path: root/src/Text/Pandoc/Class/PandocMonad.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ViewPatterns #-}
{- |
Module      : Text.Pandoc.Class.PandocMonad
Copyright   : Copyright (C) 2016-2020 Jesse Rosenthal, John MacFarlane
License     : GNU GPL, version 2 or above

Maintainer  : Jesse Rosenthal <[email protected]>
Stability   : alpha
Portability : portable

This module defines a type class, 'PandocMonad', for pandoc readers
and writers.
-}

module Text.Pandoc.Class.PandocMonad
  ( PandocMonad(..)
  , getTimestamp
  , getPOSIXTime
  , getZonedTime
  , readFileFromDirs
  , report
  , runSilently
  , setRequestHeader
  , setNoCheckCertificate
  , getLog
  , setVerbosity
  , getVerbosity
  , setTrace
  , getTrace
  , getMediaBag
  , setMediaBag
  , insertMedia
  , setUserDataDir
  , getUserDataDir
  , fetchItem
  , extractURIData
  , getInputFiles
  , setInputFiles
  , getOutputFile
  , setOutputFile
  , setResourcePath
  , getResourcePath
  , setRequestHeaders
  , getRequestHeaders
  , getSourceURL
  , readMetadataFile
  , toTextM
  , fillMediaBag
  , toLang
  , makeCanonical
  , findFileWithDataFallback
  , checkUserDataDir
  ) where

import Control.Monad.Except (MonadError (catchError, throwError))
import Control.Monad.Trans (MonadTrans, lift)
import Control.Monad (when)
import Data.Time (UTCTime)
import Data.Time.Clock.POSIX (POSIXTime, utcTimeToPOSIXSeconds,
                             posixSecondsToUTCTime)
import Data.Time.LocalTime (TimeZone, ZonedTime, utcToZonedTime)
import Network.URI ( escapeURIString, nonStrictRelativeTo,
                     unEscapeString, parseURIReference, isAllowedInURI,
                     parseURI, URI(..) )
import System.FilePath ((</>), takeExtension, dropExtension,
                        isRelative, makeRelative)
import System.Random (StdGen)
import Text.Collate.Lang (Lang(..), parseLang)
import Text.Pandoc.Class.CommonState (CommonState (..))
import Text.Pandoc.Definition
import Text.Pandoc.Error
import Text.Pandoc.Logging
import Text.Pandoc.MIME (MimeType, getMimeType)
import Text.Pandoc.MediaBag (MediaBag, lookupMedia, MediaItem(..))
import Text.Pandoc.Shared (safeRead, makeCanonical, tshow)
import Text.Pandoc.URI (uriPathToPath, pBase64DataURI)
import qualified Data.Attoparsec.Text as A
import Text.Pandoc.Walk (walkM)
import qualified Text.Pandoc.UTF8 as UTF8
import Data.ByteString.Base64 (decodeLenient)
import Text.Parsec (ParsecT, getPosition, sourceLine, sourceName)
import qualified Data.ByteString as B
import qualified Data.ByteString.Lazy as BL
import qualified Data.Text as T
import qualified Debug.Trace
import qualified Text.Pandoc.MediaBag as MB
import qualified Data.Text.Encoding as TSE
import qualified Data.Text.Encoding.Error as TSE

