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Clojure and clj-gpui in Evalight

Feasibility spike. The hosted playground stays ClojureScript + SCI. JVM Clojure is local mode only (bun run evalight / bun run local).

What already worked

  • The editor maps .clj / .cljs / .cljc / .edn to the same CodeMirror Clojure mode and Parinfer.
  • Local Evalight already speaks nREPL (bencode TCP client). Compiled ClojureScript uses a Clojure session plus shadow/nrepl-select.
  • Preview is an iframe of either SCI (preview.html) or the compiled app HTTP server. Hiding Preview is already a first-class layout mode.

What was missing

SCI cannot load JVM Clojure (interop, macros, clj-gpui). project-payload only sent .cljs / .cljc to the sandbox. isUserProject treated any evalight.edn with :main as a shadow-cljs app, so dropping Evalight into a clj-gpui template would try to start shadow-cljs watch and fail.

GPUI is a native window (Rust process, Vulkan). It is not HTML. An iframe cannot contain it.

Runtime split (this branch)

Kind How we detect it Eval Preview
ClojureScript lamp / export shadow-cljs.edn :app, or evalight.edn :main without :runtime SCI (playground) or shadow nREPL (local) iframe
Evalight checkout :workshop in shadow-cljs.edn SCI iframe of workshop
clj-gpui evalight.edn :runtime :gpui, gpui.edn, or clj-gpui / gpui.dev in deps.edn JVM nREPL (clj -M:dev or --attach) native window + status pane
JVM Clojure deps.edn / project.clj without the above JVM nREPL (nrepl.cmdline or --attach) hidden

evalight.edn :runtime always wins. :preview {:kind :native|:iframe|:none} overrides the default.

Can Preview show the GPUI window?

Not inside the iframe. The running program is the OS window that gpui.dev already opens. That is the honest preview.

This branch:

  1. Spawns or attaches to clj -M:dev so that window still appears.
  2. Replaces the iframe with a Preview · GPUI pane (nREPL port, app var, status). Live stays off: clj-gpui's file watcher already reloads.
  3. Asks GET /api/runtime/frame after nREPL connects, after Run, after a successful eval, and shortly after a save. That evals (gpui.runtime/preview-png) when the var exists. Not a poll, and Live stays off.

clj-gpui does not intern preview-png today. There is no screenshot or offscreen capture in the host protocol (v5 is UI-tree JSON). To put pixels in the Evalight pane, add something like:

;; in clj-gpui, later
(defn preview-png
  "Return a base64 PNG of the current native window, or nil."
  []
  ...)

The host would capture the GPUI framebuffer (or an OS window shot) and Clojure would return the bytes. Evalight already displays a data:image/png when that call succeeds. Reconstructing the UI tree as HTML would not be the GPUI window; this spike does not do that.

Bundling with the clj-gpui template

Evalight's export zip already vendors evalight/ (workshop JS + server.mjs). A clj-gpui app should do the same, without shadow-cljs:

;; template/evalight.edn
{:name "my-app"
 :main my.app/app
 :runtime :gpui
 :preview {:kind :native}}
{
  "name": "my-app",
  "private": true,
  "scripts": {
    "evalight": "bun evalight/server.mjs"
  }
}

evalight.template/gpui-evalight-edn and clj-package-json emit those files. Copy a packed evalight/ folder next to src/ (from bun run embed + pack, or bun run evalight after placing it).

Then, from the app directory:

bun run evalight

Needs Bun (workshop server), a JDK, the Clojure CLI, and a GPUI host binary (CLJ_GPUI_BIN or Cargo on first clj -M:dev). It does not need bun install of shadow-cljs.

--attach joins an already-running clj -M:dev nREPL and does not kill it. --preview-url is a ClojureScript flag and is unused here.

From an Evalight checkout:

bun run local /path/to/clj-gpui/examples/todomvc

That serves the :workshop release (evalight-ui/js), never public/js from bun run dev. A watch build of the IDE injects the shadow HUD. Local mode does not start that websocket, so the old path showed shadow-cljs – Reconnecting… over a SCI preview while clj -M:dev still opened the native window.

If Preview still says Live / (bump) / app.core, the browser is not on this local server (wrong port), or it is caching /js/main.js. Hard-refresh (Ctrl-Shift-R). The header should read GPUI next to the project name, and Preview should be Preview · GPUI.

Generic Clojure (not clj-gpui)

Same editor, REPL, hover, completions. No preview column (and no mobile Preview tab). Evalight is then a small Nightlight-style workshop on disk, not a browser image.

The hosted playground will not grow a JVM. SCI-in-the-browser is the wrong interpreter for real Clojure.

Risks

  • First clj -M:dev compiles the Rust host. Evalight waits up to 120s for nREPL. A cold Cargo build can exceed that; --attach after a manual first run is the workaround.
  • Linux GPUI needs a display and Vulkan (lavapipe is enough). Headless Cloud Agent VMs will connect nREPL and show the status pane, not a window.
  • nrepl.cmdline for generic Clojure ignores project :nrepl aliases. evalight.edn can grow {:nrepl {:cmd [...]}} later.
  • Windows: process-group kill matches the compiled runtime (Unix-first).

What this repo still will not do

  • SCI eval of .clj in the playground.
  • A second interpreter, LSP, or generic IDE.
  • Embedding a native window in Chromium.
  • Changing clj-gpui itself (that is a follow-up in violetpurpleish/clj-gpui).