/Text/Pandoc/Readers/DocBook.hs
index 663960a87..f82158ab4 100644
--- a/src/Text/Pandoc/Readers/DocBook.hs
+++ b/src/Text/Pandoc/Readers/DocBook.hs
@@ -15,6 +15,9 @@ import Control.Applicative ((<$>))
 import Data.List (intersperse)
 import Data.Maybe (fromMaybe)
 import Text.TeXMath (readMathML, writeTeX)
+import Text.Pandoc.Error (PandocError)
+import Text.Pandoc.Compat.Except
+import Data.Default
 
 {-
 
@@ -497,7 +500,7 @@ List of all DocBook tags, with [x] indicating implemented,
 [x] ?asciidoc-br? - line break from asciidoc docbook output
 -}
 
-type DB = State DBState
+type DB = ExceptT PandocError (State DBState)
 
 data DBState = DBState{ dbSectionLevel :: Int
                       , dbQuoteType    :: QuoteType
@@ -507,16 +510,18 @@ data DBState = DBState{ dbSectionLevel :: Int
                       , dbFigureTitle  :: Inlines
                       } deriving Show
 
-readDocBook :: ReaderOptions -> String -> Pandoc
-readDocBook _ inp  = Pandoc (dbMeta st') (toList $ mconcat bs)
-  where (bs, st') = runState (mapM parseBlock $ normalizeTree $ parseXML inp')
-                             DBState{ dbSectionLevel = 0
-                                    , dbQuoteType = DoubleQuote
-                                    , dbMeta = mempty
-                                    , dbAcceptsMeta = False
-                                    , dbBook = False
-                                    , dbFigureTitle = mempty
-                                    }
+instance Default DBState where
+  def = DBState{ dbSectionLevel = 0
+               , dbQuoteType = DoubleQuote
+               , dbMeta = mempty
+               , dbAcceptsMeta = False
+               , dbBook = False
+               , dbFigureTitle = mempty }
+
+
+readDocBook :: ReaderOptions -> String -> Either PandocError Pandoc
+readDocBook _ inp  = (\blocks -> Pandoc (dbMeta st') (toList . mconcat $ blocks)) <$>  bs
+  where (bs , st') = flip runState def . runExceptT . mapM parseBlock . normalizeTree . parseXML $ inp'
         inp' = handleInstructions inp
 
 -- We treat <?asciidoc-br?> specially (issue #1236), converting it
diff --git a/src/Text/Pandoc/Readers/Docx.hs b/src/Text/Pandoc/Readers/Docx.hs
index d61cc8b1b..67a97ae85 100644
--- a/src/Text/Pandoc/Readers/Docx.hs
+++ b/src/Text/Pandoc/Readers/Docx.hs
@@ -96,14 +96,17 @@ import Control.Applicative ((<$>))
 import Data.Sequence (ViewL(..), viewl)
 import qualified Data.Sequence as Seq (null)
 
+import Text.Pandoc.Error
+import Text.Pandoc.Compat.Except
+
 readDocx :: ReaderOptions
          -> B.ByteString
-         -> (Pandoc, MediaBag)
+         -> Either PandocError (Pandoc, MediaBag)
 readDocx opts bytes =
   case archiveToDocx (toArchive bytes) of
-    Right docx -> (Pandoc meta blks, mediaBag) where
-      (meta, blks, mediaBag) = (docxToOutput opts docx)
-    Left _   -> error $ "couldn't parse docx file"
+    Right docx -> (\(meta, blks, mediaBag) -> (Pandoc meta blks, mediaBag))
+                    <$> (docxToOutput opts docx)
+    Left _   -> Left (ParseFailure "couldn't parse docx file")
 
 data DState = DState { docxAnchorMap :: M.Map String String
                      , docxMediaBag      :: MediaBag
@@ -122,10 +125,10 @@ data DEnv = DEnv { docxOptions  :: ReaderOptions
 instance Default DEnv where
   def = DEnv def False
 
-type DocxContext = ReaderT DEnv (State DState)
+type DocxContext = ExceptT PandocError (ReaderT DEnv (State DState))
 
-evalDocxContext :: DocxContext a -> DEnv -> DState -> a
-evalDocxContext ctx env st = evalState (runReaderT ctx env) st
+evalDocxContext :: DocxContext a -> DEnv -> DState -> Either PandocError a
+evalDocxContext ctx env st = flip evalState st . flip runReaderT env . runExceptT $ ctx
 
 -- This is empty, but we put it in for future-proofing.
 spansToKeep :: [String]
@@ -551,7 +554,7 @@ bodyToOutput (Body bps) = do
             blks',
             mediaBag)
 
-docxToOutput :: ReaderOptions -> Docx -> (Meta, [Block], MediaBag)
+docxToOutput :: ReaderOptions -> Docx -> Either PandocError (Meta, [Block], MediaBag)
 docxToOutput opts (Docx (Document _ body)) =
   let dEnv   = def { docxOptions  = opts} in
    evalDocxContext (bodyToOutput body) dEnv def
diff --git a/src/Text/Pandoc/Readers/EPUB.hs b/src/Text/Pandoc/Readers/EPUB.hs
index b061d8683..338540533 100644
--- a/src/Text/Pandoc/Readers/EPUB.hs
+++ b/src/Text/Pandoc/Readers/EPUB.hs
@@ -35,18 +35,20 @@ import Control.DeepSeq.Generics (deepseq, NFData)
 
 import Debug.Trace (trace)
 