-- | The PandocMonad typeclass contains all the potentially
-- IO-related functions used in pandoc's readers and writers.
-- Instances of this typeclass may implement these functions
-- in IO (as in 'PandocIO') or using an internal state that
-- represents a file system, time, and so on (as in 'PandocPure').
class (Functor m, Applicative m, Monad m, MonadError PandocError m)
      => PandocMonad m where
  -- | Lookup an environment variable.
  lookupEnv :: T.Text -> m (Maybe T.Text)
  -- | Get the current (UTC) time.
  getCurrentTime :: m UTCTime
  -- | Get the locale's time zone.
  getCurrentTimeZone :: m TimeZone
  -- | Return a new generator for random numbers.
  newStdGen :: m StdGen
  -- | Return a new unique integer.
  newUniqueHash :: m Int
  -- | Retrieve contents and mime type from a URL, raising
  -- an error on failure.
  openURL :: T.Text -> m (B.ByteString, Maybe MimeType)
  -- | Read the lazy ByteString contents from a file path,
  -- raising an error on failure.
  readFileLazy :: FilePath -> m BL.ByteString
  -- | Read the strict ByteString contents from a file path,
  -- raising an error on failure.
  readFileStrict :: FilePath -> m B.ByteString
  -- | Read the contents of stdin as a strict ByteString, raising
  -- an error on failure.
  readStdinStrict :: m B.ByteString
  -- | Return a list of paths that match a glob, relative to
  -- the working directory.  See 'System.FilePath.Glob' for
  -- the glob syntax.
  glob :: String -> m [FilePath]
  -- | Returns True if file exists.
  fileExists :: FilePath -> m Bool
  -- | Returns the path of data file.
  getDataFileName :: FilePath -> m FilePath
  -- | Return the modification time of a file.
  getModificationTime :: FilePath -> m UTCTime
  -- | Get the value of the 'CommonState' used by all instances
  -- of 'PandocMonad'.
  getCommonState :: m CommonState
  -- | Set the value of the 'CommonState' used by all instances
  -- of 'PandocMonad'.
  -- | Get the value of a specific field of 'CommonState'.
  putCommonState :: CommonState -> m ()
  -- | Get the value of a specific field of 'CommonState'.
  getsCommonState :: (CommonState -> a) -> m a
  getsCommonState f = f <$> getCommonState
  -- | Modify the 'CommonState'.
  modifyCommonState :: (CommonState -> CommonState) -> m ()
  modifyCommonState f = getCommonState >>= putCommonState . f
  -- | Output a log message.
  logOutput :: LogMessage -> m ()
  -- | Output a debug message to sterr, using 'Debug.Trace.trace',
  -- if tracing is enabled.  Note: this writes to stderr even in
  -- pure instances.
  trace :: T.Text -> m ()
  trace msg = do
    tracing <- getsCommonState stTrace
    when tracing $ Debug.Trace.trace ("[trace] " ++ T.unpack msg) (return ())

-- * Functions defined for all PandocMonad instances

-- | Set the verbosity level.
setVerbosity :: PandocMonad m => Verbosity -> m ()
setVerbosity verbosity =
  modifyCommonState $ \st -> st{ stVerbosity = verbosity }

-- | Get the verbosity level.
getVerbosity :: PandocMonad m => m Verbosity
getVerbosity = getsCommonState stVerbosity

-- | Set tracing. This affects the behavior of 'trace'. If tracing
-- is not enabled, 'trace' does nothing.
setTrace :: PandocMonad m => Bool -> m ()
setTrace enabled = modifyCommonState $ \st -> st{ stTrace = enabled }

-- | Get tracing status.
getTrace :: PandocMonad m => m Bool
getTrace = getsCommonState stTrace

-- | Get the accumulated log messages (in temporal order).
getLog :: PandocMonad m => m [LogMessage]
getLog = reverse <$> getsCommonState stLog

-- | Log a message using 'logOutput'.  Note that 'logOutput' is
-- called only if the verbosity level exceeds the level of the
-- message, but the message is added to the list of log messages
-- that will be retrieved by 'getLog' regardless of its verbosity level.
report :: PandocMonad m => LogMessage -> m ()
report msg = do
  verbosity <- getsCommonState stVerbosity
  let level = messageVerbosity msg
  when (level <= verbosity) $ logOutput msg
  modifyCommonState $ \st -> st{ stLog = msg : stLog st }

-- | Run an action, but suppress the output of any log messages;
-- instead, all messages reported by @action@ are returned separately
-- and not added to the main log.
runSilently :: PandocMonad m => m a -> m (a, [LogMessage])
runSilently action = do
  -- get current settings
  origLog <- getsCommonState stLog
  origVerbosity <- getVerbosity
  -- reset log level and set verbosity to the minimum
  modifyCommonState (\st -> st { stVerbosity = ERROR, stLog = []})
  result <- action
  -- get log messages reported while running `action`
  newLog <- getsCommonState stLog
  modifyCommonState (\st -> st { stVerbosity = origVerbosity, stLog = origLog})

  return (result, newLog)

