Geno, written in Geno

A language is a lot more convincing once it can describe itself. The selfhost/ directory holds a Geno frontend and tree-walking interpreter written entirely in Geno: about 8,400 lines across eight modules.

selfhost/
  Tokens.geno       token definitions
  Lexer.geno        source text to tokens
  Ast.geno          syntax tree types
  Parser.geno       tokens to syntax tree
  Types.geno        type representation
  TypeChecker.geno  checking
  Interpreter.geno  evaluation
  Main.geno         demo, check, run and test commands

What it is good for

Writing a compiler is a stress test for a language: deep recursion, large algebraic data types, exhaustive matches over dozens of variants, and plenty of string handling. Several of Geno's rough edges were found because the self-hosted parser needed something the language made awkward.

Its functions carry examples, like any other Geno code, so geno test selfhost/ doubles as a regression suite for the language that runs it.

What it is not, yet

• It has no code generation backend and no bootstrap path. The Python implementation remains the reference compiler.
• It covers the subset of Geno its own modules use. Async, traits and app mode are not implemented.
• Builtin coverage is 123 of 231, about 53%.

Keeping it honest

scripts/check_selfhost_parity.py fails the build when a new keyword or builtin appears in the language without either an implementation in selfhost/ or an entry in a named gap group. The gaps are ordered so the next work goes to pure, host-independent builtins first. Nothing drifts silently.

The self-hosted CLI follows the same contracts as the real one, including 0.5's exit semantics: an inner program's Int result becomes the status cmd_run returns.