+import Text.Pandoc.Error
+
 type Items = M.Map String (FilePath, MimeType)
 
-readEPUB :: ReaderOptions -> BL.ByteString -> (Pandoc, MediaBag)
+readEPUB :: ReaderOptions -> BL.ByteString -> Either PandocError (Pandoc, MediaBag)
 readEPUB opts bytes = runEPUB (archiveToEPUB opts $ toArchive bytes)
 
-runEPUB :: Except String a -> a
-runEPUB = either error id . runExcept
+runEPUB :: Except PandocError a -> Either PandocError a
+runEPUB = runExcept
 
 -- Note that internal reference are aggresively normalised so that all ids
 -- are of the form "filename#id"
 --
-archiveToEPUB :: (MonadError String m) => ReaderOptions -> Archive -> m (Pandoc, MediaBag)
+archiveToEPUB :: (MonadError PandocError m) => ReaderOptions -> Archive -> m (Pandoc, MediaBag)
 archiveToEPUB os archive = do
   -- root is path to folder with manifest file in
   (root, content) <- getManifest archive
@@ -64,19 +66,20 @@ archiveToEPUB os archive = do
   return $ (ast, mediaBag)
   where
     os' = os {readerParseRaw = True}
-    parseSpineElem :: MonadError String m => FilePath -> (FilePath, MimeType) -> m Pandoc
+    parseSpineElem :: MonadError PandocError m => FilePath -> (FilePath, MimeType) -> m Pandoc
     parseSpineElem (normalise -> r) (normalise -> path, mime) = do
       when (readerTrace os) (traceM path)
       doc <- mimeToReader mime r path
       let docSpan = B.doc $ B.para $ B.spanWith (takeFileName path, [], []) mempty
       return $ docSpan <> doc
-    mimeToReader :: MonadError String m => MimeType -> FilePath -> FilePath -> m Pandoc
+    mimeToReader :: MonadError PandocError m => MimeType -> FilePath -> FilePath -> m Pandoc
     mimeToReader "application/xhtml+xml" (normalise -> root) (normalise -> path) = do
       fname <- findEntryByPathE (root </> path) archive
-      return $ fixInternalReferences path .
+      html <- either throwError return .
                 readHtml os' .
                   UTF8.toStringLazy $
                     fromEntry fname
+      return $ fixInternalReferences path html
     mimeToReader s _ path
       | s `elem` imageMimes = return $ imageToPandoc path
       | otherwise = return $ mempty
@@ -114,7 +117,7 @@ imageMimes = ["image/gif", "image/jpeg", "image/png"]
 
 type CoverImage = FilePath
 
-parseManifest :: (MonadError String m) => Element -> m (Maybe CoverImage, Items)
+parseManifest :: (MonadError PandocError m) => Element -> m (Maybe CoverImage, Items)
 parseManifest content = do
   manifest <- findElementE (dfName "manifest") content
   let items = findChildren (dfName "item") manifest
@@ -130,7 +133,7 @@ parseManifest content = do
       mime <- findAttrE (emptyName "media-type") e
       return (uid, (href, mime))
 
-parseSpine :: MonadError String m => Items -> Element -> m [(FilePath, MimeType)]
+parseSpine :: MonadError PandocError m => Items -> Element -> m [(FilePath, MimeType)]
 parseSpine is e = do
   spine <- findElementE (dfName "spine") e
   let itemRefs = findChildren (dfName "itemref") spine
@@ -141,7 +144,7 @@ parseSpine is e = do
       guard linear
       findAttr (emptyName "idref") ref
 
-parseMeta :: MonadError String m => Element -> m Meta
+parseMeta :: MonadError PandocError m => Element -> m Meta
 parseMeta content = do
   meta <- findElementE (dfName "metadata") content
   let dcspace (QName _ (Just "http://purl.org/dc/elements/1.1/") (Just "dc")) = True
@@ -159,7 +162,7 @@ renameMeta :: String -> String
 renameMeta "creator" = "author"
 renameMeta s = s
 
-getManifest :: MonadError String m => Archive -> m (String, Element)
+getManifest :: MonadError PandocError m => Archive -> m (String, Element)
 getManifest archive = do
   metaEntry <- findEntryByPathE ("META-INF" </> "container.xml") archive
   docElem <- (parseXMLDocE . UTF8.toStringLazy . fromEntry) metaEntry
@@ -266,18 +269,18 @@ emptyName s = QName s Nothing Nothing
 
 -- Convert Maybe interface to Either
 
-findAttrE :: MonadError String m => QName -> Element -> m String
+findAttrE :: MonadError PandocError m => QName -> Element -> m String
 findAttrE q e = mkE "findAttr" $ findAttr q e
 
-findEntryByPathE :: MonadError String m => FilePath -> Archive -> m Entry
+findEntryByPathE :: MonadError PandocError m => FilePath -> Archive -> m Entry
 findEntryByPathE (normalise -> path) a =
   mkE ("No entry on path: " ++ path) $ findEntryByPath path a
 