-- | Set request header to use in HTTP requests.
setRequestHeader :: PandocMonad m
                 => T.Text  -- ^ Header name
                 -> T.Text  -- ^ Value
                 -> m ()
setRequestHeader name val = modifyCommonState $ \st ->
  st{ stRequestHeaders =
       (name, val) : filter (\(n,_) -> n /= name) (stRequestHeaders st)  }

-- | Determine whether certificate validation is disabled
setNoCheckCertificate :: PandocMonad m => Bool -> m ()
setNoCheckCertificate noCheckCertificate = modifyCommonState $ \st -> st{stNoCheckCertificate = noCheckCertificate}

-- | Initialize the media bag.
setMediaBag :: PandocMonad m => MediaBag -> m ()
setMediaBag mb = modifyCommonState $ \st -> st{stMediaBag = mb}

-- | Retrieve the media bag.
getMediaBag :: PandocMonad m => m MediaBag
getMediaBag = getsCommonState stMediaBag

-- | Insert an item into the media bag.
insertMedia :: PandocMonad m => FilePath -> Maybe MimeType -> BL.ByteString -> m ()
insertMedia fp mime bs = do
  mb <- getMediaBag
  let mb' = MB.insertMedia fp mime bs mb
  setMediaBag mb'

-- | Retrieve the input filenames.
getInputFiles :: PandocMonad m => m [FilePath]
getInputFiles = getsCommonState stInputFiles

-- | Set the input filenames.
setInputFiles :: PandocMonad m => [FilePath] -> m ()
setInputFiles fs = do
  let sourceURL = case fs of
                    []    -> Nothing
                    (x:_) -> case parseURI x of
                                Just u
                                  | uriScheme u `elem` ["http:","https:"] ->
                                      Just $ show u{ uriQuery = "",
                                                     uriFragment = "" }
                                _ -> Nothing

  modifyCommonState $ \st -> st{ stInputFiles = fs
                               , stSourceURL = T.pack <$> sourceURL }

-- | Retrieve the output filename.
getOutputFile :: PandocMonad m => m (Maybe FilePath)
getOutputFile = getsCommonState stOutputFile

-- | Set the output filename.
setOutputFile :: PandocMonad m => Maybe FilePath -> m ()
setOutputFile mbf = modifyCommonState $ \st -> st{ stOutputFile = mbf }

-- | Retrieve the resource path searched by 'fetchItem'.
getResourcePath :: PandocMonad m => m [FilePath]
getResourcePath = getsCommonState stResourcePath

-- | Set the resource path searched by 'fetchItem'.
setResourcePath :: PandocMonad m => [FilePath] -> m ()
setResourcePath ps = modifyCommonState $ \st -> st{stResourcePath = ps}

-- | Retrieve the request headers to add for HTTP requests.
getRequestHeaders :: PandocMonad m => m [(T.Text, T.Text)]
getRequestHeaders = getsCommonState stRequestHeaders

-- | Set the request headers to add for HTTP requests.
setRequestHeaders :: PandocMonad m => [(T.Text, T.Text)] -> m ()
setRequestHeaders hs = modifyCommonState $ \st -> st{ stRequestHeaders = hs }

-- | Get the absolute UL or directory of first source file.
getSourceURL :: PandocMonad m => m (Maybe T.Text)
getSourceURL = getsCommonState stSourceURL

-- | Get the current UTC time. If the @SOURCE_DATE_EPOCH@ environment
-- variable is set to a unix time (number of seconds since midnight
-- Jan 01 1970 UTC), it is used instead of the current time, to support
-- reproducible builds.
getTimestamp :: PandocMonad m => m UTCTime
getTimestamp = do
  mbSourceDateEpoch <- lookupEnv "SOURCE_DATE_EPOCH"
  case mbSourceDateEpoch >>= safeRead of
    Just (epoch :: Integer) ->
      return $ posixSecondsToUTCTime $ fromIntegral epoch
    Nothing -> getCurrentTime

-- | Get the POSIX time. If @SOURCE_DATE_EPOCH@ is set to a unix time,
-- it is used instead of the current time.
getPOSIXTime :: PandocMonad m => m POSIXTime
getPOSIXTime = utcTimeToPOSIXSeconds <$> getTimestamp

-- | Get the zoned time. If @SOURCE_DATE_EPOCH@ is set to a unix time,
-- value (POSIX time), it is used instead of the current time.
getZonedTime :: PandocMonad m => m ZonedTime
getZonedTime = do
  t <- getTimestamp
  tz <- getCurrentTimeZone
  return $ utcToZonedTime tz t

