Lua: deprecate classic custom writers.

doc/custom-writers.md

@@ -16,16 +16,61 @@ install any additional software to do this.
 [Lua]: https://www.lua.org
 
 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.
+document. Writers must define just a single function named
+`Writer`, which gets passed the document and writer options, and
+then handles the conversion of the document, rendering it into a
+string. This interface was introduced in pandoc 2.17.2.
+
+Pandoc also supports "classic" custom writers, where a Lua
+function must be defined for each AST element type. Classic style
+writers are *deprecated* and should be replaced with new-style
+writers if possible.
+
+# Writers
+
+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
+UTF-8 encoded 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
 
 # Classic style
 
 A writer using the classic style defines rendering functions for
-each element of the pandoc AST.
+each element of the pandoc AST. Note that this style is
+*deprecated* and may be removed in later versions.
 
 For example,
 
@@ -76,50 +121,19 @@ function Doc (body, meta, vars)
 end
 ```
 
-# New style
+## Changes in pandoc 3.0
 
-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
-UTF-8 encoded string.
+Custom writers were reworked in pandoc 3.0. For technical reasons,
+the global variables `PANDOC_DOCUMENT` and `PANDOC_WRITER_OPTIONS`
+are set to the empty document and default values, respectively.
+The old behavior can be restored by adding the following snippet,
+which turns a classic into a new style writer.
 
 ``` lua
 function Writer (doc, opts)
-  -- ...
+  PANDOC_DOCUMENT = doc
+  PANDOC_WRITER_OPTIONS = opts
+  loadfile(PANDOC_SCRIPT_FILE)()
+  return pandoc.write_classic(doc, opts)
 end
 ```
-
-Writers that do not return text but binary data should define a
-function with name `BinaryWriter` instead. The function must still
-return a string, but it does not have to be UTF-8 encoded and can
-contain arbitrary binary data.
-
-If both `Writer` and `BinaryWriter` functions are defined, then
-only the `Writer` function will be used.
-
-## 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