Add and apply filters in order (not reversed).

This changes `applyFilters` from Text.Pandoc.Filter so
that it does a left fold rather than a right fold, applying
the filters in the order listed. [behavior change]

The command-line arguments are accumulated in order instead
of reverse order.

A first step twoards #5881.

src/Text/Pandoc/App.hs

@@ -85,8 +85,7 @@ convertWithOpts opts = do
                                 (optMetadata opts)) &&
                       optCiteMethod opts `notElem` [Natbib, Biblatex] &&
                       all (not . isPandocCiteproc) filters
-  let filters' = if needsCiteproc then JSONFilter "pandoc-citeproc" : filters
-                                  else filters
+  let filters' = filters ++ [ JSONFilter "pandoc-citeproc" | needsCiteproc ]
 
   let sources = case optInputFiles opts of
                      []  -> ["-"]

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -436,16 +436,14 @@ options =
     , Option "F" ["filter"]
                  (ReqArg
                   (\arg opt -> return opt { optFilters =
-                                    JSONFilter (normalizePath arg) :
-                                    optFilters opt })
+                      optFilters opt ++ [JSONFilter (normalizePath arg)] })
                   "PROGRAM")
                  "" -- "External JSON filter"
 
     , Option "L" ["lua-filter"]
                  (ReqArg
                   (\arg opt -> return opt { optFilters =
-                                    LuaFilter (normalizePath arg) :
-                                    optFilters opt })
+                      optFilters opt ++ [LuaFilter (normalizePath arg)] })
                   "SCRIPTPATH")
                  "" -- "Lua filter"
 

src/Text/Pandoc/Filter.hs

@@ -21,7 +21,6 @@ module Text.Pandoc.Filter
 
 import Prelude
 import Data.Aeson.TH (deriveJSON, defaultOptions)
-import Data.Foldable (foldrM)
 import GHC.Generics (Generic)
 import Text.Pandoc.Class (PandocIO)
 import Text.Pandoc.Definition (Pandoc)
@@ -33,6 +32,7 @@ import Data.YAML
 import qualified Data.Text as T
 import System.FilePath (takeExtension)
 import Control.Applicative ((<|>))
+import Control.Monad (foldM)
 
 -- | Type of filter and path to filter file.
 data Filter = LuaFilter FilePath
@@ -63,10 +63,10 @@ applyFilters :: ReaderOptions
              -> PandocIO Pandoc
 applyFilters ropts filters args d = do
   expandedFilters <- mapM expandFilterPath filters
-  foldrM ($) d $ map applyFilter expandedFilters
+  foldM applyFilter d expandedFilters
  where
-  applyFilter (JSONFilter f) = JSONFilter.apply ropts args f
-  applyFilter (LuaFilter f)  = LuaFilter.apply ropts args f
+  applyFilter doc (JSONFilter f) = JSONFilter.apply ropts args f doc
+  applyFilter doc (LuaFilter f)  = LuaFilter.apply ropts args f doc
 
 -- | Expand paths of filters, searching the data directory.
 expandFilterPath :: Filter -> PandocIO Filter