This was a programming language I made for a weekend-long hackathon where the theme was "first class comments"
Mark Sideways is a literate programming language that uses plain markdown files as its input source type. Function and class definitions are defined by # headers, arguments are defined by bulleted lists, and the code itself appears as code block segments. Everything else is a "comment".
The compiler and interpreter itself was implemented in Python. It also supports basic 2D drawing using SDL via PyGame.
For example, here is a program that calculates Pi. This is NOT a readme file about the program. This markdown file IS the program itself.
This is a video by the hackathon organizer demonstrating MarkSideways at 11:13: