Warning: This is pre-release software. You should probably not use it. Features described below may be incomplete.
m4rkdown extends CommonMark with a terrible macro system.
These can be defined in Haskell or as simple text replacement.
It is intended to allow you to build convoluted authoring systems on top of standard Markdown syntax. This may or may not be a good idea.
The project is named after M4, the venerable macro processing system. It does not really resemble M4, besides being an overpowered tool for undisciplined hackery.