Lepta is an embryonic domain specific language and library for writing domain specific languages (including itself) and source code generators. At the moment it consists only as a prototype, written in a predecessor language (part of a tool I call WizardsWorkbench) and transpiled into PHP code. The associated library is of course also currently written in PHP. The plan is to incrementally rewrite Lepta in Lepta, and switch the generated code language to C++, with the library also being rewritten in C++. (The source code generater part of Lepta is not tied to outputting any particular language.)
A paper on WizardsWorkbench is available here.
The demonstration of WizardsWorkbench for that paper.