Async operations should be first-class citizens in Vera, modelled as an algebraic effect. An <Async> effect with async and await operations fits naturally:

fn fetch_both(@String, @String -> @Tuple<Json, Json>)
  requires(true)
  ensures(true)
  effects(<Http, Async>)
{
  let @Future<Json> = async(http_get(@String.0));
  let @Future<Json> = async(http_get(@String.1));
  let @Json = await(@Future<Json>.1);
  let @Json = await(@Future<Json>.0);
  Tuple(@Json.1, @Json.0)
}

Key design points:

• async(expr) wraps an effectful computation in a Future<T>, starting it concurrently
• await(@Future<T>.n) suspends until the future resolves
• The <Async> effect must be declared, making concurrency explicit
• Handlers can provide different scheduling strategies (thread pool, event loop, sequential)
• Avoids coloured-function problems because algebraic effects already separate description from execution

Spec reference: spec/00-introduction.md § 0.8 "Asynchronous Promises/Futures"