Add interface for custom readers written in Lua. (#7671)

New module Text.Pandoc.Readers.Custom, exporting
readCustom [API change].

Users can now do `-f myreader.lua` and pandoc will treat the
script myreader.lua as a custom reader, which parses an input
string to a pandoc AST, using the pandoc module defined for
Lua filters.

A sample custom reader can be found in data/reader.lua.

Closes #7669.

1 files changed, 27 insertions, 9 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index 019d80bf0..7e9f9f85a 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -266,6 +266,7 @@ header when requesting a document from a URL:
     - `tikiwiki` ([TikiWiki markup])
     - `twiki` ([TWiki markup])
     - `vimwiki` ([Vimwiki])
+    - the path of a custom Lua reader, see [Custom readers and writers] below
     :::
 
     Extensions can be individually enabled or disabled by
@@ -338,7 +339,7 @@ header when requesting a document from a URL:
     - `tei` ([TEI Simple])
     - `xwiki` ([XWiki markup])
     - `zimwiki` ([ZimWiki markup])
-    - the path of a custom Lua writer, see [Custom writers] below
+    - the path of a custom Lua writer, see [Custom readers and writers] below
     :::
 
     Note that `odt`, `docx`, `epub`, and `pdf` output will not be directed
@@ -6574,19 +6575,35 @@ With these custom styles, you can use your input document as a
 reference-doc while creating docx output (see below), and maintain the
 same styles in your input and output files.
 
-# Custom writers
+# Custom readers and writers
 
-Pandoc can be extended with custom writers written in [Lua].  (Pandoc
-includes a Lua interpreter, so Lua need not be installed separately.)
+Pandoc can be extended with custom readers and writers written
+in [Lua].  (Pandoc includes a Lua interpreter, so Lua need not
+be installed separately.)
 
-To use a custom writer, simply specify the path to the Lua script
-in place of the output format. For example:
+To use a custom reader or writer, simply specify the path to the
+Lua script in place of the input or output format. For example:
 
     pandoc -t data/sample.lua
+    pandoc -f my_custom_markup_language.lua -t latex -s
 
-Creating a custom writer requires writing a Lua function for each
-possible element in a pandoc document.  To get a documented example
-which you can modify according to your needs, do
+A custom reader is a Lua script that defines one function,
+Reader, which takes a string as input and returns a Pandoc
+AST.  See the [Lua filters documentation] for documentation
+of the functions that are available for creating pandoc
+AST elements.  For parsing, the [lpeg] parsing library
+is available by default. To see a sample custom reader:
+
+    pandoc --print-default-data-file reader.lua
+
+Reader options are available via the global variable
+`PANDOC_READER_OPTIONS`, as expalined in the [Lua filters
+documentation].
+
+A custom writer is a Lua script that defines a function
+that specifies how to render each element in a Pandoc AST.
+To see a documented example which you can modify according
+to your needs:
 
     pandoc --print-default-data-file sample.lua
 
@@ -6598,6 +6615,7 @@ default template with the name
 subdirectory of your user data directory (see [Templates]).
 
 [Lua]: https://www.lua.org
+[lpeg]:  http://www.inf.puc-rio.br/~roberto/lpeg/
 
 # Reproducible builds