README.md

rust-primes

Prime counter in Rust/WASM demonstrating async Rust futures compiled to WebAssembly. Exported functions return Promise<number> via wasm-bindgen-futures.

Deploy

ribo deploy    # runs wasm-pack build --target web

Invoke

ADDR=<addr>

# count primes in [2, 100000)
curl -X POST http://localhost:8080/invoke/$ADDR/count_primes_async \
     -d '{"args":[2,100000]}'

# count two ranges "concurrently" (they run sequentially — see below)
curl -X POST http://localhost:8080/invoke/$ADDR/count_primes_joined \
     -d '{"args":[2,50000,50000,100000]}'

What this demonstrates

• wasm_bindgen_futures::future_to_promise — wrapping a Rust async fn as a JS Promise
• CPU-bound futures are not parallel: future::join on synchronous work runs range 1 to completion before range 2 starts, because neither future ever yields mid-computation
• Async Rust in WASM is cooperative, not parallel — true in any single-threaded V8 environment
• Compare with rust-analyst which uses yield_now() to make interleaving explicit