Custom writer: support new-style Writer function.

2 files changed, 76 insertions, 8 deletions

diff --git a/doc/custom-writers.md b/doc/custom-writers.md
index fb08cd120..a1ce1ca56 100644
--- a/doc/custom-writers.md
+++ b/doc/custom-writers.md
@@ -15,8 +15,17 @@ install any additional software to do this.
 
 [Lua]: https://www.lua.org
 
-A custom writer is a Lua file that defines functions for
-rendering each element of a pandoc AST.
+A custom writer is a Lua file that defines how to render the
+document. Two styles of custom writers are supported: classic
+custom writers must define rendering functions for each AST
+element. New style writers, available since pandoc 2.17.2, must
+define just a single function `Writer`, which gets passed the
+document and writer options, and then does all rendering.
+
+# Classic style
+
+A writer using the classic style defines rendering functions for
+all elements of pandoc's AST.
 
 For example,
 
@@ -26,15 +35,15 @@ function Para(s)
 end
 ```
 
-The best way to go about creating a custom writer is to modify
-the example that comes with pandoc.  To get the example, you
-can do
+The best way to go about creating a classic custom writer is to
+modify the example that comes with pandoc. To get the example,
+you can do
 
 ```
 pandoc --print-default-data-file sample.lua > sample.lua
 ```
 
-# A custom HTML writer
+## A custom HTML writer
 
 `sample.lua` is a full-features HTML writer, with explanatory
 comments. To use it, just use the path to the custom writer as
@@ -51,7 +60,7 @@ the functions in `sample.lua` according to your needs.
 ``` {.lua include="sample.lua"}
 ```
 
-# Template variables
+## Template variables
 
 New template variables can be added, or existing ones
 modified, by returning a second value from function `Doc`.
@@ -66,3 +75,43 @@ function Doc (body, meta, vars)
   return body, vars
 end
 ```
+
+# New style
+
+Custom writers using the new style must contain a global function
+named `Writer`. Pandoc calls this function with the document and
+writer options as arguments, and expects the function to return a
+string.
+
+``` lua
+function Writer (doc, opts)
+  -- ...
+end
+```
+
+## Example: modified Markdown writer
+
+Writers have access to all modules described in the [Lua filters
+documentation][]. This includes `pandoc.write`, which can be used
+to render a document in a format already supported by pandoc. The
+document can be modified before this conversion, as demonstrated
+in the following short example. It renders a document as GitHub
+Flavored Markdown, but always uses fenced code blocks, never
+indented code.
+
+``` lua
+function Writer (doc, opts)
+  local filter = {
+    CodeBlock = function (cb)
+      -- only modify if code block has no attributes
+      if cb.attr == pandoc.Attr() then
+        local delimited = '```\n' .. cb.text .. '\n```'
+        return pandoc.RawBlock('markdown', delimited)
+      end
+    end
+  }
+  return pandoc.write(doc:walk(filter), 'gfm', opts)
+end
+```
+
+[Lua filters documentation]: https://pandoc.org/lua-filters.html
diff --git a/src/Text/Pandoc/Writers/Custom.hs b/src/Text/Pandoc/Writers/Custom.hs
index e2b8bddf6..af3fd8306 100644
--- a/src/Text/Pandoc/Writers/Custom.hs
+++ b/src/Text/Pandoc/Writers/Custom.hs
@@ -33,4 +33,23 @@ writeCustom luaFile opts doc = either throw pure <=< runLua $ do
   dofileTrace luaFile >>= \case
     OK -> pure ()
     _  -> throwErrorAsException
-  Classic.runCustom opts doc
+  -- Most classic writers contain code that throws an error if a global
+  -- is not present. This would break our check for the existence of a
+  -- "Writer" function. We resort to raw access for that reason, but
+  -- could also catch the error instead.
+  let rawgetglobal x = do
+        pushglobaltable
+        pushName x
+        rawget (nth 2) <* remove (nth 2) -- remove global table
+
+  rawgetglobal "Writer" >>= \case
+    TypeNil -> do
+      pop 1  -- remove nil
+      Classic.runCustom opts doc
+    _       -> do
+      -- Writer on top of the stack. Call it with document and writer
+      -- options as arguments.
+      push doc
+      push opts
+      call 2 1
+      forcePeek $ peekText top