Functions for manipulating Pandoc documents or extracting information from them by walking the Pandoc structure (or intermediate structures like '[Block]' or '[Inline]'. These are faster (by a factor of four or five) than the generic functions defined in Text.Pandoc.Generic.

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.Walk
import Data.Char (toUpper)

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

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

changeHeaders :: Pandoc -> Pandoc
changeHeaders = walk modHeader

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

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

extractURLs :: Pandoc -> [Text]
extractURLs = query extractURL