-parseXMLDocE :: MonadError String m => String -> m Element
+parseXMLDocE :: MonadError PandocError m => String -> m Element
 parseXMLDocE doc = mkE "Unable to parse XML doc" $ parseXMLDoc doc
 
-findElementE :: MonadError String m => QName -> Element -> m Element
+findElementE :: MonadError PandocError m => QName -> Element -> m Element
 findElementE e x = mkE ("Unable to find element: " ++ show e) $ findElement e x
 
-mkE :: MonadError String m => String -> Maybe a -> m a
-mkE s = maybe (throwError s) return
+mkE :: MonadError PandocError m => String -> Maybe a -> m a
+mkE s = maybe (throwError . ParseFailure $ s) return
diff --git a/src/Text/Pandoc/Readers/HTML.hs b/src/Text/Pandoc/Readers/HTML.hs
index 02ff07e73..59f71589e 100644
--- a/src/Text/Pandoc/Readers/HTML.hs
+++ b/src/Text/Pandoc/Readers/HTML.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses,
+ViewPatterns#-}
 {-
 Copyright (C) 2006-2014 John MacFarlane <jgm@berkeley.edu>
 
@@ -43,7 +44,7 @@ import Text.Pandoc.Definition
 import qualified Text.Pandoc.Builder as B
 import Text.Pandoc.Builder (Blocks, Inlines, trimInlines, HasMeta(..))
 import Text.Pandoc.Shared ( extractSpaces, renderTags'
-                          , escapeURI, safeRead )
+                          , escapeURI, safeRead, mapLeft )
 import Text.Pandoc.Options (ReaderOptions(readerParseRaw, readerTrace)
                            , Extension (Ext_epub_html_exts,
                                Ext_native_divs, Ext_native_spans))
@@ -62,15 +63,18 @@ import Text.TeXMath (readMathML, writeTeX)
 import Data.Default (Default (..), def)
 import Control.Monad.Reader (Reader,ask, asks, local, runReader)
 
+import Text.Pandoc.Error
+
+import Text.Parsec.Error
+
 
 -- | Convert HTML-formatted string to 'Pandoc' document.
 readHtml :: ReaderOptions -- ^ Reader options
          -> String        -- ^ String to parse (assumes @'\n'@ line endings)
-         -> Pandoc
+         -> Either PandocError Pandoc
 readHtml opts inp =
-  case flip runReader def $ runParserT parseDoc (HTMLState def{ stateOptions = opts } [])  "source" tags of
-          Left err'    -> error $ "\nError at " ++ show  err'
-          Right result -> result
+    mapLeft (ParseFailure . getError) . flip runReader def $
+      runParserT parseDoc (HTMLState def{ stateOptions = opts } []) "source" tags
     where tags = stripPrefixes . canonicalizeTags $
                    parseTagsOptions parseOptions{ optTagPosition = True } inp
           parseDoc = do
@@ -78,6 +82,9 @@ readHtml opts inp =
              meta <- stateMeta . parserState <$> getState
              bs' <- replaceNotes (B.toList blocks)
              return $ Pandoc meta bs'
+          getError (errorMessages -> ms) = case ms of
+                                                []    -> ""
+                                                (m:_) -> messageString m
 
 replaceNotes :: [Block] -> TagParser [Block]
 replaceNotes = walkM replaceNotes'
diff --git a/src/Text/Pandoc/Readers/Haddock.hs b/src/Text/Pandoc/Readers/Haddock.hs
index c03382c17..aa2534afc 100644
--- a/src/Text/Pandoc/Readers/Haddock.hs
+++ b/src/Text/Pandoc/Readers/Haddock.hs
@@ -26,15 +26,17 @@ import Documentation.Haddock.Parser
 import Documentation.Haddock.Types
 import Debug.Trace (trace)
 
+import Text.Pandoc.Error
+
 -- | Parse Haddock markup and return a 'Pandoc' document.
 readHaddock :: ReaderOptions -- ^ Reader options
             -> String        -- ^ String to parse
-            -> Pandoc
+            -> Either PandocError Pandoc
 readHaddock opts =
 #if MIN_VERSION_haddock_library(1,2,0)
-  B.doc . docHToBlocks . trace' . _doc . parseParas
+  Right . B.doc . docHToBlocks . trace' . _doc . parseParas
 #else
-  B.doc . docHToBlocks . trace' . parseParas
+  Right .  B.doc . docHToBlocks . trace' . parseParas
 #endif
   where trace' x = if readerTrace opts
                       then trace (show x) x
diff --git a/src/Text/Pandoc/Readers/LaTeX.hs b/src/Text/Pandoc/Readers/LaTeX.hs
index 31ac37bf1..a517f9566 100644
--- a/src/Text/Pandoc/Readers/LaTeX.hs
+++ b/src/Text/Pandoc/Readers/LaTeX.hs
@@ -57,11 +57,12 @@ import qualified Data.Map as M
 import qualified Control.Exception as E
 import System.FilePath (takeExtension, addExtension)
 import Text.Pandoc.Highlighting (fromListingsLanguage)
+import Text.Pandoc.Error
 
