Provide Data.Default instances for ParserState and WriterOptions.

Now you can use def (which is re-exported by Text.Pandoc) instead of
defaultParserState or defaultWriterOptions.  For now, these
are still defined too, so existing code need not change.

Closes #546.

pandoc.cabal

@@ -223,6 +223,7 @@ Library
                  base64-bytestring >= 0.1 && < 0.2,
                  zlib >= 0.5 && < 0.6,
                  highlighting-kate >= 0.5.1 && < 0.6,
+                 data-default >= 0.4 && < 0.6,
                  temporary >= 1.1 && < 1.2
   if flag(blaze_html_0_5)
     build-depends:
@@ -331,6 +332,7 @@ Executable pandoc
                  base64-bytestring >= 0.1 && < 0.2,
                  zlib >= 0.5 && < 0.6,
                  highlighting-kate >= 0.5.1 && < 0.6,
+                 data-default >= 0.4 && < 0.6,
                  temporary >= 1.1 && < 1.2
   if flag(blaze_html_0_5)
     build-depends:
@@ -398,6 +400,7 @@ Executable test-pandoc
                  base64-bytestring >= 0.1 && < 0.2,
                  zlib >= 0.5 && < 0.6,
                  highlighting-kate >= 0.5.1 && < 0.6,
+                 data-default >= 0.4 && < 0.6,
                  temporary >= 1.1 && < 1.2
   if flag(blaze_html_0_5)
     build-depends:

src/Text/Pandoc.hs

@@ -114,6 +114,8 @@ module Text.Pandoc
                , rtfEmbedImage
                , jsonFilter
                , ToJsonFilter(..)
+               -- * From Data.Default
+               , def
              ) where
 
 import Text.Pandoc.Definition
@@ -149,6 +151,7 @@ import Text.Pandoc.Shared
 import Data.Version (showVersion)
 import Text.JSON.Generic
 import Paths_pandoc (version)
+import Data.Default
 
 -- | Version number of pandoc library.
 pandocVersion :: String

src/Text/Pandoc/Parsing.hs

@@ -88,6 +88,7 @@ import Text.Pandoc.Shared
 import qualified Data.Map as M
 import Text.TeXMath.Macros (applyMacros, Macro, parseMacroDefinitions)
 import Text.HTML.TagSoup.Entity ( lookupEntity )
+import Data.Default
 
 -- | Like >>, but returns the operation on the left.
 -- (Suggested by Tillmann Rendel on Haskell-cafe list.)
@@ -658,6 +659,9 @@ data ParserState = ParserState
     }
     deriving Show
 
+instance Default ParserState where
+  def = defaultParserState
+
 defaultParserState :: ParserState
 defaultParserState = 
     ParserState { stateParseRaw        = False,
@@ -872,13 +876,13 @@ macro = do
   inp <- getInput
   case parseMacroDefinitions inp of
        ([], _)    -> pzero
-       (ms, rest) -> do def <- count (length inp - length rest) anyChar
+       (ms, rest) -> do def' <- count (length inp - length rest) anyChar
                         if apply
                            then do
                              updateState $ \st ->
                                st { stateMacros = ms ++ stateMacros st }
                              return Null
-                           else return $ RawBlock "latex" def
+                           else return $ RawBlock "latex" def'
 
 -- | Apply current macros to string.
 applyMacros' :: String -> GenParser Char ParserState String

src/Text/Pandoc/Shared.hs

@@ -72,7 +72,7 @@ module Text.Pandoc.Shared (
                      readDataFile,
                      -- * Error handling
                      err,
-                     warn,
+                     warn
                     ) where
 
 import Text.Pandoc.Definition
@@ -94,6 +94,7 @@ import Text.Pandoc.Highlighting (Style, pygments)
 import Text.Pandoc.Pretty (charWidth)
 import System.Locale (defaultTimeLocale)
 import Data.Time
+import Data.Default
 import System.IO (stderr)
 
 --
@@ -524,6 +525,9 @@ data WriterOptions = WriterOptions
   , writerTeXLigatures     :: Bool       -- ^ Use tex ligatures quotes, dashes in latex
   } deriving Show
 
+instance Default WriterOptions where
+  def = defaultWriterOptions
+
 {-# DEPRECATED writerXeTeX "writerXeTeX no longer does anything" #-}
 -- | Default writer options.
 defaultWriterOptions :: WriterOptions