-- | Read file, checking in any number of directories.
readFileFromDirs :: PandocMonad m => [FilePath] -> FilePath -> m (Maybe T.Text)
readFileFromDirs [] _ = return Nothing
readFileFromDirs (d:ds) f = catchError
    (Just <$> (readFileStrict (d </> f) >>= toTextM (d </> f)))
    (\_ -> readFileFromDirs ds f)

-- | Convert BCP47 string to a Lang, issuing warning
-- if there are problems.
toLang :: PandocMonad m => Maybe T.Text -> m (Maybe Lang)
toLang Nothing = return Nothing
toLang (Just s) =
  case parseLang s of
       Left _ -> do
         report $ InvalidLang s
         return Nothing
       Right l -> return (Just l)

-- | Specialized version of parseURIReference that disallows
-- single-letter schemes.  Reason:  these are usually windows absolute
-- paths.
parseURIReference' :: T.Text -> Maybe URI
parseURIReference' s = do
  u <- parseURIReference (T.unpack s)
  case uriScheme u of
       [_] -> Nothing
       _   -> Just u

-- | Set the user data directory in common state.
setUserDataDir :: PandocMonad m
               => Maybe FilePath
               -> m ()
setUserDataDir mbfp = modifyCommonState $ \st -> st{ stUserDataDir = mbfp }

-- | Get the user data directory from common state.
getUserDataDir :: PandocMonad m
               => m (Maybe FilePath)
getUserDataDir = getsCommonState stUserDataDir

-- | Fetch an image or other item from the local filesystem or the net.
-- Returns raw content and maybe mime type.
fetchItem :: PandocMonad m
          => T.Text
          -> m (B.ByteString, Maybe MimeType)
fetchItem s = do
  mediabag <- getMediaBag
  case lookupMedia (T.unpack s) mediabag of
    Just item -> return (BL.toStrict (mediaContents item),
                         Just (mediaMimeType item))
    Nothing -> downloadOrRead s

-- | Returns the content and, if available, the MIME type of a resource.
-- If the given resource location is a valid URI, then download the
-- resource from that URI. Otherwise, treat the resource identifier as a
-- local file name.
--
-- Note that resources are treated relative to the URL of the first
-- input source, if any.
downloadOrRead :: PandocMonad m
               => T.Text
               -> m (B.ByteString, Maybe MimeType)
downloadOrRead s
 | "data:" `T.isPrefixOf` s,
   Right (bs, mt) <- A.parseOnly (pBase64DataURI <* A.endOfInput) s
   = pure (bs, Just mt)
 | otherwise = do
  sourceURL <- getsCommonState stSourceURL
  case (sourceURL >>= parseURIReference' . ensureEscaped, ensureEscaped s) of
    (Just u, s') -> -- try fetching from relative path at source
       case parseURIReference' s' of
            Just u' -> openURL $ T.pack $ show $ u' `nonStrictRelativeTo` u
            Nothing -> openURL s' -- will throw error
    (Nothing, s'@(T.unpack -> ('/':'/':c:_))) | c /= '?' ->  -- protocol-relative URI
                -- we exclude //? because of //?UNC/ on Windows
       case parseURIReference' s' of
            Just u' -> openURL $ T.pack $ show $ u' `nonStrictRelativeTo` httpcolon
            Nothing -> openURL s' -- will throw error
    (Nothing, s') ->
       case parseURI (T.unpack s') of  -- requires absolute URI
            Just URI{ uriScheme = "file:", uriPath = upath}
              -> readLocalFile $ uriPathToPath (T.pack upath)
            Just URI{ uriScheme = "data:", uriPath = upath}
              -> pure $ extractURIData upath
            -- We don't want to treat C:/ as a scheme:
            Just u' | length (uriScheme u') > 2 -> openURL (T.pack $ show u')
            _ -> readLocalFile fp -- get from local file system
   where readLocalFile f = do
             resourcePath <- getResourcePath
             (fp', cont) <- if isRelative f
                               then withPaths resourcePath readFileStrict f
                               else (f,) <$> readFileStrict f
             report $ LoadedResource f (makeRelative "." fp')
             return (cont, mime)
         httpcolon = URI{ uriScheme = "http:",
                          uriAuthority = Nothing,
                          uriPath = "",
                          uriQuery = "",
                          uriFragment = "" }
         dropFragmentAndQuery = T.takeWhile (\c -> c /= '?' && c /= '#')
         fp = unEscapeString $ T.unpack $ dropFragmentAndQuery s
         mime = getMimeType $ case takeExtension fp of
                     ".gz" -> dropExtension fp
                     ".svgz" -> dropExtension fp ++ ".svg"
                     x     -> x
         ensureEscaped = T.pack . escapeURIString isAllowedInURI . T.unpack . T.map convertSlash
         convertSlash '\\' = '/'
         convertSlash x    = x

