Lua filters: use script to initialize the interpreter

The file `init.lua` is used to initialize the Lua interpreter which is
used in Lua filters. This gives users the option to require libraries
which they want to use in all of their filters, and to extend default
modules.

data/init.lua

@@ -0,0 +1,6 @@
+-- This Lua script is run every time the Lua interpreter is started when running
+-- a Lua filter. It can be customized to load additional modules or to alter the
+-- default modules.
+
+pandoc = require 'pandoc'
+pandoc.mediabag = require 'pandoc.mediabag'

doc/lua-filters.md

@@ -3,7 +3,7 @@ title: Pandoc Lua Filters
 author:
 - Albert Krewinkel
 - John MacFarlane
-date: 'November 20, 2017'
+date: 'December 6, 2017'
 ---
 
 # Introduction
@@ -176,6 +176,34 @@ Some pandoc functions have been made available in lua:
   which stores binary content such as images that may be
   included in the final document.
 
+# Lua interpreter initialization
+
+The way the Lua interpreter is set-up can be controlled by
+placing a file `init.lua` in pandoc's data directory. The
+default init file loads the `pandoc` and `pandoc.mediabag`
+modules:
+
+``` lua
+pandoc = require 'pandoc'
+pandoc.mediabag = require 'pandoc.mediabag'
+```
+
+A common use-case would be to add code to load additional
+modules or to alter default modules. E.g., the following snippet
+adds all unicode-aware functions defined in the [`text`
+module](#module-text) to the default `string` module, prefixed
+with the string `uc_`.
+
+```lua
+for name, fn in pairs(require 'text') do
+  string['uc_' .. name] = fn
+end
+```
+
+This makes it possible to apply these functions on strings using
+colon syntax (`mystring:uc_upper()`).
+
+
 # Examples
 
 ## Macro substitution.

pandoc.cabal

@@ -108,6 +108,8 @@ data-files:
                  data/abbreviations
                  -- sample lua custom writer
                  data/sample.lua
+                 -- lua init script
+                 data/init.lua
                  -- pandoc lua module
                  data/pandoc.lua
                  -- lua List module

src/Text/Pandoc/Lua.hs

@@ -48,10 +48,11 @@ import Foreign.Lua (FromLuaStack (peek), Lua, LuaException (..),
                     Status (OK), ToLuaStack (push))
 import Text.Pandoc.Class (PandocIO, getCommonState, getMediaBag, setMediaBag)
 import Text.Pandoc.Definition
+import Text.Pandoc.Lua.Filter (LuaFilter, walkMWithLuaFilter)
 import Text.Pandoc.Lua.Packages (LuaPackageParams (..),
                                  installPandocPackageSearcher)
-import Text.Pandoc.Lua.PandocModule (pushMediaBagModule, pushPandocModule)
-import Text.Pandoc.Lua.Filter (LuaFilter, walkMWithLuaFilter)
+import Text.Pandoc.Lua.PandocModule (pushPandocModule) -- TODO: remove
+import Text.Pandoc.Lua.Util (loadScriptFromDataDir)
 import qualified Foreign.Lua as Lua
 import qualified Foreign.Lua.Module.Text as Lua
 
@@ -101,17 +102,11 @@ luaPackageParams datadir = do
 
 -- Initialize the lua state with all required values
 initLuaState :: LuaPackageParams -> Lua ()
-initLuaState luaPkgParams@(LuaPackageParams commonState datadir mbRef) = do
+initLuaState luaPkgParams = do
   Lua.openlibs
   Lua.preloadTextModule "text"
   installPandocPackageSearcher luaPkgParams
-  pushPandocModule datadir
-  -- add MediaBag module
-  push "mediabag"
-  pushMediaBagModule commonState mbRef
-  Lua.rawset (-3)
-  Lua.setglobal "pandoc"
-  return ()
+  loadScriptFromDataDir (luaPkgDataDir luaPkgParams) "init.lua"
 
 pushGlobalFilter :: Lua ()
 pushGlobalFilter = do

src/Text/Pandoc/Lua/PandocModule.hs

@@ -60,7 +60,7 @@ import qualified Text.Pandoc.MediaBag as MB
 
 -- | Push the "pandoc" on the lua stack. Requires the `list` module to be
 -- loaded.
-pushPandocModule :: Maybe FilePath -> Lua ()
+pushPandocModule :: Maybe FilePath -> Lua NumResults
 pushPandocModule datadir = do
   loadScriptFromDataDir datadir "pandoc.lua"
   addFunction "_pipe" pipeFn
@@ -68,6 +68,7 @@ pushPandocModule datadir = do
   addFunction "sha1" sha1HashFn
   addFunction "walk_block" walkBlock
   addFunction "walk_inline" walkInline
+  return 1
 
 walkElement :: (ToLuaStack a, Walkable [Inline] a, Walkable [Block] a)
             => a -> LuaFilter -> Lua NumResults
@@ -99,14 +100,14 @@ readDoc formatSpec content = do
 --
 -- MediaBag submodule
 --
-pushMediaBagModule :: CommonState -> IORef MB.MediaBag -> Lua ()
+pushMediaBagModule :: CommonState -> IORef MB.MediaBag -> Lua NumResults
 pushMediaBagModule commonState mediaBagRef = do
   Lua.newtable
   addFunction "insert" (insertMediaFn mediaBagRef)
   addFunction "lookup" (lookupMediaFn mediaBagRef)
   addFunction "list" (mediaDirectoryFn mediaBagRef)
   addFunction "fetch" (fetch commonState mediaBagRef)
-  return ()
+  return 1
 
 addFunction :: ToHaskellFunction a => String -> a -> Lua ()
 addFunction name fn = do