By admitting input, a program acquires a control language by which a user can guide the program through a maze of possibilities. — Chuck Moore

what: Model |> View |> Self-modify architecture

An approach to interactive apps/games I'd wanted to try in a toy language but realized is largely language-agnostic, and easy to explain to anybody who understands Redux/Elm. I had a week+ to prototype it in JS.

a proof-of-concept live coding JS environment, where user actions call WRITE(...) to modify app source

This is a bad idea in many ways (⚠ including security!) but it challenges assumptions on essential complexity and walls between language/env authors | developer | end-user.

• Browsers popularized what user sees being a pure function of DOM.

• React popularized DOM being pure function of your data/state ("model").

• We need a live coding environment where the code you edit gets re-evaluated on every edit. (TODO: to scale this needs caching of intermediate results, but full re-run works for a PoC).

• Now add functions that take model and return new model.

• If user were willing to type function calls into editor, you're done ;-) This is an internal DSL approach to interaction: Model + View is all you need, and you can be purely functional without any in-language approach to state (like in spreadsheets).

• To make it friendlier, classical UI explicitly reads events, translates them to state changes — i.e. treats user input as external DSL. Don't do that!
  Instead, translate user actions to changes in the source editor — which trigger re-computation.

• Traditional developer (especially one attempting event sourcing / time travel / record-replay live coding) needs two concepts of stateful change: changing state inside the app, but also changing the app source. This approach reduces it to only one.

  • Think of an event-sourcing DB migration changing the format of past events, or a refactor changing Redux actions structure, invalidating the recorded history. Fixing those requires thinking of both "change" concepts at once :-/

    In this self-modify paradigm you get same issues — but history is regular code, so regular "refactor after a funciton interface changed" skills apply.

HOW TO TRY

1. https://model-view-self-modify.netlify.app/ (or open index.html locally, or serve it by e.g. python3 -m http.server).
2. Paste the content of tetris.js into the editor.
3. Start moving "TIME TRAVEL" line up.
4. Put editor cursor before it and start clicking [left] [right] [down] buttons to play from that moment.
5. Put cursor inside RCSet([...]) in newGame.board. Start clicking board cells to mark them occupied.

Implementation things I learnet in 1st week of prototyping

• localStorage is awesome for edit/reload development! 2 lines for major quality of life improvement

• JS without bundler/transpiler is fun again.

  • CDNs serving npm packages: unpkg.com, skypack.dev, esm.sh

    • If you don't want CDNs, there is snowpack.dev (didn't try it)

  • Can use ESM imports inside <script type="module">

    • may also need a <script type="importmap">

  • htm library: JSX-like notation in JS tagged templates

• JS eval() facilities are surprisingly junky :-( I spent most of the week tweaking my MVP environment instead of using it...

  • literal eval() has magic access to current scope
  • myEval = eval; myEval() doesn't. It's considered "safer" because the scope it'll pollute is — reliably — only the GLOBAL scope :-D
  • new Function() constructor is better, separates parsing from running & clean passing of values.
  • SyntaxError within eval() / Function() doesn't report what line it happened (only Firefox does, non-standard) => KLUDGE: keep removing lines from the end until the error changes/goes away
  • //# sourceURL= directive for cleaner stack traces
  • TODO: dynamically creating <script> tags looks promising?

Paradigm things I discovered

• It's not just for end-user interaction! WRITE() makes it easy to scaffold helpers you use while coding.

• I hoped to build Light Table-like env that magically renders values under the cursor, i.e. time travel would be "stop evaluation at this point". I didn't get to that, but the twist is that's not good enough anyway — time travel requires stopping Model manipulation early, but still running later View code!

  => I experimented with a kludge: eval skipping editor selection. Unusual and causes lots of flicker.
  => Dropped that in favor of moving /* ... */ markers – zero magic just regular editor shortcuts.

• JS syntax is hard to slice into safe-for-partial execution chunks. A nd

• is React conceptual overkill here?
  For the most part, instead of html`<${View} ...${model}/>` it's simpler to write View(model).
  The deep benefit of React.createElement is separation from "mount component now" vs. "render it later", and supporting a stateful lifecycle...
  But if we [pretend to] re-evaluate everything all the time, we can just call functions (like in Elm!)

  • It does give opportunity for some caching.
  • More importantly, it provides well-understood seam between stateless parts and off-the-shelf stateful-lifecycle parts.

Future

• finish the Tetris

• look for max opportunities to use WRITE() during coding - "moldable development"

  • "level editor" kind of stuff
  • color picker

• It's fragile to place editor cursor correctly before interaction. Add a way to target a fixed place in code.

  • MVP: BEFORE_COMMENT('FOO').WRITE(...) targetting //FOO or /*FOO*/?
    (It's important to avoid target the code itself that mentions the target name :-)

• go meta: Shift parts of the live env e.g. into the env itself so they can be edited?

  • serialize CodeMirror edit actions to a text stream, allow time travel there too?!

but more important:

• computation caching! Don't re-run code from start, esp. when appending at the end.

  • check out https://tomasp.net/academic/papers/live/
  • mobx?

• don't reinvent the env — build on observablehq or similar. (https://github.com/asg017/dataflow ?)

  • Observable notebook already parses separate JS cells, computes data dependencies and manages re-computation...

• Try React "fast refresh" API to replace re-defined components in-place?