pandoc.mediabag module: add items function iterating over mediabag

A new function `pandoc.mediabag.items` was added to Lua module
pandoc.mediabag. This allows users to lazily iterate over all media bag
items, loading items into Lua one-by-one. Example:

    for filename, mime_type, content in pandoc.mediabag.items() do
      -- use media bag item.
    end

This is a convenient alternative to using `mediabag.list` in combination
with `mediabag.lookup`.

1 files changed, 75 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Lua/Marshaling/MediaBag.hs b/src/Text/Pandoc/Lua/Marshaling/MediaBag.hs
new file mode 100644
index 000000000..816843c1d
--- /dev/null
+++ b/src/Text/Pandoc/Lua/Marshaling/MediaBag.hs
@@ -0,0 +1,75 @@
+{-# LANGUAGE NoImplicitPrelude    #-}
+{- |
+   Module      : Text.Pandoc.Lua.Marshaling.MediaBag
+   Copyright   : © 2012-2019 John MacFarlane
+                 © 2017-2019 Albert Krewinkel
+   License     : GNU GPL, version 2 or above
+   Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+   Stability   : alpha
+
+Instances to marshal (push) and unmarshal (peek) media data.
+-}
+module Text.Pandoc.Lua.Marshaling.MediaBag (pushIterator) where
+
+import Prelude
+import Foreign.Ptr (Ptr)
+import Foreign.StablePtr (StablePtr, deRefStablePtr, newStablePtr)
+import Foreign.Lua (Lua, NumResults, Peekable, Pushable, StackIndex)
+import Foreign.Lua.Types.Peekable (reportValueOnFailure)
+import Foreign.Lua.Userdata (ensureUserdataMetatable, pushAnyWithMetatable,
+                             toAnyWithName)
+import Text.Pandoc.MediaBag (MediaBag, mediaItems)
+import Text.Pandoc.MIME (MimeType)
+import Text.Pandoc.Lua.Marshaling.AnyValue (AnyValue (..))
+
+import qualified Data.ByteString.Lazy as BL
+import qualified Foreign.Lua as Lua
+import qualified Foreign.Storable as Storable
+
+-- | A list of 'MediaBag' items.
+newtype MediaItems = MediaItems [(String, MimeType, BL.ByteString)]
+
+instance Pushable MediaItems where
+  push = pushMediaItems
+
+instance Peekable MediaItems where
+  peek = peekMediaItems
+
+-- | Push an iterator triple to be used with Lua's @for@ loop construct.
+-- Each iterator invokation returns a tripple consisting of an item's
+-- filename, MIME type, and content.
+pushIterator :: MediaBag -> Lua NumResults
+pushIterator mb = do
+  Lua.pushHaskellFunction nextItem
+  Lua.push (MediaItems $ mediaItems mb)
+  Lua.pushnil
+  return 3
+
+-- | Lua type name for @'MediaItems'@.
+mediaItemsTypeName :: String
+mediaItemsTypeName = "pandoc MediaItems"
+
+-- | Push a @MediaItems@ element to the stack.
+pushMediaItems :: MediaItems -> Lua ()
+pushMediaItems xs = pushAnyWithMetatable pushMT xs
+ where
+  pushMT = ensureUserdataMetatable mediaItemsTypeName (return ())
+
+-- | Retrieve a @MediaItems@ element from the stack.
+peekMediaItems :: StackIndex -> Lua MediaItems
+peekMediaItems = reportValueOnFailure mediaItemsTypeName
+                 (`toAnyWithName` mediaItemsTypeName)
+
+-- | Retrieve a list of items from an iterator state, return the first
+-- item (if present), and advance the state.
+nextItem :: Ptr (StablePtr MediaItems) -> AnyValue -> Lua NumResults
+nextItem ptr _ = do
+  (MediaItems items) <- Lua.liftIO $ deRefStablePtr =<< Storable.peek ptr
+  case items of
+    [] -> 2 <$ (Lua.pushnil *> Lua.pushnil)
+    (key, mt, content):xs -> do
+      Lua.liftIO $ Storable.poke ptr =<< newStablePtr (MediaItems xs)
+      Lua.push key
+      Lua.push mt
+      Lua.push content
+      return 3