Added ToJsonFilter class, deprecated old jsonFilter function.

src/Text/Pandoc.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE ScopedTypeVariables, FlexibleInstances #-}
 {-
 Copyright (C) 2006-2010 John MacFarlane <jgm@berkeley.edu>
 
@@ -111,6 +112,7 @@ module Text.Pandoc
                -- * Miscellaneous
                , rtfEmbedImage
                , jsonFilter
+               , ToJsonFilter(..)
              ) where
 
 import Text.Pandoc.Definition
@@ -205,8 +207,43 @@ writers = [("native"       , writeNative)
           ,("asciidoc"     , writeAsciiDoc)
           ]
 
+{-# DEPRECATED jsonFilter "Use toJsonFilter instead" #-}
 -- | Converts a transformation on the Pandoc AST into a function
 -- that reads and writes a JSON-encoded string.  This is useful
 -- for writing small scripts.
 jsonFilter :: (Pandoc -> Pandoc) -> String -> String
 jsonFilter f = encodeJSON . f . decodeJSON
+
+-- | 'toJsonFilter' convert a function into a filter that reads pandoc's json output
+-- from stdin, transforms it, and writes it to stdout.  Usage example:
+--
+-- > -- capitalize.hs
+-- > -- compile with:
+-- > --     ghc --make capitalize
+-- > -- run with:
+-- > --     pandoc -t json | ./capitalize | pandoc -f json
+-- >
+-- > import Text.Pandoc
+-- > import Data.Char (toUpper)
+-- >
+-- > main :: IO ()
+-- > main = toJsonFilter capitalizeStrings
+-- >
+-- > capitalizeStrings :: Inline -> Inline
+-- > capitalizeStrings (Str s) = Str $ map toUpper s
+-- > capitalizeStrings x       = x
+--
+-- The function can be any type @(a -> a)@ or @(a -> IO a)@, where @a@
+-- is an instance of 'Data'.  So, for example, @a@ can be 'Pandoc',
+-- 'Inline', 'Block', ['Inline'], ['Block'], 'Meta', 'ListNumberStyle',
+-- 'Alignment', 'ListNumberDelim', 'QuoteType', etc. See 'Text.Pandoc.Definition'.
+class ToJsonFilter a where
+  toJsonFilter :: a -> IO ()
+
+instance (Data a) => ToJsonFilter (a -> a) where
+  toJsonFilter f = getContents
+    >>= putStr . encodeJSON . (bottomUp f :: Pandoc -> Pandoc) . decodeJSON
+
+instance (Data a) => ToJsonFilter (a -> IO a) where
+  toJsonFilter f = getContents >>= (bottomUpM f :: Pandoc -> IO Pandoc) . decodeJSON
+    >>= putStr . encodeJSON