-- Extract data from a data URI's path component.
extractURIData :: String -> (B.ByteString, Maybe MimeType)
extractURIData upath =
  case break (== ';') (filter (/= ' ') mimespec) of
     (mime', ";base64") -> (decodeLenient contents, Just (T.pack mime'))
     (mime', _) -> (contents, Just (T.pack mime'))
  where
    (mimespec, rest) = break (== ',') $ unEscapeString upath
    contents = UTF8.fromString $ drop 1 rest

-- | Checks if the file path is relative to a parent directory.
isRelativeToParentDir :: FilePath -> Bool
isRelativeToParentDir fname =
  let canonical = makeCanonical fname
   in length canonical >= 2 && take 2 canonical == ".."

-- | Returns possible user data directory if the file path refers to a file or
-- subdirectory within it.
checkUserDataDir :: PandocMonad m => FilePath -> m (Maybe FilePath)
checkUserDataDir fname =
  if isRelative fname && not (isRelativeToParentDir fname)
     then getUserDataDir
     else return Nothing

-- | Read metadata file from the working directory or, if not found there, from
-- the metadata subdirectory of the user data directory.
readMetadataFile :: PandocMonad m => FilePath -> m B.ByteString
readMetadataFile fname = findFileWithDataFallback "metadata" fname >>= \case
  Nothing -> throwError $ PandocCouldNotFindMetadataFileError (T.pack fname)
  Just metadataFile -> readFileStrict metadataFile

-- | Tries to run an action on a file: for each directory given, a
-- filepath is created from the given filename, and the action is run on
-- that filepath. Returns the result of the first successful execution
-- of the action, or throws a @PandocResourceNotFound@ exception if the
-- action errors for all filepaths.
withPaths :: PandocMonad m
          => [FilePath] -> (FilePath -> m a) -> FilePath -> m (FilePath, a)
withPaths [] _ fp = throwError $ PandocResourceNotFound $ T.pack fp
withPaths (p:ps) action fp =
  catchError ((p </> fp,) <$> action (p </> fp))
             (\_ -> withPaths ps action fp)

-- | A variant of Text.Pandoc.UTF8.toText that takes a FilePath
-- as well as the file's contents as parameter, and traps UTF8
-- decoding errors so it can issue a more informative PandocUTF8DecodingError
-- with source position.
toTextM :: PandocMonad m => FilePath -> B.ByteString -> m T.Text
toTextM fp bs =
  case TSE.decodeUtf8' . filterCRs . dropBOM $ bs of
    Left (TSE.DecodeError _ (Just w)) ->
      case B.elemIndex w bs of
        Just offset ->
          throwError $ PandocUTF8DecodingError (T.pack fp) offset w
        Nothing -> throwError $ PandocUTF8DecodingError (T.pack fp) 0 w
    Left e -> throwError $ PandocAppError (tshow e)
    Right t -> return t
 where
   dropBOM bs' =
     if "\xEF\xBB\xBF" `B.isPrefixOf` bs'
        then B.drop 3 bs'
        else bs'
   filterCRs = B.filter (/=13)

