Help me find: interactive tutorial for a JavaScript scheme interpreter

Does anybody remember this, posted on HN?

It was an interactive tutorial on a single page web app, where you have a code editor, some explanation for what the code is doing, and arrows to skip forward or backwards in the tutorial. You can then edit the code and see different results.

IIRC the code was in JavaScript and it created a basic lisp/scheme interpreter.

It explained succinctly concepts of tokenizing/parsing, lexing, AST, etc. with these examples.

I've googled for:

- web app showing how to build an interpreter / compiler, complete with lexer, parser, ast - hacker news interactive interpreter lexer ast tutorial javascript - hacker news writing a compiler tutorial spa interactive ast lexer tokenizer parser

with no luck.

Thank you for any help!