Cope and Drag for Alloy

Pairs the Alloy Analyzer with the Cope and Drag (CnD) visualizer inside VS Code. Run a command in a .als model and explore the instance in a webview with full interactivity — instances, evaluator, and next/previous enumeration — straight from Alloy's own engine.

It forks nothing: it drives upstream Alloy through its public API via a tiny bridge, and embeds stock Cope and Drag unmodified.

Features

• Visualize in Cope and Drag — open any .als and run a command; the instance renders in a CnD webview beside your editor.
• Full interactivity — pick commands, evaluate expressions in CnD's REPL, and step through instances (next/previous) using Alloy's own solver.
• Run / Check CodeLens — a one-click ▶ above every run / check command.
• Custom layouts via sidecar .cnd files — describe how an instance should be drawn (the same spec language Forge embeds in its XML). A model foo.als automatically pairs with foo.cnd; save it and the open visualization re-lays-out live.
• .cnd syntax highlighting — YAML structure plus highlighting for CnD constraints, directives, and options, with comment/bracket editing support.
• On-save error checking — Alloy's own parser/type-checker surfaces errors and warnings as squiggles.
• Editor navigation — symbols, definitions, references, and rename via Alloy's bundled language server.
• Bring your own Alloy — no jar is bundled; a compatible Alloy 6+ install is auto-detected, or downloaded on demand (cached), or pointed at via a setting.

Requirements

• JDK 17+ — auto-detected (JAVA_HOME, then /usr/libexec/java_home / common JVM locations), or set alloy.javaPath. The Alloy release is Java-17 bytecode, so a 17+ runtime is required even if your default java is older.
• Alloy 6+ — auto-detected from your workspace, ~/Downloads, ~/Desktop, /Applications, or a prior download; otherwise the extension offers to download a pinned release once, or set alloy.jarPath.

Quick start

1. Open a .als file.
2. Click Open Cope and Drag (the graph icon in the editor title bar), or use the ▶ Run CodeLens above a command.
3. Explore the instance in the CnD panel — evaluate expressions, switch commands, and step through instances.

Custom layouts (.cnd sidecar)

Create a foo.cnd next to foo.als to control the visualization — for example:

constraints:
  - cyclic:
      selector: next
      direction: clockwise
directives:
  - flag: hideDisconnectedBuiltIns
  - attribute:
      field: value

The spec is attached to every instance (and persists across enumeration). Saving the .cnd re-applies the layout to the open visualization; the Alloy: Reload Cope and Drag Layout command does the same on demand.

Settings

• alloy.javaPath — explicit Java 17+ executable (otherwise auto-detected).
• alloy.jarPath — explicit Alloy 6+ jar (otherwise auto-detected, then downloaded on demand).

How it works

.als ─"Open Cope and Drag"─▶ extension spawns
        java -cp <alloy.jar>:<cnd-alloy-server.jar> org.alloytools.cnd.CnDServer <port> <file>
                                          │  (calls the Alloy API: run / fork / eval)
                                          │  line-based JSON socket
   Sterling provider (Node, in-extension) ── Sterling ws ─▶ <iframe> Cope and Drag

The bridge (alloy-bridge/CnDServer.java) only calls Alloy's public API (execute_commandFromBook, A4Solution.fork(int)/.eval(), A4SolutionWriter.writeInstance), so it runs against any compatible Alloy jar. Enumeration goes through fork(int), the primitive behind the Analyzer's trace buttons (next() is fork(-3)); for temporal models the explorer offers New Config, New Trace, and New Init. The extension translates between that socket and the Sterling websocket protocol Cope and Drag speaks.

The bundled Cope and Drag is its Alloy build — the explorer drawer is collapsed by default, so a run opens straight to the graph. That build hardcodes its websocket URL, so the extension rewrites it to the live provider port as it serves the bundle.

Building from source

Prerequisites: JDK 17+, Node 18+, and a Cope and Drag build:alloy bundle.

npm install
# CND_DIST is a Cope and Drag dist built with `yarn build:alloy`
JAVA_HOME=/path/to/jdk-17 CND_DIST=/path/to/copeanddrag/dist npm run bundle
npm run compile

npm run bundle writes server/cnd-alloy-server.jar (the bridge) and media/copeanddrag/. No Alloy jar is packaged — it's downloaded at runtime or auto-detected. Press F5 to launch an Extension Development Host.

License

MIT