Generic functions for manipulating Pandoc documents.

Here's a simple example, defining a function that replaces all the level 3+ headers in a document with regular paragraphs in ALL CAPS:

 import Text.Pandoc.Definition
 import Text.Pandoc.Generic
 import Data.Char (toUpper)

 modHeader :: Block -> Block
 modHeader (Header n xs) | n >= 3 = Para $ bottomUp allCaps xs
 modHeader x = x

 allCaps :: Inline -> Inline
 allCaps (Str xs) = Str $ map toUpper xs
 allCaps x = x

 changeHeaders :: Pandoc -> Pandoc
 changeHeaders = bottomUp modHeader

bottomUp is so called because it traverses the Pandoc structure from bottom up. topDown goes the other way. The difference between them can be seen from this example:

 normal :: [Inline] -> [Inline]
 normal (Space : Space : xs) = Space : xs
 normal (Emph xs : Emph ys : zs) = Emph (xs ++ ys) : zs
 normal xs = xs

 myDoc :: Pandoc
 myDoc =  Pandoc (Meta {docTitle = [], docAuthors = [], docDate = []})
  [Para [Str "Hi",Space,Emph [Str "world",Space],Emph [Space,Str "emphasized"]]]

Here we want to use topDown to lift normal to Pandoc -> Pandoc. The top down strategy will collapse the two adjacent Emphs first, then collapse the resulting adjacent Spaces, as desired. If we used bottomUp, we would end up with two adjacent Spaces, since the contents of the two Emph inlines would be processed before the Emphs were collapsed into one.

 topDown normal myDoc ==
   Pandoc (Meta {docTitle = [], docAuthors = [], docDate = []})
    [Para [Str "Hi",Space,Emph [Str "world",Space,Str "emphasized"]]]

 bottomUp normal myDoc ==
   Pandoc (Meta {docTitle = [], docAuthors = [], docDate = []})
    [Para [Str "Hi",Space,Emph [Str "world",Space,Space,Str "emphasized"]]]

bottomUpM is a monadic version of bottomUp. It could be used, for example, to replace the contents of delimited code blocks with attribute include=FILENAME with the contents of FILENAME:

 doInclude :: Block -> IO Block
 doInclude cb@(CodeBlock (id, classes, namevals) contents) =
   case lookup "include" namevals of
        Just f  -> return . (CodeBlock (id, classes, namevals)) =<< readFile f
        Nothing -> return cb
 doInclude x = return x

 processIncludes :: Pandoc -> IO Pandoc
 processIncludes = bottomUpM doInclude

queryWith can be used, for example, to compile a list of URLs linked to in a document:

 extractURL :: Inline -> [String]
 extractURL (Link _ (u,_)) = [u]
 extractURL (Image _ (u,_)) = [u]
 extractURL _ = []

 extractURLs :: Pandoc -> [String]
 extractURLs = queryWith extractURL