 -- | Parse LaTeX from string and return 'Pandoc' document.
 readLaTeX :: ReaderOptions -- ^ Reader options
           -> String        -- ^ String to parse (assumes @'\n'@ line endings)
-          -> Pandoc
+          -> Either PandocError Pandoc
 readLaTeX opts = readWith parseLaTeX def{ stateOptions = opts }
 
 parseLaTeX :: LP Pandoc
@@ -853,12 +854,8 @@ rawEnv name = do
 type IncludeParser = ParserT [Char] [String] IO String
 
 -- | Replace "include" commands with file contents.
-handleIncludes :: String -> IO String
-handleIncludes s = do
-  res <- runParserT includeParser' [] "input" s
-  case res of
-       Right s'    -> return s'
-       Left e      -> error $ show e
+handleIncludes :: String -> IO (Either PandocError String)
+handleIncludes s =  mapLeft (ParsecError s) <$> runParserT includeParser' [] "input" s
 
 includeParser' :: IncludeParser
 includeParser' =
diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 8892f60fb..369c889d1 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE RelaxedPolyRec #-} -- needed for inlinesBetween on GHC < 7
+{-# LANGUAGE ScopedTypeVariables #-}
 {-
 Copyright (C) 2006-2014 John MacFarlane <jgm@berkeley.edu>
 
@@ -64,13 +65,14 @@ import Text.HTML.TagSoup.Match (tagOpen)
 import qualified Data.Set as Set
 import Text.Printf (printf)
 import Debug.Trace (trace)
+import Text.Pandoc.Error
 
 type MarkdownParser a = ParserT [Char] ParserState (Reader ParserState) a
 
 -- | Read markdown from an input string and return a Pandoc document.
 readMarkdown :: ReaderOptions -- ^ Reader options
              -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-             -> Pandoc
+             -> Either PandocError Pandoc
 readMarkdown opts s =
   runMarkdown opts s parseMarkdown
 
@@ -78,16 +80,17 @@ readMarkdown opts s =
 -- and a list of warnings.
 readMarkdownWithWarnings :: ReaderOptions -- ^ Reader options
                          -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-                         -> (Pandoc, [String])
+                        -> Either PandocError (Pandoc, [String])
 readMarkdownWithWarnings opts s = runMarkdown opts s (returnWarnings parseMarkdown)
 
-runMarkdown :: ReaderOptions -> String -> MarkdownParser a -> a
-runMarkdown opts inp p = fst res
+runMarkdown :: forall a . ReaderOptions -> String -> MarkdownParser a -> Either PandocError a
+runMarkdown opts inp p = fst <$> res
   where
     imd = readWithM (returnState p) def{ stateOptions = opts } (inp ++ "\n\n")
+    res :: Either PandocError (a, ParserState)
     res = runReader imd s
     s :: ParserState
-    s   = snd $ runReader imd s
+    s   = either def snd res
 
 --
 -- Constants and data structure definitions
@@ -246,8 +249,9 @@ yamlMetaBlock = try $ do
                          H.foldrWithKey (\k v m ->
                               if ignorable k
                                  then m
-                                 else B.setMeta (T.unpack k)
-                                            (yamlToMeta opts v) m)
+                                 else case yamlToMeta opts v of
+                                        Left _  -> m
+                                        Right v' -> B.setMeta (T.unpack k) v' m)
                            nullMeta hashmap
                 Right Yaml.Null -> return nullMeta
                 Right _ -> do
@@ -279,38 +283,42 @@ yamlMetaBlock = try $ do
 ignorable :: Text -> Bool
 ignorable t = T.pack "_" `T.isSuffixOf` t
 
