Integrate DeferredMediaBag into CommonState

The DeferredMediaBag is now the object that is held in state. It should
not be visible to users, who will still deal with MediaBag through
exported getters and setters.

We now have a function `fetchDeferredMedia` which returns () but
downloads/reads in all of the deferred media.

Note that getMediaBag first fetches all deferred media.

src/Text/Pandoc/Class.hs

@@ -135,15 +135,17 @@ getWarnings :: PandocMonad m => m [String]
 getWarnings = getsCommonState stWarnings
 
 setMediaBag :: PandocMonad m => MediaBag -> m ()
-setMediaBag mb = modifyCommonState $ \st -> st{stMediaBag = mb}
+setMediaBag mb = modifyCommonState $
+                 \st -> st{stDeferredMediaBag = DeferredMediaBag mb mempty}
 
 getMediaBag :: PandocMonad m => m MediaBag
-getMediaBag = getsCommonState stMediaBag
+getMediaBag = fetchDeferredMedia >> (dropDeferredMedia <$> getsCommonState stDeferredMediaBag)
 
 insertMedia :: PandocMonad m => FilePath -> Maybe MimeType -> BL.ByteString -> m ()
-insertMedia fp mime bs =
-    modifyCommonState $ \st ->
-      st{stMediaBag = MB.insertMedia fp mime bs (stMediaBag st) }
+insertMedia fp mime bs = do
+  (DeferredMediaBag mb dm) <- getsCommonState stDeferredMediaBag
+  let mb' = MB.insertMedia fp mime bs mb
+  modifyCommonState $ \st -> st{stDeferredMediaBag =DeferredMediaBag mb' dm }
 
 getInputFiles :: PandocMonad m => m (Maybe [FilePath])
 getInputFiles = getsCommonState stInputFiles
@@ -179,8 +181,9 @@ instance Monoid DeferredMediaBag where
   mappend (DeferredMediaBag mb lst) (DeferredMediaBag mb' lst') =
     DeferredMediaBag (mb <> mb') (lst <> lst')
 
-getDeferredMedia :: PandocMonad m => DeferredMediaBag -> m MediaBag
-getDeferredMedia (DeferredMediaBag mb defMedia) = do
+fetchDeferredMedia' :: PandocMonad m => m MediaBag
+fetchDeferredMedia' = do
+  (DeferredMediaBag mb defMedia) <- getsCommonState stDeferredMediaBag
   fetchedMedia <- mapM (\dfp -> fetchItem Nothing (unDefer dfp)) defMedia
   return $ foldr
     (\(dfp, (bs, mbMime)) mb' ->
@@ -188,18 +191,22 @@ getDeferredMedia (DeferredMediaBag mb defMedia) = do
     mb
     (zip defMedia fetchedMedia)
 
+fetchDeferredMedia :: PandocMonad m => m ()
+fetchDeferredMedia = fetchDeferredMedia' >>= setMediaBag
+
 dropDeferredMedia :: DeferredMediaBag -> MediaBag
 dropDeferredMedia (DeferredMediaBag mb _) = mb
 
+
 data CommonState = CommonState { stWarnings :: [String]
-                               , stMediaBag :: MediaBag
+                               , stDeferredMediaBag :: DeferredMediaBag
                                , stInputFiles :: Maybe [FilePath]
                                , stOutputFile :: Maybe FilePath
                                }
 
 instance Default CommonState where
   def = CommonState { stWarnings = []
-                    , stMediaBag = mempty
+                    , stDeferredMediaBag = mempty
                     , stInputFiles = Nothing
                     , stOutputFile = Nothing
                     }
@@ -284,7 +291,7 @@ fetchItem :: PandocMonad m
           -> String
           -> m (B.ByteString, Maybe MimeType)
 fetchItem sourceURL s = do
-  mediabag <- getMediaBag
+  mediabag <- dropDeferredMedia <$> getsCommonState stDeferredMediaBag
   case lookupMedia s mediabag of
        Just (mime, bs) -> return $ (BL.toStrict bs, Just mime)
        Nothing ->