-- | Returns @fp@ if the file exists in the current directory; otherwise
-- searches for the data file relative to @/subdir/@. Returns @Nothing@
-- if neither file exists.
findFileWithDataFallback :: PandocMonad m
                         => FilePath  -- ^ subdir
                         -> FilePath  -- ^ fp
                         -> m (Maybe FilePath)
findFileWithDataFallback subdir fp = do
  -- First we check to see if the file is found. If not, and if it's not
  -- an absolute path, we check to see whether it's in @userdir/@. If
  -- not, we leave it unchanged.
  existsInWorkingDir <- fileExists fp
  if existsInWorkingDir
     then return $ Just fp
     else do
       mbDataDir <- checkUserDataDir fp
       case mbDataDir of
         Nothing -> return Nothing
         Just datadir -> do
           let datafp = datadir </> subdir </> fp
           existsInDataDir <- fileExists datafp
           return $ if existsInDataDir
                    then Just datafp
                    else Nothing

-- | Traverse tree, filling media bag for any images that
-- aren't already in the media bag.
fillMediaBag :: PandocMonad m => Pandoc -> m Pandoc
fillMediaBag d = walkM handleImage d
  where handleImage :: PandocMonad m => Inline -> m Inline
        handleImage (Image attr lab (src, tit)) = catchError
          (do mediabag <- getMediaBag
              let fp = T.unpack src
              case lookupMedia fp mediabag of
                Just _ -> return ()
                Nothing -> do
                  (bs, mt) <- fetchItem src
                  insertMedia fp mt (BL.fromStrict bs)
              return $ Image attr lab (src, tit))
          (\e ->
              case e of
                PandocIOError text err -> do
                  report $ CouldNotFetchResource text . T.pack $
                            (show err ++ "\nReplacing image with description.")
                  -- emit alt text
                  return $ replacementSpan attr src tit lab
                PandocResourceNotFound _ -> do
                  report $ CouldNotFetchResource src
                            "replacing image with description"
                  -- emit alt text
                  return $ replacementSpan attr src tit lab
                PandocHttpError u er -> do
                  report $ CouldNotFetchResource u
                            (T.pack $ show er ++ "\rReplacing image with description.")
                  -- emit alt text
                  return $ replacementSpan attr src tit lab
                _ -> throwError e)
        handleImage x = return x

        replacementSpan (ident, classes, attribs) src title descr =
          Span ( ident
               , "image":"placeholder":classes
               , ("original-image-src", src) :
                 ("original-image-title", title) :
                 attribs
               )
               descr

-- This requires UndecidableInstances.  We could avoid that
-- by repeating the definitions below for every monad transformer
-- we use: ReaderT, WriterT, StateT, RWST.  But this seems to
-- be harmless.
instance (MonadTrans t, PandocMonad m, Functor (t m),
          MonadError PandocError (t m), Monad (t m),
          Applicative (t m)) => PandocMonad (t m) where
  lookupEnv = lift . lookupEnv
  getCurrentTime = lift getCurrentTime
  getCurrentTimeZone = lift getCurrentTimeZone
  newStdGen = lift newStdGen
  newUniqueHash = lift newUniqueHash
  openURL = lift . openURL
  readFileLazy = lift . readFileLazy
  readFileStrict = lift . readFileStrict
  readStdinStrict = lift readStdinStrict
  glob = lift . glob
  fileExists = lift . fileExists
  getDataFileName = lift . getDataFileName
  getModificationTime = lift . getModificationTime
  getCommonState = lift getCommonState
  putCommonState = lift . putCommonState
  logOutput = lift . logOutput

instance {-# OVERLAPS #-} PandocMonad m => PandocMonad (ParsecT s st m) where
  lookupEnv = lift . lookupEnv
  getCurrentTime = lift getCurrentTime
  getCurrentTimeZone = lift getCurrentTimeZone
  newStdGen = lift newStdGen
  newUniqueHash = lift newUniqueHash
  openURL = lift . openURL
  readFileLazy = lift . readFileLazy
  readFileStrict = lift . readFileStrict
  readStdinStrict = lift readStdinStrict
  glob = lift . glob
  fileExists = lift . fileExists
  getDataFileName = lift . getDataFileName
  getModificationTime = lift . getModificationTime
  getCommonState = lift getCommonState
  putCommonState = lift . putCommonState
  trace msg = do
    tracing <- getsCommonState stTrace
    when tracing $ do
      pos <- getPosition
      Debug.Trace.trace
        ("[trace] Parsed " ++ T.unpack msg ++ " at line " ++
            show (sourceLine pos) ++
            if sourceName pos == "chunk"
               then " of chunk"
               else "")
        (return ())
  logOutput = lift . logOutput