content: {-# LANGUAGE CPP #-}
  {-
  Copyright (c) 2006-2023, John MacFarlane
  
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  -}
  
  {- |
     Module      : Text.Pandoc.Generic
     Copyright   : Copyright (C) 2006-2023 John MacFarlane
     License     : BSD3
  
     Maintainer  : John MacFarlane <jgm@berkeley.edu>
     Stability   : alpha
     Portability : portable
  
  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 @Emph@s first, then
  collapse the resulting adjacent @Space@s, as desired. If we used 'bottomUp',
  we would end up with two adjacent @Space@s, since the contents of the
  two @Emph@ inlines would be processed before the @Emph@s 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
  
  -}
  module Text.Pandoc.Generic where
  
  import Data.Generics
  
  -- | Applies a transformation on @a@s to matching elements in a @b@,
  -- moving from the bottom of the structure up.
  bottomUp :: (Data a, Data b) => (a -> a) -> b -> b
  bottomUp f = everywhere (mkT f)
  
  -- | Applies a transformation on @a@s to matching elements in a @b@,
  -- moving from the top of the structure down.
  topDown :: (Data a, Data b) => (a -> a) -> b -> b
  topDown f = everywhere' (mkT f)
  
  -- | Like 'bottomUp', but with monadic transformations.
  bottomUpM :: (Monad m, Data a, Data b) => (a -> m a) -> b -> m b
  bottomUpM f = everywhereM (mkM f)
  
  -- | Runs a query on matching @a@ elements in a @c@.  The results
  -- of the queries are combined using 'mappend'.
  queryWith :: (Data a, Monoid b, Data c) => (a -> b) -> c -> b
  queryWith f = everything mappend (mempty `mkQ` f)
  