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Try Capy in your browser

Capy compiles to WebAssembly. The playground below runs the real compiler — no server round-trip — on six curated sample DSLs you can edit live. Press Run (or ⌘/Ctrl + Enter) to regenerate; press Download to grab the result.

If the iframe doesn't load, open the playground in a new tab.

What you can do here

  • Pick a sample from the dropdown (recipe / invitation / meal plan / reading log / Breakout / Snake).
  • Edit the script in the left pane. Press Run; the right pane updates.
  • Preview HTML output inline (recipe cards, invitation, games all render directly).
  • Download what you made — single file or zip archive for multi-file outputs.
  • Open the library (click the disclosure at the bottom) to see or modify the grammar that powers the sample.

How it works

your browser
    │
    ├─ index.html  ── the playground UI
    ├─ wasm_exec.js ── Go's standard wasm runtime
    └─ capy.wasm   ── the Capy compiler, ~6.2 MB,
                       loaded once, cached forever

The capy.wasm module is the same engine that ships with the CLI and the MCP server, compiled with GOOS=js GOARCH=wasm. It exposes a single function capyRun(lib, format, script) returning the rendered output and (for multi-file libraries) the full file map.

Source: cmd/capy-wasm/main.go.

Run the playground locally

git clone https://github.com/olivierdevelops/capy
cd capy
scripts/build-playground.sh        # builds capy.wasm + bundles samples.json
python3 -m http.server -d docs/assets/playground 8000
# open http://localhost:8000

The scripts/build-playground.sh script does three things:

  1. Compiles cmd/capy-wasm to capy.wasm with GOOS=js GOARCH=wasm.
  2. Copies Go's wasm_exec.js runtime alongside it.
  3. Runs cmd/playground-bundle to bake the six curated samples into samples.json.

CI re-runs all three on every deploy.

What if I want my OWN library in the playground?

Edit the library pane (expand the disclosure at the bottom) and the playground will use it instead. Or fork the repo and add an entry to the CURATED list in cmd/playground-bundle/main.go.

What it can't do (yet)

  • No source @import "..." — the playground has no filesystem. Inline everything into one script.capy.
  • No file system writes — multi-file libraries still work (you'll see one tab per file in the output), but the files only exist in the browser until you click Download.

Limits

The wasm bundle is ~6 MB. It loads once and is cached by the browser. After that, every Run is a few milliseconds (the compilation is pure CPU, no network).

Most libraries run in well under 100 ms even for the largest samples in the picker. If you're using your own multi-thousand-line library, expect proportionally longer compile times.