-toMetaValue :: ReaderOptions -> Text -> MetaValue
-toMetaValue opts x =
-  case readMarkdown opts' (T.unpack x) of
-       Pandoc _ [Plain xs] -> MetaInlines xs
-       Pandoc _ [Para xs]
+toMetaValue :: ReaderOptions -> Text -> Either PandocError MetaValue
+toMetaValue opts x = toMeta <$> readMarkdown opts' (T.unpack x)
+  where
+    toMeta p =
+      case p of
+        Pandoc _ [Plain xs]  -> MetaInlines xs
+        Pandoc _ [Para xs]
          | endsWithNewline x -> MetaBlocks [Para xs]
          | otherwise         -> MetaInlines xs
-       Pandoc _ bs           -> MetaBlocks bs
-  where endsWithNewline t = T.pack "\n" `T.isSuffixOf` t
-        opts' = opts{readerExtensions=readerExtensions opts `Set.difference` meta_exts}
-        meta_exts = Set.fromList [ Ext_pandoc_title_block
-                                 , Ext_mmd_title_block
-                                 , Ext_yaml_metadata_block
-                                 ]
-
-yamlToMeta :: ReaderOptions -> Yaml.Value -> MetaValue
+        Pandoc _ bs           -> MetaBlocks bs
+    endsWithNewline t = T.pack "\n" `T.isSuffixOf` t
+    opts' = opts{readerExtensions=readerExtensions opts `Set.difference` meta_exts}
+    meta_exts = Set.fromList [ Ext_pandoc_title_block
+                             , Ext_mmd_title_block
+                             , Ext_yaml_metadata_block
+                             ]
+
+yamlToMeta :: ReaderOptions -> Yaml.Value -> Either PandocError MetaValue
 yamlToMeta opts (Yaml.String t) = toMetaValue opts t
 yamlToMeta _    (Yaml.Number n)
   -- avoid decimal points for numbers that don't need them:
-  | base10Exponent n >= 0     = MetaString $ show
+  | base10Exponent n >= 0     = return $ MetaString $ show
                                 $ coefficient n * (10 ^ base10Exponent n)
-  | otherwise                 = MetaString $ show n
-yamlToMeta _    (Yaml.Bool b) = MetaBool b
-yamlToMeta opts (Yaml.Array xs) = B.toMetaValue $ map (yamlToMeta opts)
-                                                $ V.toList xs
-yamlToMeta opts (Yaml.Object o) = MetaMap $ H.foldrWithKey (\k v m ->
+  | otherwise                 = return $ MetaString $ show n
+yamlToMeta _    (Yaml.Bool b) = return $ MetaBool b
+yamlToMeta opts (Yaml.Array xs) = B.toMetaValue <$> mapM (yamlToMeta opts)
+                                                  (V.toList xs)
+yamlToMeta opts (Yaml.Object o) = MetaMap <$> H.foldrWithKey (\k v m ->
                                 if ignorable k
                                    then m
-                                   else M.insert (T.unpack k)
-                                           (yamlToMeta opts v) m)
-                               M.empty o
-yamlToMeta _ _ = MetaString ""
+                                   else (do
+                                    v' <- yamlToMeta opts v
+                                    m' <- m
+                                    return (M.insert (T.unpack k) v' m')))
+                                (return M.empty) o
+yamlToMeta _ _ = return $ MetaString ""
 
 stopLine :: MarkdownParser ()
 stopLine = try $ (string "---" <|> string "...") >> blankline >> return ()
@@ -466,6 +474,7 @@ block = do
   res <- choice [ mempty <$ blanklines
                , codeBlockFenced
                , yamlMetaBlock
+               , guardEnabled Ext_latex_macros *> macro
                -- note: bulletList needs to be before header because of
                -- the possibility of empty list items: -
                , bulletList
@@ -475,7 +484,6 @@ block = do
                , htmlBlock
                , table
                , codeBlockIndented
-               , guardEnabled Ext_latex_macros *> macro
                , rawTeXBlock
                , lineBlock
                , blockQuote
diff --git a/src/Text/Pandoc/Readers/MediaWiki.hs b/src/Text/Pandoc/Readers/MediaWiki.hs
index d1ba35ba0..939d10fb2 100644
--- a/src/Text/Pandoc/Readers/MediaWiki.hs
+++ b/src/Text/Pandoc/Readers/MediaWiki.hs
@@ -58,21 +58,21 @@ import Data.Maybe (fromMaybe)
 import Text.Printf (printf)
 import Debug.Trace (trace)
 
+import Text.Pandoc.Error
+
 -- | Read mediawiki from an input string and return a Pandoc document.
 readMediaWiki :: ReaderOptions -- ^ Reader options
               -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-              -> Pandoc
+              -> Either PandocError Pandoc
 readMediaWiki opts s =
-  case runParser parseMediaWiki MWState{ mwOptions = opts
+  readWith parseMediaWiki MWState{ mwOptions = opts
                                        , mwMaxNestingLevel = 4
                                        , mwNextLinkNumber  = 1
                                        , mwCategoryLinks = []
                                        , mwHeaderMap = M.empty
                                        , mwIdentifierList = []
                                        }
-       "source" (s ++ "\n") of
-          Left err'    -> error $ "\nError:\n" ++ show err'
-          Right result -> result
+           (s ++ "\n")
 
 data MWState = MWState { mwOptions         :: ReaderOptions
                        , mwMaxNestingLevel :: Int
diff --git a/src/Text/Pandoc/Readers/Native.hs b/src/Text/Pandoc/Readers/Native.hs
index f4dfa62c1..fc6b3362a 100644
--- a/src/Text/Pandoc/Readers/Native.hs
+++ b/src/Text/Pandoc/Readers/Native.hs
@@ -3,7 +3,7 @@ Copyright (C) 2011-2014 John MacFarlane <jgm@berkeley.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
+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,
@@ -33,6 +33,9 @@ module Text.Pandoc.Readers.Native ( readNative ) where
 import Text.Pandoc.Definition
 import Text.Pandoc.Shared (safeRead)
 
+import Text.Pandoc.Error
+import Control.Applicative
+
 -- | Read native formatted text and return a Pandoc document.
 -- The input may be a full pandoc document, a block list, a block,
 -- an inline list, or an inline.  Thus, for example,
@@ -44,33 +47,18 @@ import Text.Pandoc.Shared (safeRead)
 -- > Pandoc nullMeta [Plain [Str "hi"]]
 --
 readNative :: String      -- ^ String to parse (assuming @'\n'@ line endings)
-           -> Pandoc
-readNative s =
-  case safeRead s of
-       Just d    -> d
-       Nothing   -> Pandoc nullMeta $ readBlocks s
+           -> Either PandocError Pandoc
+readNative s = maybe (Pandoc nullMeta <$> readBlocks s) Right (safeRead s)
 
