Generic functions for manipulating Pandoc documents. (Note: the functions defined in Text.Pandoc.Walk should be used instead, when possible, as they are much faster.)

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 nullMeta
 [ 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 nullMeta
   [Para [Str "Hi",Space,Emph [Str "world",Space,Str "emphasized"]]]

bottomUp normal myDoc ==
  Pandoc nullMeta
   [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