WebAssembly

Flow compiles to WebAssembly through the C backend and emits a runnable page alongside the module.

Requires FLOW_HOST=python and Emscripten on the path.

Build

FLOW_HOST=python flow wasm program.flow --backend=c --out build/wasm

Serve the output directory over HTTP. Opening the page from file:// fails on module fetch.

cd build/wasm && python3 -m http.server 8000

Graphics in the browser

stdlib/gfx.flow targets canvas on the web, so a gfx program builds for the browser without changes. gfx_time_ms becomes performance.now() and gfx_wait_frame becomes a no-op, because gfx_present already waits on requestAnimationFrame. Pace the loop with gfx_wait_frame anyway so the same source behaves correctly in a native window.

Read first

ls examples/wasm/
ls scripts/ | grep wasm
cat docs/demos/wasm.md

scripts/build_wasm.sh, scripts/build_all_wasm.sh, and scripts/build_wasm_gallery.py are the pipelines the project uses for the published gallery. Follow them rather than inventing a build.

Verify

A WebAssembly build is only done when the page runs. Check:

• the module loads with no console error;
• the program produces the same output it produced natively, for at least one input;
• the frame loop advances rather than rendering once and stopping.

scripts/verify_browser_interp.py exists for this. Say which check you ran. Do not report a working demo you have not loaded.

Pitfalls

• Emscripten missing produces a confusing failure early. Check emcc --version first.
• Files a program opens must be preloaded with --preload; there is no host filesystem.
• Threads need cross-origin isolation headers, which a plain http.server does not send.
• Size grows quickly with linked runtimes. Report the module size.