-readBlocks :: String -> [Block]
-readBlocks s =
-  case safeRead s of
-       Just d    -> d
-       Nothing   -> [readBlock s]
+readBlocks :: String -> Either PandocError [Block]
+readBlocks s = maybe ((:[]) <$> readBlock s) Right (safeRead s)
 
-readBlock :: String -> Block
-readBlock s =
-  case safeRead s of
-       Just d    -> d
-       Nothing   -> Plain $ readInlines s
+readBlock :: String -> Either PandocError Block
+readBlock s = maybe (Plain <$> readInlines s) Right (safeRead s)
 
-readInlines :: String -> [Inline]
-readInlines s =
-  case safeRead s of
-       Just d     -> d
-       Nothing    -> [readInline s]
+readInlines :: String -> Either PandocError [Inline]
+readInlines s = maybe ((:[]) <$> readInline s) Right (safeRead s)
 
-readInline :: String -> Inline
-readInline s =
-  case safeRead s of
-       Just d     -> d
-       Nothing    -> error "Cannot parse document"
+readInline :: String -> Either PandocError Inline
+readInline s = maybe (Left . ParseFailure $ "Could not read: " ++ s) Right (safeRead s)
 
diff --git a/src/Text/Pandoc/Readers/OPML.hs b/src/Text/Pandoc/Readers/OPML.hs
index 35d01e877..19ddba36b 100644
--- a/src/Text/Pandoc/Readers/OPML.hs
+++ b/src/Text/Pandoc/Readers/OPML.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE FlexibleContexts #-}
 module Text.Pandoc.Readers.OPML ( readOPML ) where
 import Data.Char (toUpper)
 import Text.Pandoc.Options
@@ -11,8 +12,11 @@ import Data.Generics
 import Data.Monoid
 import Control.Monad.State
 import Control.Applicative ((<$>), (<$))
+import Data.Default
+import Text.Pandoc.Compat.Except
+import Text.Pandoc.Error
 
-type OPML = State OPMLState
+type OPML = ExceptT PandocError (State OPMLState)
 
 data OPMLState = OPMLState{
                         opmlSectionLevel :: Int
@@ -21,17 +25,19 @@ data OPMLState = OPMLState{
                       , opmlDocDate      :: Inlines
                       } deriving Show
 
-readOPML :: ReaderOptions -> String -> Pandoc
+instance Default OPMLState where
+  def = OPMLState{ opmlSectionLevel = 0
+                 , opmlDocTitle = mempty
+                 , opmlDocAuthors = []
+                 , opmlDocDate = mempty
+                  }
+
+readOPML :: ReaderOptions -> String -> Either PandocError Pandoc
 readOPML _ inp  = setTitle (opmlDocTitle st')
-                   $ setAuthors (opmlDocAuthors st')
-                   $ setDate (opmlDocDate st')
-                   $ doc $ mconcat bs
-  where (bs, st') = runState (mapM parseBlock $ normalizeTree $ parseXML inp)
-                             OPMLState{ opmlSectionLevel = 0
-                                    , opmlDocTitle = mempty
-                                    , opmlDocAuthors = []
-                                    , opmlDocDate = mempty
-                                    }
+                   . setAuthors (opmlDocAuthors st')
+                   . setDate (opmlDocDate st')
+                   . doc . mconcat <$> bs
+  where (bs, st') = flip runState def . runExceptT $ (mapM parseBlock $ normalizeTree $ parseXML inp)
 
 -- normalize input, consolidating adjacent Text and CRef elements
 normalizeTree :: [Content] -> [Content]
@@ -58,14 +64,16 @@ attrValue attr elt =
     Just z  -> z
     Nothing -> ""
 
-asHtml :: String -> Inlines
-asHtml s = case readHtml def s of
-                Pandoc _ [Plain ils] -> fromList ils
-                _ -> mempty
+exceptT :: Either PandocError a -> OPML a
+exceptT = either throwError return
+
+asHtml :: String -> OPML Inlines
+asHtml s = (\(Pandoc _ bs) -> case bs of
+                                [Plain ils] -> fromList ils
+                                _ -> mempty) <$> exceptT (readHtml def s)
 
-asMarkdown :: String -> Blocks
-asMarkdown s = fromList bs
-  where Pandoc _ bs = readMarkdown def s
+asMarkdown :: String -> OPML Blocks
+asMarkdown s = (\(Pandoc _ bs) -> fromList bs) <$> exceptT (readMarkdown def s)
 
