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 / next / 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.next()/.eval(), A4SolutionWriter.writeInstance), so it runs against any compatible Alloy jar. The extension translates between that socket and the Sterling websocket protocol Cope and Drag speaks.

Building from source

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

npm install
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