pandoc.mediabag module: add function delete

Function `pandoc.mediabag.delete` allows to remove a single item of the given
name from the media bag.

doc/lua-filters.md

@@ -2450,6 +2450,18 @@ e.g.:
 
     local mb = require 'pandoc.mediabag'
 
+### delete {#mediabag-delete}
+
+`delete (filepath)`
+
+Removes a single entry from the media bag.
+
+Parameters:
+
+`filepath`:
+:   filename of the item to be deleted. The media bag will be
+    left unchanged if no entry with the given filename exists.
+
 ### empty {#mediabag-empty}
 
 `empty ()`

src/Text/Pandoc/Lua/Module/MediaBag.hs

@@ -34,6 +34,7 @@ import qualified Text.Pandoc.MediaBag as MB
 pushModule :: Lua NumResults
 pushModule = do
   Lua.newtable
+  addFunction "delete" delete
   addFunction "empty" empty
   addFunction "insert" insertMediaFn
   addFunction "items" items
@@ -61,6 +62,11 @@ setCommonState st = do
 modifyCommonState :: (CommonState -> CommonState) -> Lua ()
 modifyCommonState f = getCommonState >>= setCommonState . f
 
+-- | Delete a single item from the media bag.
+delete :: FilePath -> Lua NumResults
+delete fp = 0 <$ modifyCommonState
+  (\st -> st { stMediaBag = MB.deleteMedia fp (stMediaBag st) })
+
 -- | Delete all items from the media bag.
 empty :: Lua NumResults
 empty = 0 <$ modifyCommonState (\st -> st { stMediaBag = mempty })
@@ -86,7 +92,7 @@ lookupMediaFn :: FilePath
 lookupMediaFn fp = do
   res <- MB.lookupMedia fp . stMediaBag <$> getCommonState
   case res of
-    Nothing -> Lua.pushnil *> return 1
+    Nothing -> 1 <$ Lua.pushnil
     Just (mimeType, contents) -> do
       Lua.push mimeType
       Lua.push contents

src/Text/Pandoc/MediaBag.hs

@@ -16,6 +16,7 @@ interface for interacting with it.
 -}
 module Text.Pandoc.MediaBag (
                      MediaBag,
+                     deleteMedia,
                      lookupMedia,
                      insertMedia,
                      mediaDirectory,
@@ -41,6 +42,14 @@ newtype MediaBag = MediaBag (M.Map [String] (MimeType, BL.ByteString))
 instance Show MediaBag where
   show bag = "MediaBag " ++ show (mediaDirectory bag)
 
+-- | Delete a media item from a 'MediaBag', or do nothing if no item corresponds
+-- to the given path.
+deleteMedia :: FilePath       -- ^ relative path and canonical name of resource
+            -> MediaBag
+            -> MediaBag
+deleteMedia fp (MediaBag mediamap) =
+  MediaBag $ M.delete (splitDirectories fp) mediamap
+
 -- | Insert a media item into a 'MediaBag', replacing any existing
 -- value with the same name.
 insertMedia :: FilePath       -- ^ relative path and canonical name of resource