 getBlocks :: Element -> OPML Blocks
 getBlocks e =  mconcat <$> (mapM parseBlock $ elContent e)
@@ -82,8 +90,8 @@ parseBlock (Elem e) =
         "outline" -> gets opmlSectionLevel >>= sect . (+1)
         "?xml"  -> return mempty
         _       -> getBlocks e
-   where sect n = do let headerText = asHtml $ attrValue "text" e
-                     let noteBlocks = asMarkdown $ attrValue "_note" e
+   where sect n = do headerText <- asHtml $ attrValue "text" e
+                     noteBlocks <- asMarkdown $ attrValue "_note" e
                      modify $ \st -> st{ opmlSectionLevel = n }
                      bs <- getBlocks e
                      modify $ \st -> st{ opmlSectionLevel = n - 1 }
diff --git a/src/Text/Pandoc/Readers/Org.hs b/src/Text/Pandoc/Readers/Org.hs
index 457db200b..1dfbdd700 100644
--- a/src/Text/Pandoc/Readers/Org.hs
+++ b/src/Text/Pandoc/Readers/Org.hs
@@ -60,10 +60,12 @@ import           Data.Maybe (fromMaybe, isJust)
 import           Data.Monoid (mconcat, mempty, mappend)
 import           Network.HTTP (urlEncode)
 
+import           Text.Pandoc.Error
+
 -- | Parse org-mode string and return a Pandoc document.
 readOrg :: ReaderOptions -- ^ Reader options
         -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-        -> Pandoc
+        -> Either PandocError Pandoc
 readOrg opts s = runOrg opts s parseOrg
 
 data OrgParserLocal = OrgParserLocal { orgLocalQuoteContext :: QuoteContext
@@ -71,13 +73,13 @@ data OrgParserLocal = OrgParserLocal { orgLocalQuoteContext :: QuoteContext
 
 type OrgParser = ParserT [Char] OrgParserState (Reader OrgParserLocal)
 
-runOrg :: ReaderOptions -> String -> OrgParser a -> a
-runOrg opts inp p = fst res
+runOrg :: ReaderOptions -> String -> OrgParser a -> Either PandocError a
+runOrg opts inp p = fst <$> res
   where
     imd = readWithM (returnState p) def{ orgStateOptions = opts } (inp ++ "\n\n")
     res = runReader imd def { finalState = s }
     s :: OrgParserState
-    s   = snd $ runReader imd (def { finalState = s })
+    s   = either def snd res
 
 parseOrg :: OrgParser Pandoc
 parseOrg = do
@@ -1259,17 +1261,15 @@ math      = B.math      <$> choice [ math1CharBetween '$'
 
 displayMath :: OrgParser Inlines
 displayMath = B.displayMath <$> choice [ rawMathBetween "\\[" "\\]"
-                                       , rawMathBetween "$$"  "$$" ]
-
-updatePositions :: Char
-                -> OrgParser (Char)
-updatePositions c = do
-  when (c `elem` emphasisPreChars) updateLastPreCharPos
-  when (c `elem` emphasisForbiddenBorderChars) updateLastForbiddenCharPos
-  return c
+                                       , rawMathBetween "$$"  "$$"
+                                       ]
 
 symbol :: OrgParser Inlines
 symbol = B.str . (: "") <$> (oneOf specialChars >>= updatePositions)
+ where updatePositions c = do
+         when (c `elem` emphasisPreChars) updateLastPreCharPos
+         when (c `elem` emphasisForbiddenBorderChars) updateLastForbiddenCharPos
+         return c
 
 emphasisBetween :: Char
                 -> OrgParser Inlines
diff --git a/src/Text/Pandoc/Readers/RST.hs b/src/Text/Pandoc/Readers/RST.hs
index 4ae9d52ae..a8112bc81 100644
--- a/src/Text/Pandoc/Readers/RST.hs
+++ b/src/Text/Pandoc/Readers/RST.hs
@@ -51,13 +51,15 @@ import Data.Monoid (mconcat, mempty)
 import Data.Sequence (viewr, ViewR(..))
 import Data.Char (toLower, isHexDigit, isSpace)
 
+import Text.Pandoc.Error
+
 -- | Parse reStructuredText string and return Pandoc document.
 readRST :: ReaderOptions -- ^ Reader options
         -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-        -> Pandoc
+        -> Either PandocError Pandoc
 readRST opts s = (readWith parseRST) def{ stateOptions = opts } (s ++ "\n\n")
 
-readRSTWithWarnings :: ReaderOptions -> String -> (Pandoc, [String])
+readRSTWithWarnings :: ReaderOptions -> String -> Either PandocError (Pandoc, [String])
 readRSTWithWarnings opts s = (readWith (returnWarnings parseRST)) def{ stateOptions = opts } (s ++ "\n\n")
 
 type RSTParser = Parser [Char] ParserState
diff --git a/src/Text/Pandoc/Readers/TWiki.hs b/src/Text/Pandoc/Readers/TWiki.hs
index 9f5738478..07b414431 100644
--- a/src/Text/Pandoc/Readers/TWiki.hs
+++ b/src/Text/Pandoc/Readers/TWiki.hs
@@ -48,17 +48,18 @@ import Data.Maybe (fromMaybe)
 import Text.HTML.TagSoup
 import Data.Char (isAlphaNum)
 import qualified Data.Foldable as F
+import Text.Pandoc.Error
 
