From 4cf6ddb027cdc262e34dea50ec77f3fcdbc517d2 Mon Sep 17 00:00:00 2001
From: Albert Krewinkel <albert@zeitkraut.de>
Date: Tue, 1 Oct 2024 12:08:00 +0200
Subject: Lua: allow returning a single filter from filter files

It is now possible to return a single filter from a filter file, e.g.

``` lua
-- Switch single- and double quotes
return {
  Quoted = function (q)
    elem.quotetype = elem.quotetype == 'SingleQuote'
      and 'DoubleQuote' or 'SingleQuote'
    return elem
  end
}

The filter must not contain numerical indexes, or it might be treated as
a list of filters.
---
 pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

(limited to 'pandoc-lua-engine/src/Text')

diff --git a/pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs b/pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs
index 4728f961e..40ccb2f8f 100644
--- a/pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs
+++ b/pandoc-lua-engine/src/Text/Pandoc/Lua/Filter.hs
@@ -43,7 +43,10 @@ runFilterFile' envIdx filterPath doc = do
       -- filter if nothing was returned.
       luaFilters <- forcePeek $
         if newtop - oldtop >= 1
-        then peekList peekFilter top      -- get from explicit filter table
+        then liftLua (rawlen top) >>= \case
+          -- explicitly returned filter, either a single one or a list
+          0 -> (:[]) <$!> peekFilter top  -- single filter
+          _ -> peekList peekFilter top    -- list of explicit filters
         else (:[]) <$!> peekFilter envIdx -- get the implicit filter in _ENV
       settop oldtop
       runAll luaFilters doc
-- 
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