Lua filters: allow filtering of element lists (#6040)

Lists of Inline and Block elements can now be filtered via `Inlines` and
`Blocks` functions, respectively. This is helpful if a filter conversion
depends on the order of elements rather than a single element.

For example, the following filter can be used to remove all spaces
before a citation:

    function isSpaceBeforeCite (spc, cite)
      return spc and spc.t == 'Space'
       and cite and cite.t == 'Cite'
    end

    function Inlines (inlines)
      for i = #inlines-1,1,-1 do
        if isSpaceBeforeCite(inlines[i], inlines[i+1]) then
          inlines:remove(i)
        end
      end
      return inlines
    end

Closes: #6038

1 files changed, 45 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Lua/Marshaling/List.hs b/src/Text/Pandoc/Lua/Marshaling/List.hs
new file mode 100644
index 000000000..e4fbfc200
--- /dev/null
+++ b/src/Text/Pandoc/Lua/Marshaling/List.hs
@@ -0,0 +1,45 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE DeriveDataTypeable   #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE NoImplicitPrelude    #-}
+{-# LANGUAGE UndecidableInstances #-}
+{- |
+Module      : Text.Pandoc.Lua.Marshaling.List
+Copyright   : © 2012-2020 John MacFarlane
+              © 2017-2020 Albert Krewinkel
+License     : GNU GPL, version 2 or above
+Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+Stability   : alpha
+
+Marshaling/unmarshaling instances for @pandoc.List@s.
+-}
+module Text.Pandoc.Lua.Marshaling.List
+  ( List (..)
+  ) where
+
+import Prelude
+import Data.Data (Data)
+import Foreign.Lua (Peekable, Pushable)
+import Text.Pandoc.Walk (Walkable (..))
+import Text.Pandoc.Lua.Util (defineHowTo, pushViaConstructor)
+
+import qualified Foreign.Lua as Lua
+
+-- | List wrapper which is marshalled as @pandoc.List@.
+newtype List a = List { fromList :: [a] }
+  deriving (Data, Eq, Show)
+
+instance Pushable a => Pushable (List a) where
+  push (List xs) =
+    pushViaConstructor "List" xs
+
+instance Peekable a => Peekable (List a) where
+  peek idx = defineHowTo "get List" $ do
+    xs <- Lua.peek idx
+    return $ List xs
+
+-- List is just a wrapper, so we can reuse the walk instance for
+-- unwrapped Hasekll lists.
+instance Walkable [a] b => Walkable (List a) b where
+  walkM f = walkM (fmap fromList . f . List)
+  query f = query (f . List)