 -- | Read twiki from an input string and return a Pandoc document.
 readTWiki :: ReaderOptions -- ^ Reader options
           -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-          -> Pandoc
+          -> Either PandocError Pandoc
 readTWiki opts s =
   (readWith parseTWiki) def{ stateOptions = opts } (s ++ "\n\n")
 
 readTWikiWithWarnings :: ReaderOptions -- ^ Reader options
                       -> String        -- ^ String to parse (assuming @'\n'@ line endings)
-                      -> (Pandoc, [String])
+                      -> Either PandocError (Pandoc, [String])
 readTWikiWithWarnings opts s =
   (readWith parseTWikiWithWarnings) def{ stateOptions = opts } (s ++ "\n\n")
  where parseTWikiWithWarnings = do
diff --git a/src/Text/Pandoc/Readers/Textile.hs b/src/Text/Pandoc/Readers/Textile.hs
index 63ab80eb9..4565b26a1 100644
--- a/src/Text/Pandoc/Readers/Textile.hs
+++ b/src/Text/Pandoc/Readers/Textile.hs
@@ -68,11 +68,12 @@ import Text.Printf
 import Control.Applicative ((<$>), (*>), (<*), (<$))
 import Data.Monoid
 import Debug.Trace (trace)
+import Text.Pandoc.Error
 
 -- | Parse a Textile text and return a Pandoc document.
 readTextile :: ReaderOptions -- ^ Reader options
             -> String       -- ^ String to parse (assuming @'\n'@ line endings)
-            -> Pandoc
+            -> Either PandocError Pandoc
 readTextile opts s =
   (readWith parseTextile) def{ stateOptions = opts } (s ++ "\n\n")
 
diff --git a/src/Text/Pandoc/Readers/Txt2Tags.hs b/src/Text/Pandoc/Readers/Txt2Tags.hs
index 834d18c5c..304d6d4c5 100644
--- a/src/Text/Pandoc/Readers/Txt2Tags.hs
+++ b/src/Text/Pandoc/Readers/Txt2Tags.hs
@@ -48,6 +48,7 @@ import Data.Monoid (Monoid, mconcat, mempty, mappend)
 import Control.Monad (void, guard, when)
 import Data.Default
 import Control.Monad.Reader (Reader, runReader, asks)
+import Text.Pandoc.Error
 
 import Data.Time.LocalTime (getZonedTime)
 import Text.Pandoc.Compat.Directory(getModificationTime)
@@ -83,12 +84,12 @@ getT2TMeta inps out = do
     return $ T2TMeta curDate curMtime (intercalate ", " inps) out
 
 -- | Read Txt2Tags from an input string returning a Pandoc document
-readTxt2Tags :: T2TMeta -> ReaderOptions -> String -> Pandoc
+readTxt2Tags :: T2TMeta -> ReaderOptions -> String -> Either PandocError Pandoc
 readTxt2Tags t opts s = flip runReader t $ readWithM parseT2T (def {stateOptions = opts}) (s ++ "\n\n")
 
 -- | Read Txt2Tags (ignoring all macros) from an input string returning
 -- a Pandoc document
-readTxt2TagsNoMacros :: ReaderOptions -> String -> Pandoc
+readTxt2TagsNoMacros :: ReaderOptions -> String -> Either PandocError Pandoc
 readTxt2TagsNoMacros = readTxt2Tags def
 
 parseT2T :: T2T Pandoc
diff --git a/src/Text/Pandoc/Shared.hs b/src/Text/Pandoc/Shared.hs
index bc960fd38..e0460c66e 100644
--- a/src/Text/Pandoc/Shared.hs
+++ b/src/Text/Pandoc/Shared.hs
@@ -85,6 +85,8 @@ module Text.Pandoc.Shared (
                      -- * Error handling
                      err,
                      warn,
+                     mapLeft,
+                     hush,
                      -- * Safe read
                      safeRead,
                      -- * Temp directory
@@ -113,7 +115,7 @@ import System.FilePath ( (</>), takeExtension, dropExtension)
 import Data.Generics (Typeable, Data)
 import qualified Control.Monad.State as S
 import qualified Control.Exception as E
-import Control.Monad (msum, unless)
+import Control.Monad (msum, unless, MonadPlus(..))
 import Text.Pandoc.Pretty (charWidth)
 import Text.Pandoc.Compat.Locale (defaultTimeLocale)
 import Data.Time
@@ -855,6 +857,14 @@ warn msg = do
   name <- getProgName
   UTF8.hPutStrLn stderr $ name ++ ": " ++ msg
 
+mapLeft :: (a -> b) -> Either a c -> Either b c
+mapLeft f (Left x) = Left (f x)
+mapLeft _ (Right x) = Right x
+
+hush :: Either a b -> Maybe b
+hush (Left _) = Nothing
+hush (Right x) = Just x
+
 -- | Remove intermediate "." and ".." directories from a path.
 --
 -- > collapseFilePath "./foo" == "foo"
@@ -883,11 +893,11 @@ collapseFilePath = joinPath . reverse . foldl go [] . splitDirectories
 -- Safe read
 --
 
-safeRead :: (Monad m, Read a) => String -> m a
+safeRead :: (MonadPlus m, Read a) => String -> m a
 safeRead s = case reads s of
                   (d,x):_
                     | all isSpace x -> return d
-                  _                 -> fail $ "Could not read `" ++ s ++ "'"
+                  _                 -> mzero
 
 --
 -- Temp directory